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Hardware support => Projects => Topic started by: Lukapple on April 04, 2015, 06:02:29 AM

Title: Driving watering electronic timer with Moteino
Post by: Lukapple on April 04, 2015, 06:02:29 AM
Hi.
I bought electronic timer for watering the garden. What I want to do is controlling it with Moteino.
Timer module can be seperated from the part with valve. Those two modules are connected with cinch.
I've connected multimeter to that cinch connector on timer module and measured voltage when ON/OFF button is pressed
However, my multimeter doesn't support "max peak measuring", so on the attached pictures are seen approximate measuring results (ON = +0.5V, OFF= - 1.5V).

Any idea what kind of valve is used in watering timers? What is the best way to control that valve with Moteino? Can it be done only with Moteino or do I need any additional electronic components?

Here are some photos of the electronic timer:
(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/timer_box.jpg)
(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/timer.jpg)
(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/timer_off.jpg)
(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/timer_on.jpg)


Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 04, 2015, 08:23:28 AM
Typically those valves are controlled by a low voltage DC motor that drives a gear train which brings the speed down to a couple RPM and increases the torque needed to open a ball valve.  The tricky part is knowing EXACTLY when the valve is open and when it is closed.  On the devices I've taken apart, the shaft of the ball valve had a cam that tripped a microswitch so the controller knew exactly what state it was in.  Unless there is some electronics in the valve housing, I'm not sure how the controller knows that the valve is open or closed.

Since you only have a meter (not a scope), it would be difficult to know exactly what protocol is used.  However, since a low power controller is unlikely to operate above 5V and most logic these days will operate with 3V signals, you might be able to experiment to see if it uses some DC state to control the valve.  Given that you get both positive and negative signals, the controller signal might be capacitively coupled to the valve.  A simple experiment might be to use a 10-100uF and a 3volt battery.   Tie the - side of the capacitor to your valve input and the - side of the battery to the common terminal.

To turn on the valve, try connecting the + side of the capacitor to + side of battery, leave it there for a couple of seconds (so the capacitor charges).   
To turn it off,  take the + side of the charged capacitor to the common (- side of the battery).  Does the valve turn off?

If you were in the US I'd offer to scope it for you (because I'm curious myself), but I see the French packaging so that would be difficult and expensive to send back and forth (unless you are in Canada).

Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: Lukapple on April 04, 2015, 02:39:29 PM
Hi.
Thanks for your answer. Next week I'll get osciloscope and will do some measures and post results. I'll also take apart valve module, to see what's inside.
Title: Re: Driving watering electronic timer with Moteino
Post by: Lukapple on April 08, 2015, 02:22:40 PM
Update:
Here is how does ON/OFF signal looks like:

(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/on_off_signal.png)

Any hints how to create that signal with Moteino ?
Title: Re: Driving watering electronic timer with Moteino
Post by: Lukapple on April 09, 2015, 04:47:15 PM
Update:
In valve module there is just dc motor.
(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/valve_motor.jpg)
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 10, 2015, 05:26:56 PM
Sorry for not replying sooner, I thought I had, but I guess it was the time I found myself face down, sleeping on the keyboard (bnbnbnbnbnbbnbnbnbbnbn).  :-[

It looks like the 'motor' isn't a motor at all and is a DC Latching Solenoid!  Cool!  How much does this valve/controller cost???

In any case, have I got a circuit for you!  I haven't deployed this project yet, so the SW isn't final, but attached is a schematic that will generate the voltage (12V for my solenoid but can easily be shifted in SW to 9V) AND give you a healthy pulse to drive the solenoid to one state or the other.  It takes the Moteino about one second to charge up the cap to 12V and only 10mS to trip my solenoid (Hunter DC Latching).

I'll post the complete design some time next week.

Tom

Title: Re: Driving watering electronic timer with Moteino
Post by: Lukapple on April 11, 2015, 04:07:00 AM
Hi Tom.
Woow, you're genius! It's true, valve is using DC Latching Solenoid (like part from old pinball machine).
I went deeper and disassembled that valve.
Here are additional details:
(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/valve1.jpg)
(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/valve2.jpg)
(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/valve3.jpg)
(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/valve4.jpg)

There is some rubber seal attached on solenoid and it seals hole on that white square part, so water stops.

Here is video  (https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/%20Latching_Solenoid_demo.mov) of that solenoid in action  ;D

That water timer was not so expensive, abut $30.

Thanks a lot for schematics, I'll study it and will be back with more questions :)

L
Title: Re: Driving watering electronic timer with Moteino
Post by: Lukapple on April 13, 2015, 03:34:58 PM
Hi Tom.
Any updates on your project yet? :) Can't wait to order components and put pcb board together.
Could you explain to me how does your circuit diagram work?
How much current does that "healthy pulse" need?

Thanks
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 13, 2015, 05:01:09 PM
Hi Tom.
Any updates on your project yet? :) Can't wait to order components and put pcb board together.
Could you explain to me how does your circuit diagram work?
How much current does that "healthy pulse" need?

Thanks
I just started debugging late this afternoon so haven't made much progress on it yet.  It'll be a couple of days yet.

The large capacitor is charged to the solenoid voltage and then dumped into the solenoid so all the heavy current is supplied by the large cap, not the battery.  IIRC the peak current from the battery was less than 65mA, but I'll have to measure it again, once it's running.

Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: Lukapple on April 16, 2015, 10:51:34 AM
Hi Tom.
Few questions:
- why do you need BATT1 (2xAA). Is this the power for Moteino? If yes, then it should be probably connected to PWR Moteino pin (currently it's connected to 3.3v output)?
- you're using D5 pin to open Q5, and when Q5 is in open state, C3 is charging. With D5 you control how much V you charge Q5. Is that right?
- C1 and L1 are there for protection, right?
- v_solenoid is voltage of C3, which is dumped into the solenoid, right?

Could you explain to me why did you choose that approach. It would be probably easier, if you use just that transistor H-bridge, with gates connected to Moteino and power solenoid with 9v battery (for my case) and then controlling pulse length with software timer in moteino? Would that work?
What is the advantage of using your version with caps?

Thanks
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 16, 2015, 07:36:31 PM
My posting for this project will explain the updated circuit, but a couple of answers below...
Hi Tom.
Few questions:
- why do you need BATT1 (2xAA). Is this the power for Moteino? If yes, then it should be probably connected to PWR Moteino pin (currently it's connected to 3.3v output)?
If you look at the note inside the Moteino Module block, you'll see that I remove the VR from the module so that I can drive the 3.3V pin as a power input and, yes, the 2XAA drives the Moteino directly.
Quote
- you're using D5 pin to open Q5, and when Q5 is in open state, C3 is charging. With D5 you control how much V you charge Q5. Is that right?
- C1 and L1 are there for protection, right?
- v_solenoid is voltage of C3, which is dumped into the solenoid, right?
Nope, Nope, Nope. C3 is simply a filter cap on the power supply (VBat).  Q5 supplies current into L1 when D5 is low and that current is blocked by D1. When Q5 is turned off, however, that's when the interesting stuff begins... Think about it...
Quote
Could you explain to me why did you choose that approach. It would be probably easier, if you use just that transistor H-bridge, with gates connected to Moteino and power solenoid with 9v battery (for my case) and then controlling pulse length with software timer in moteino? Would that work?
What is the advantage of using your version with caps?
'Work' is a matter of definition.  Yes, it would work.  Would it be easier?  Sure.  Would it be efficient?  Not at all.   Think about what power this device needs and when.  The device needs ~3.3V ALL the time.  It needs a High Voltage some of the time (rarely, actually, a few seconds every few days).   What is the mAH rating of your anemic 9V battery, most of which is being thrown away regulating down to 3.3V.  What is the mAH rating of a couple of hefty AA batteries, which can supply the power directly to the Moteino without conversion or loss...
Quote

Thanks
You're welcome...   :D
Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 17, 2015, 05:40:07 PM
FYI, I started the description of my Sprinkler Valve in its own thread: https://lowpowerlab.com/forum/index.php/topic,1046.msg6718.html#msg6718

I should be able to provide full documentation over this weekend.

Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: Lukapple on April 17, 2015, 05:45:22 PM
Hi Tom.
Thanks for answers and for sharing your work with us. Can't wait for full documentation.
 :D
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 18, 2015, 02:21:23 PM
Well, as luck would have it, my own hose water timer died just as we're having this discussion.  It didn't 'die' exactly, but it began leaking out of the housing.  The unit is an Orbit Single Dial Water timer (http://www.amazon.com/Orbit-62061N-91213-Single-Dial-Water-Timer/dp/B004INGS8S) and I replaced it with a Nelson Large Timer w/ LCD Screen (http://www.amazon.com/gp/product/B007FG7TWO).  As far as I can tell, the two units are identical except in appearance and quality - the Nelson appears to be better quality (despite its lower price) - it's more solidly built and has a metal coupling in the inlet side vs plastic on the Orbit.

In any case, I replaced the Orbit and, having had this discussion, decided to take it apart (I probably would have done that anyway, but was even more 'inspired' now).

Four very interesting discoveries:

1. The place where it was leaking was actually poorly assembled and after reseating the cover on the valve (and generous application of silicone grease) the unit no longer leaks and is still usable.
2. This (and the Nelson) use a DC Latching Solenoid as well!!!  My, my.  I thought these were like another valve I took apart that had gear motor driven ball valve.  Nope. 
3. When I took the valve apart to reseat it (and learn about its insides) I was surprised to find that the metal plunger inside did not 'toggle' between two magnetic ends as I had imagined.  The plunger moved freely inside the coil shell with only a spring to keep it closed.  Hmmmm, is it powered all the time it's on?
4. Nope!  It has a similar type of bipolar drive as the DC Latching solenoid I've been testing recently and the type Lukapple discovered in his own device.  A lot higher voltage and current than my valve, but bipolar nonetheless!

After learning #4 above, I've concluded that the plunger must get magnetized from the heavy pulse and, consequently 'sticks' in the open position until it's demagnetized (or reverse polarized) by the close pulse.  Regardless, my conclusion is that a similar type of drive as I use on my own sprinkler control could be used for this type of solenoid as well.  The only caveat is that the voltage and current is much higher.

I measured pulses that peak at about 22 volts and over 3 1/2 Amps!  That's a lot of energy!

Attached below are my measurements.  Yellow trace is current at 1A/V and the blue trace is voltage (10V/div).

Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: EloyP on April 19, 2015, 02:45:14 AM
Well, as luck would have it, my own hose water timer died just as we're having this discussion.  It didn't 'die' exactly, but it began leaking out of the housing.  The unit is an Orbit Single Dial Water timer (http://www.amazon.com/Orbit-62061N-91213-Single-Dial-Water-Timer/dp/B004INGS8S)

[...]

In any case, I replaced the Orbit and, having had this discussion, decided to take it apart (I probably would have done that anyway, but was even more 'inspired' now).

[...]

After learning #4 above, I've concluded that the plunger must get magnetized from the heavy pulse and, consequently 'sticks' in the open position until it's demagnetized (or reverse polarized) by the close pulse.  Regardless, my conclusion is that a similar type of drive as I use on my own sprinkler control could be used for this type of solenoid as well.  The only caveat is that the voltage and current is much higher.

I measured pulses that peak at about 22 volts and over 3 1/2 Amps!  That's a lot of energy!

Attached below are my measurements.  Yellow trace is current at 1A/V and the blue trace is voltage (10V/div).

Wow. Just wow! I can't believe my luck and how timely this is! :) This forum is great for sharing ideas and knowledge.

For a long time (a couple of years, on and off, but definitely for *too* long) I have been working on a DC latching solenoid-based water valve controller. The valves have always being made by Orbit. Initially, they were of this type:

http://www.amazon.com/Orbit-62035-Extra-Watering-System/dp/B0016HQOYC/

About 8 years ago I bought two Orbit kits, each with a couple of these valves and a timer. These valves are now discontinued, and while the timers are pretty decent, their limitations (mainly that I hated when it rained in the middle of the night and there was no way for the timer to know there was no need to turn on the valves right before sunrise) is what planted in me an obsession to built a better controller for the type of water valves that can be operated by batteries, i.e. DC latching solenoid-based water valves.

So, about two years ago I started to research, standing on the shoulders of giants, how to drive the above valves:

http://blog.chapus.net/orbit-62035-redux/

Using data from this research I started the design of my own controller. Along the way I found out that the above valves were discontinued and replaced by the valves that (I think) your valve controller drives, Tom. Am I correct to infer that your controller drives valves like these?

http://www.amazon.com/dp/B002HJOUYY?psc=1

Because my original valves were discontinued, I designed my project (borrowing ideas from some other people's work) so it could handle both types of valves (either/or) -- one side of the PCB would handle the old valves, and the other side would handle the new valves. My thought was that eventually my old valves were going to die and I was going to have to replace them with the new generation valves, so I decided it was better to design for the future.

Fast forward to today and you will find me doing "field tests" of this project. The problem I ran into is that things "kind of" work, meaning the valves (the new generation, which starting this year I am using exclusively) would open/close "most of the time". This is obviously catastrophic in this scenario -- imagine a valve that fails to close when you are not around to notice.

Why do I think I am so lucky and the timing of Tom's post is perfect? Well, I can't find any major flaws in my design, both hardware and software (perhaps some of you will, and I'd love to hear about it). And yet, the darn valves won't activate sometimes, so I have been scratching my head for a while. My next move was going to purchase a timer that can control the new generation valves to scope the output and make my own controller behave the same way. In fact, I was planning to go to Lowe's this weekend to get me one of these controllers. But, that won't be necessary now because Tom has just provided the measurements that I need!

Is that time base 4 milliseconds, for a pulse length of about 16  milliseconds,Tom? If so, I think my problem is too low initial voltage and two long of a pulse (I was using around 18 VDC, and afraid of going higher out of fear of damaging the valve, and 20 milliseconds), which are easy to change by modifying my device's configuration.

Once I am sure everything is working well 100% I will publish the software, the Eagle files, and documentation. As a preview, I am attaching a PNG of my design (it does not use a Moteino but the next generation will), a picture of a partially assembled board and a board before going into the reflow oven, and a picture of the board I have in the yard for my field tests (the twisted pair wires are for measurements with a scope on the other side of the brick foundation).

Thanks again Tom!

Cheers,

Eloy Paris.-
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 19, 2015, 10:16:07 AM
<snip>

Wow. Just wow! I can't believe my luck and how timely this is! :) This forum is great for sharing ideas and knowledge.
No kidding!  I just got a very valuable piece of information from you!   And this is it:
Quote
<snip>
http://www.amazon.com/dp/B002HJOUYY?psc=1
I wasn't aware that they sold the valves separately!  Thanks for the link!  I just ordered one of these to check it out, but I'm sure it's simply a repackaging of the same valve used in the timer I referenced in my previous post.  I've attached a photo of the 'insides'. 

Quote
<snip>
Fast forward to today and you will find me doing "field tests" of this project. The problem I ran into is that things "kind of" work, meaning the valves (the new generation, which starting this year I am using exclusively) would open/close "most of the time". This is obviously catastrophic in this scenario -- imagine a valve that fails to close when you are not around to notice.
<g>  If you look very closely at the two traces below, you will see why you might have a problem and Orbit doesn't (as often).  Notice the first trace has a 'wiggle' a 4.5 mS into the pulse?  That is a result of the metal piece moving inside the core.  See in the second trace there is no such 'wiggle'.  Why?  Because the metal piece has already moved.  Orbit actually sends a second pulse about 10 seconds later than the first (probably the time it takes them to rebuild the charge), probably as a safety measure.  If it didn't go on the first one (sticky maybe) it will probably go on the second, would be my guess.
Quote

Why do I think I am so lucky and the timing of Tom's post is perfect? Well, I can't find any major flaws in my design, both hardware and software (perhaps some of you will, and I'd love to hear about it). And yet, the darn valves won't activate sometimes, so I have been scratching my head for a while. My next move was going to purchase a timer that can control the new generation valves to scope the output and make my own controller behave the same way. In fact, I was planning to go to Lowe's this weekend to get me one of these controllers. But, that won't be necessary now because Tom has just provided the measurements that I need!

Is that time base 4 milliseconds, for a pulse length of about 16  milliseconds,Tom? If so, I think my problem is too low initial voltage and two long of a pulse (I was using around 18 VDC, and afraid of going higher out of fear of damaging the valve, and 20 milliseconds), which are easy to change by modifying my device's configuration.

Once I am sure everything is working well 100% I will publish the software, the Eagle files, and documentation. As a preview, I am attaching a PNG of my design (it does not use a Moteino but the next generation will), a picture of a partially assembled board and a board before going into the reflow oven, and a picture of the board I have in the yard for my field tests (the twisted pair wires are for measurements with a scope on the other side of the brick foundation).

Thanks again Tom!

Cheers,

Eloy Paris.-
From what I've seen of this Orbit valve, there are three things you need to have to make sure you have a sufficient signal:
1. Sufficient voltage on the boost.  At least 22V, maybe even 25V.  The resistance of the coil was 4.6 Ohm IIRC, coupled with the high inductance you need a lot of volts to build up the current quickly.
2. It really needs 3.5A initially, at least for the first 7 mS the current needs to be very strong.  Your 2200uF should be enough, but can your H bridge drive this much current with very little voltage drop.
3. Duration of the pulse should be at least 16mS, 20mS should be fine.

My circuit, as it is, won't drive this relay.  I used 20V transistors and the cap is only 16V.  Both of those would need to change (in the H bridge anyway) and change the resistor divider on the voltage sense.  However, all of those would require only component replacement.  There shouldn't need to be any other circuit change.  I would need to tweak the SW, but I think I can handle that  ;)

On your circuit, I would put a current limit resistor in series with L1 'cause a SW controlled switch is bound to get stuck on at some point...  Otherwise it looks pretty good.  I like that you support multiple channels (especially for a hose type valve).  Your H-Bridge drivers have a higher Vgsth than mine, but probably at the point that the VBoost collapses to where this would be an issue, all the work has been done.

I really like your water-resistant enclosure with its customizable hanging system!

As you said, the beauty of a forum is sharing information.  Thanks for sharing!

Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: EloyP on April 22, 2015, 01:02:14 PM
Hi Tom!

I wasn't aware that they sold the valves separately!  Thanks for the link!  I just ordered one of these to check it out, but I'm sure it's simply a repackaging of the same valve used in the timer I referenced in my previous post.

Ah, sorry, I forgot that you said you are using an integrated timer/valve. The standalone valves that I am using are different from a product point of view. But as you say, from an electrical point of view I am pretty sure that the valves work the same way. The standalone valves are available at Lowes, and probably at Home Depot as well.

Quote
Orbit actually sends a second pulse about 10 seconds later than the first (probably the time it takes them to rebuild the charge), probably as a safety measure.  If it didn't go on the first one (sticky maybe) it will probably go on the second, would be my guess.

Ahh, very interesting. That's kind of "cheating", but it's actually a good idea. I'll implement the same thing. Better safe (and cheater) than sorry.

Quote
From what I've seen of this Orbit valve, there are three things you need to have to make sure you have a sufficient signal:
1. Sufficient voltage on the boost.  At least 22V, maybe even 25V.  The resistance of the coil was 4.6 Ohm IIRC, coupled with the high inductance you need a lot of volts to build up the current quickly.

Yes, my voltage booster has no problem charging to 25 VDC. The actual design is borrowed from here:

http://www.arduino.cc/playground/Main/RegulatedPositiveVoltageBooster

The capacitor is cutting it close -- it's rated for 25 VDC. I need to check the tolerance but I probably should not go up to 25 VDC just to be safe.

Quote
2. It really needs 3.5A initially, at least for the first 7 mS the current needs to be very strong.  Your 2200uF should be enough, but can your H bridge drive this much current with very little voltage drop.

Indeed, transistor selection is a key part of this design. I think the H-bridge transistors I am using should be able to handle the task -- they are rated at up to 7.3 Amps continuous current and a max VDS voltage of 30 Volts.

https://www.fairchildsemi.com/datasheets/FD/FDS8858CZ.pdf

Quote
My circuit, as it is, won't drive this relay.  I used 20V transistors and the cap is only 16V.  Both of those would need to change (in the H bridge anyway) and change the resistor divider on the voltage sense.  However, all of those would require only component replacement.  There shouldn't need to be any other circuit change.  I would need to tweak the SW, but I think I can handle that  ;)

Agreed. The circuit itself is actually not too complicated. The key is component selection.

For the next version of my project I am planning to take the time to understand your H-bridge as it uses no transistors for driving it, which would reduce component count.

Quote
On your circuit, I would put a current limit resistor in series with L1 'cause a SW controlled switch is bound to get stuck on at some point...  Otherwise it looks pretty good.  I like that you support multiple channels (especially for a hose type valve).  Your H-Bridge drivers have a higher Vgsth than mine, but probably at the point that the VBoost collapses to where this would be an issue, all the work has been done.

Current limiting resistor in series with L1 is a good idea -- in fact, a software glitch is not even necessary for the switch to get stuck: If the pulse duration is too long the capacitor will discharge completely and at that time your power supply will start providing all the current. If the power supply is 5 VDC and the solenoid has a resistance of 4.6 Ohms then the current sucked by the solenoid will be around 5/4.6 =  1 Amp.

Could you elaborate on your comment regarding H-bridge drivers and Vgsth? Is a higher Vgsth not a good thing because the transistor goes (and remain) into saturation more easily?

Quote
I really like your water-resistant enclosure with its customizable hanging system!

Well, I am going one thing at a time :-) The enclosure is from a Hummus plastic container. It's not permanent but I needed to focus on circuit itself first and not on the enclosure. I also view this as an opportunity to do some stress-testing of the circuit -- it's not exposed directly to water even if the enclosure gets wet with heavy rain, but humidity definitely gets in there all the time.

Quote
As you said, the beauty of a forum is sharing information.  Thanks for sharing!

And thank you as well for your information sharing! :-)

Cheers,

Eloy Paris.-
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 22, 2015, 06:16:12 PM
Hi Tom!

Ah, sorry, I forgot that you said you are using an integrated timer/valve. The standalone valves that I am using are different from a product point of view. But as you say, from an electrical point of view I am pretty sure that the valves work the same way. The standalone valves are available at Lowes, and probably at Home Depot as well.
I bought one from Amazon.  It's sitting in my workshop now.  May get to test it tomorrow.  My primary interest was in permanently installed irrigation valves, but I have a need for a number of 'hose end' type sprinkler systems and it's tough to beat the cost and convenience of the Orbit add-on valve.   I'm very glad you pointed it out to me.
Quote

Ahh, very interesting. That's kind of "cheating", but it's actually a good idea. I'll implement the same thing. Better safe (and cheater) than sorry.

The only drawback is the extra power it takes to run another charge cycle.  This will effectively cut your battery life in half (since almost all of the overall power is consumed in the charging).  However, as you say, reliability is critical in this application.

Quote
Yes, my voltage booster has no problem charging to 25 VDC. The actual design is borrowed from here:

http://www.arduino.cc/playground/Main/RegulatedPositiveVoltageBooster

The capacitor is cutting it close -- it's rated for 25 VDC. I need to check the tolerance but I probably should not go up to 25 VDC just to be safe.
It would ok for a test, but I'd use 35V for 'real life'.
Quote
<snip>

Agreed. The circuit itself is actually not too complicated. The key is component selection.

For the next version of my project I am planning to take the time to understand your H-bridge as it uses no transistors for driving it, which would reduce component count.
I can drive directly because the low Vgsth I'm using (0.85V to begin conduction), but that's a double edged sword: low Vgsth, low Vgs breakdown.  And, on further evaluation (looking boosting to support the Orbit), I realized that the Vgs breakdown on the si2302s is only 8V (it's that on the si2365 as well, but the voltage is never more than 3.3V).  With this cross coupled approach I'm driving full VSolenoid into the gate!  Might be ok for 9V but very NOT good at 25V! 

For my initial tests I'm going to try to get away with putting a resistor in series with the gates and hope that they break down gracefully (I think it does).
Quote
<snip>
Could you elaborate on your comment regarding H-bridge drivers and Vgsth? Is a higher Vgsth not a good thing because the transistor goes (and remain) into saturation more easily?
Simple, your gate bias on both top and bottom transistor comes entirely from VBoost which collapses as the cap discharges, at some point it will collapse below the Vgs threshold and the transistors will go linear.  Fortunately, as I said, 'all the work' is probably done by then, so no major problems, assuming you've delivered all the energy needed to trip the solenoid.  It's a similar problem in my case, but it's only the bottom transistors driven from VBoost and the threshold is lower voltage.
Quote
Quote
I really like your water-resistant enclosure with its customizable hanging system!

Well, I am going one thing at a time :-) The enclosure is from a Hummus plastic container. It's not permanent but I needed to focus on circuit itself first and not on the enclosure. I also view this as an opportunity to do some stress-testing of the circuit -- it's not exposed directly to water even if the enclosure gets wet with heavy rain, but humidity definitely gets in there all the time.
I wasn't criticizing, I really do like the pragmatic approach.  You focused on getting the job done and your work shows that you'll take care of the 'other' stuff later.

Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: EloyP on April 24, 2015, 03:20:45 PM
Hi Tom!

This thread has been great, and is helping me realize the errors of my ways :-)

I bought one from Amazon.  It's sitting in my workshop now.  May get to test it tomorrow.  My primary interest was in permanently installed irrigation valves, but I have a need for a number of 'hose end' type sprinkler systems and it's tough to beat the cost and convenience of the Orbit add-on valve.   I'm very glad you pointed it out to me.

Unrelated (to this discussion) question -- are your permanently installed irrigation valves of the 24 VAC type? Just curious because it is my understanding that 24 VAC valves are what real irrigation systems (not of the garden hose type) use.

Quote
I can drive directly because the low Vgsth I'm using (0.85V to begin conduction), but that's a double edged sword: low Vgsth, low Vgs breakdown.  And, on further evaluation (looking boosting to support the Orbit), I realized that the Vgs breakdown on the si2302s is only 8V (it's that on the si2365 as well, but the voltage is never more than 3.3V).  With this cross coupled approach I'm driving full VSolenoid into the gate!  Might be ok for 9V but very NOT good at 25V! 

Aha! I completely missed the breakdown voltage! Whoops! This means I need to scrap my current design and re-design -- my P-channel transistors have max. VGS voltage of 25 volts. However, the N-channels have a max. VGS of 20 volts. And not only that, but the gate of the N-channel transistors is connected via pull-up resistor to VBOOSTED, so if I boost voltage goes higher than 20 volts (which is a requirement for those Orbit valves) then I'm in trouble. So, this design is not good and I need to fix it (I won't give up!).

To start, I can drive the low-side N-channel transistors directly from the microcontroller I/O pin. That fixes the problem of too high VGS on the N-channel transistors.

The high-side P-channel transistors also have a problem -- the transistors that I am using can go up to 25 volts VGS. However, that is cutting it too close, and if I want to use another transistor with a lower max. VGS, then the design doesn't work. So, to address this, I'll use a voltage divider. That'll give the P-channel transistor some breathing room.

My new design is attached. Any thoughts are more than welcome.

Quote
For my initial tests I'm going to try to get away with putting a resistor in series with the gates and hope that they break down gracefully (I think it does).

The gate current is very small; how does a resistor in series with the gate help with the problem of too high of a VGS voltage?

Quote
Simple, your gate bias on both top and bottom transistor comes entirely from VBoost which collapses as the cap discharges, at some point it will collapse below the Vgs threshold and the transistors will go linear.  Fortunately, as I said, 'all the work' is probably done by then, so no major problems, assuming you've delivered all the energy needed to trip the solenoid.  It's a similar problem in my case, but it's only the bottom transistors driven from VBoost and the threshold is lower voltage.

I understand now; thanks for explaining. Driving the low-side transistors directly from the microcontroller in the new design will decouple boosted voltage from gate driving, but the problem will not go away for the high-side transistors. I do think you are right in that by then all the energy has been supplied to the solenoid so this is not a problem.

Quote
I wasn't criticizing, I really do like the pragmatic approach.  You focused on getting the job done and your work shows that you'll take care of the 'other' stuff later.

Oh, I didn't think you were criticizing. I actually thought it was funny that you noticed. Besides, constructive criticism is always good (and I most definitely welcome it!).

Thanks for a great discussion!

Cheers!

Eloy Paris.-
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 24, 2015, 03:46:49 PM

Unrelated (to this discussion) question -- are your permanently installed irrigation valves of the 24 VAC type? Just curious because it is my understanding that 24 VAC valves are what real irrigation systems (not of the garden hose type) use.
In the installed system, yes, they are standard 24VAC solenoids that I control via an 8 way X10 controller (http://www.wgldesigns.com/rain8.html).  These new valves will be permanently installed as well, I just set up the first one as hose end because it was easier to test and that's what I needed for my stream auto-filler.  If, no, 'when' I get the Orbit valve working, then I'll switchover the auto filler to the Orbit and move this other one underground.

Quote
<snip>
My new design is attached. Any thoughts are more than welcome.
I'll look at your new design over a glass of wine later...
Quote
Quote
For my initial tests I'm going to try to get away with putting a resistor in series with the gates and hope that they break down gracefully (I think it does).

The gate current is very small; how does a resistor in series with the gate help with the problem of too high of a VGS voltage?
IF the gate breaks down, I want some kind of current limit on it. I'm hoping I can just lift the gate off the pad and stick in an 803 resistor.  Worse things have been done  ::)

Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: EloyP on April 24, 2015, 03:50:15 PM
The high-side P-channel transistors also have a problem -- the transistors that I am using can go up to 25 volts VGS. However, that is cutting it too close, and if I want to use another transistor with a lower max. VGS, then the design doesn't work. So, to address this, I'll use a voltage divider. That'll give the P-channel transistor some breathing room.

My new design is attached. Any thoughts are more than welcome.

Hhhhmmm. Your initial design and that voltage divider I mention above got me thinking... What about using the boosted voltage to drive the high-side transistor gates (like in your initial design) but using a voltage divider to bring down the VGS voltage of the high-side transistors; do you think that might work? I am very tempted to test this because if it works it would reduce the number of parts by two NPN transistors and two resistors, and make for a very elegant solution (thanks to you ;-) ).

Design attached.

Cheers,

Eloy Paris.-
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 24, 2015, 04:51:01 PM
Eloy,
I'd rather modify your previous design for one very simple reason, I am not confident that a 3.3V processor can reliably drive your N channel transistors.
If you look at the FDS8858 datasheet, it is very tempting to focus on the current vs Vgs curve (Figure 1, attached below).  Here you see gobs of current with a Vgs of 3.0V.  However, if you look at Figure 4, also attached below, you'll see the Rds(on) vs Vgs showing that the knee of the control voltage is at around 4V, well above the processor output voltage (unless you're using 5V Arduinos, in which case, ignore all this  :D

Compare that to the si2302 (which I'm not recommending, just pointing out the difference) you'll see the Rds(on) vs Vgs knee is below 3V, safely within the processor's output.

Now, how does that affect your circuit?  In the latest one and the previous one you're driving the N transistors from the processor and, as I say, I'm not sure that's a good thing.  One thing to notice on the FDS8858 datasheet, however, is that they explicitly show the 'zener' diode between the gate and source pins.  This implies that you can reliably overdrive if you current limit and I think that's a safe bet, especially given that the Vgs breakdown is 20V for the N device.

So, on your various implementations:
1. for your original, I would simply put a 10K resistor in series with all the gates.  This will limit the current at breakdown but still provide enough drive on the P channel gate.
2. for your circuit #2, I'd change R19 and R22 to a much higher value so that your P channel gate doesn't get cut off too soon as VBoost collapses (right now it only gets 1/2 of VBoost).  For the N channels, cross couple like I did but use 10K resistors in series with the gates.
3. for your circuit #3, Uh, don't do this one... unless, as I said, the processor is 5V.

Tom
Now on to Happy Hour...
Title: Re: Driving watering electronic timer with Moteino
Post by: EloyP on April 24, 2015, 05:58:13 PM
Hi Tom,

[...]
So, on your various implementations:
1. for your original, I would simply put a 10K resistor in series with all the gates.  This will limit the current at breakdown but still provide enough drive on the P channel gate.
2. for your circuit #2, I'd change R19 and R22 to a much higher value so that your P channel gate doesn't get cut off too soon as VBoost collapses (right now it only gets 1/2 of VBoost).  For the N channels, cross couple like I did but use 10K resistors in series with the gates.
3. for your circuit #3, Uh, don't do this one... unless, as I said, the processor is 5V.

Thanks a lot for the insight and suggestions. Decisions, decisions, decisions. Lots of things to consider!

By the way, I am not married to the FDS8858CZ -- there are several options for dual N & P-Channel in SOIC-8 package (and it seems like with compatible pinouts), which I think is very convenient for putting four valve drivers in one board. The FDS8858CZ just happens to be the first one that I looked at and bought a few samples of.

Enjoy happy hour!

Cheers,

Eloy Paris.-
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 26, 2015, 12:19:34 PM
I finally got a chance to test the Orbit AddOn valve (not with my circuit, just a breadboard setup).

Here's what I've learned:
Note that I didn't have 1000 or 2000uF caps at this voltage so paralleled multiple 470s, so the measurements should be reliable at the stated capacitance.

Given this, you MIGHT be able to get away with 20V transistors and 2000uF cap if you pump the VBOOST to >18V.  In the tests, even with 2000uF, the voltage was pretty much discharged within 20mS so I'd use this length pulse.

I will probably design the next version to operate up to 24V with the option to use 2000 or 1000uF cap.  I'd rather use a 1000uF cap since it will only take 1/2 the time to charge and that's where my load is...

Here's the next iteration of the circuit.  I swapped the polarity.  The previous version was an attempt to avoid the transistor cutting off supply to the inductor, but that failed so, back to plan A.  This version uses dual transistors like you do, only different ones to keep the N channel Vgsth within safe limit.  I might go back to discrete transistors since these dually's are more expensive and don't seem to save any space.  Don't know yet...

Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: EloyP on April 26, 2015, 01:51:17 PM
Hi Tom!

Great testing! I've been thinking about this all weekend so your update is very timely...

I finally got a chance to test the Orbit AddOn valve (not with my circuit, just a breadboard setup).

Here's what I've learned:
[...]
Given this, you MIGHT be able to get away with 20V transistors and 2000uF cap if you pump the VBOOST to >18V.  In the tests, even with 2000uF, the voltage was pretty much discharged within 20mS so I'd use this length pulse.

Agreed, that is what I have seen in the oscilloscope traces -- after 20 msecs that 2200 uF capacitor is pretty much discharged.

I have not had a chance to do live tests using a boosted voltage greater than 18 volts but in the few tests that I have done things seems to have worked well when using 22 volts. I started to use 22 volts for testing without water after you posted the oscilloscope traces for your broken integrated timer/valve.

Also, keep in mind that for a couple of weeks I was operating the valves for real (I needed to keep wet some new seed I put down) using 18 volts -- I mentioned that every now and then the valves failed to open. I think that was because the voltage I was using for the pulse was too low. But now I am also thinking that my H-bridge driver circuitry was a bit flawed (specifically, exceeding the maximum VGS on the N-channel transistors).

Quote
I will probably design the next version to operate up to 24V with the option to use 2000 or 1000uF cap.  I'd rather use a 1000uF cap since it will only take 1/2 the time to charge and that's where my load is...

Yes, I think it is a good idea to design so one can go up to 24 volts. You never know what solenoid you may encounter down the road :-)

Quote
Here's the next iteration of the circuit.  I swapped the polarity.  The previous version was an attempt to avoid the transistor cutting off supply to the inductor, but that failed so, back to plan A.  This version uses dual transistors like you do, only different ones to keep the N channel Vgsth within safe limit.  I might go back to discrete transistors since these dually's are more expensive and don't seem to save any space.  Don't know yet...

I like it! I see that you put a voltage divider to bring down the voltage used to drive the P-channel transistors. Great! That's what I had in mind too. And driving the P-channels through the H-bridge N-channels is great too. That eliminates the NPN transistor I had in my 1st and 2nd designs. I think I will go with this design as well!

This weekend I've been looking for better transistors for the H-bridge. Like you, I will be using Moteinos for this so I don't have 5 volts to drive the H-bridge (you mentioned this in your post from Friday where you offered suggestions for my three possible designs). For some reason that dual N-channel/P-channel that you are planning to use is not showing up in my search at mouser.com.

You do have a point about the dual transistors in a single package... Options are also more limited. It might make sense to use discreet components. I'll look into that as well.

Other things I like about your design:

- Gate pull down resistors. Good best practice. My H-bridge is driven by a serial register, which produces logical 0 at power up, but I still think it is a good idea to put them there.
- Transistor to turn on the voltage booster subsystem. Saves battery. Good to have it there. I think I'll borrow this idea.
- Driving the H-bridge directly from a microcontroller GPIO pin -- that is what I have wanted to do all along but as you pointed out to me, I chose the wrong power transistor to begin with.
- The on-board antenna saves space. Did you build it yourself?
- Soil moisture sensor: I have a rain/freeze sensor input (for this: http://www.lowes.com/pd_172955-74985-57069_0__?productId=1043275) but I think a moisture sensor is also a must. What are you planning to use? I am thinking about using something based on this: http://wemakethings.net/chirp/. I'd probably build something based on that without using the microcontroller he has because the ATmega on the Moteino should be able to take on that job.

I am planning to add a .1 uF in parallel with the big capacitor to try to control a bit of high-frequency noise that I have seen there when the voltage booster is on.

I think I will be done with my redesign soon. Hopefully I'll have to time to finish all this and receive prototype PCBs before watering season goes into full swing.

Speaking of EAGLE work... would you happen to have an updated library part for the Moteino? I have the one you pointed me to previously but it seems like the one you are using now is more polished.

Thanks again for sharing your knowledge and design ideas!

Cheers,

Eloy Paris.-
Title: Re: Driving watering electronic timer with Moteino
Post by: EloyP on April 26, 2015, 02:16:30 PM
Hi Tom,

I've been thinking about this a lot this weekend so I thought I'd share my thoughts on a couple of points...

Eloy,
I'd rather modify your previous design for one very simple reason, I am not confident that a 3.3V processor can reliably drive your N channel transistors.

Right, this is something that I overlooked big time. Thanks for pointing that out to me. However, I rather use different transistors so I can drive the N-channel transistors from a 3.3V signal than figuring out a way to keep the initial transistors that I chose *and* drive them from 3.3V.

Quote

Now, how does that affect your circuit?  In the latest one and the previous one you're driving the N transistors from the processor and, as I say, I'm not sure that's a good thing.  One thing to notice on the FDS8858 datasheet, however, is that they explicitly show the 'zener' diode between the gate and source pins.  This implies that you can reliably overdrive if you current limit and I think that's a safe bet, especially given that the Vgs breakdown is 20V for the N device.

This is the other thing that has kept me thinking all weekend -- I did see the clamping zener diodes in the FDS8858CZ data sheet but the data sheet also says that the +/- 20V VGS is a maximum. I suspect the transistor might get destroyed if the VGS voltage exceeds the maximum indicated in the data sheet. I have an open query with the manufacturer to see what they say regarding exceeding that voltage even with the clamping diodes there but I think they'll say the value is what the data sheet says it is, i.e. the maximum value, after which the transistor can be destroyed.

Quote
So, on your various implementations:
1. for your original, I would simply put a 10K resistor in series with all the gates.  This will limit the current at breakdown but still provide enough drive on the P channel gate.

Resistors in series with the gates would still not protect them against voltages exceeding the maximum VGS voltage, right? If so, I rather not use this implementation.

Quote
2. for your circuit #2, I'd change R19 and R22 to a much higher value so that your P channel gate doesn't get cut off too soon as VBoost collapses (right now it only gets 1/2 of VBoost).  For the N channels, cross couple like I did but use 10K resistors in series with the gates.

Right, I did not do any work to calculate the right values for the voltage divider resistors. This design could work if I find a transistor that can be driven with 3.3 volts.

Quote
3. for your circuit #3, Uh, don't do this one... unless, as I said, the processor is 5V.

This design is the one that I like the best because it uses one less transistor. However, I would need to use different transistors for the H-bridge, as you helped me realize, but I am perfectly fine with that. The way I see it, at this stage, it is better to scrap a flawed design and start over than trying to make the flawed design work.

Thanks again for your insight and ideas.

Cheers,

Eloy Paris.-
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 26, 2015, 03:42:16 PM
<snip>I have not had a chance to do live tests using a boosted voltage greater than 18 volts but in the few tests that I have done things seems to have worked well when using 22 volts. I started to use 22 volts for testing without water after you posted the oscilloscope traces for your broken integrated timer/valve.
You've made me realize that there is a flaw in my measurements!  I had been thinking that this solenoid valve is merely a low cost version of what I've been using, but your comment about testing without water reminded me that there is a FUNDAMENTAL difference between these two solenoids!  The Hunter solenoid, like virtually all commercial solenoids have a diaphram/gasket, that isolates the plunger from the water.  The Orbit plunger is in contact with the water and, as a result, the timing on the plunger is going to be affected by the viscosity of the water and any crud that's built up over the years.   If you do use this valve, I wouldn't use it at anything less than 24V and 2000uF.  I'm beginning to ask myself if I want to bother with the Orbit...
Quote

I like it! I see that you put a voltage divider to bring down the voltage used to drive the P-channel transistors. Great!
The resistors on the gate are really to current limit the gate voltage when On, and bias off the transistor when Off - not really a voltage divider (hence the large difference in resistance.
Quote
<snip>
For some reason that dual N-channel/P-channel that you are planning to use is not showing up in my search at mouser.com.
Mouser PN: 781-SI4564DY-T1-GE3, http://www.mouser.com/ProductDetail/Vishay/SI4564DY-T1-GE3

The N-Channel discrete I'm looking at is: http://www.mouser.com/ProductDetail/Vishay/SI2356DS-T1-GE3, and P Channel is: http://www.mouser.com/ProductDetail/Vishay/SI2319CDS-T1-GE3

All 40V parts...

Quote
<snip>
- The on-board antenna saves space. Did you build it yourself?
- Soil moisture sensor: I have a rain/freeze sensor input (for this: http://www.lowes.com/pd_172955-74985-57069_0__?productId=1043275) but I think a moisture sensor is also a must. What are you planning to use? I am thinking about using something based on this: http://wemakethings.net/chirp/.
The antenna I used on this prototype was a commercial one that I bought from Linx Technologies a while ago.  But winding your own is pretty easy.

LOL on the Lowes 'digital rain/freeze sensor'!  Did you ever take one of those apart?  They have a fiber washer that expands when its wet and contracts when it dries and probably expands when it freezes.  The expanding washer pushes the conductive washer on the top of it to close a circuit. You adjust the 'rainfall amount' by mechanically changing the gap between the contacts...

The Soil Moisture circuit, which you perceptively noticed the hooks in this updated schematic, is one I've used for years.  It is very reliable although you won't like the price of the probe (WaterMark Soil Sensor http://www.irrometer.com/pdf/sensors/403%20Sensor%20%20Web5.pdf).  If you're nice to me I just might send the other page of the schematic...  :D 
Actually I had planned to post my individual moisture sensor this past week but had a mishap that prevented that posting (I accidentally got the insides wet and it crapped out after a day!)  It's going back on the 'air' today.  If all is well over the next couple of days I'll be posting that.  The new Sprinkler Valve circuit has this and a ground temperature probe included...

Quote
Speaking of EAGLE work... would you happen to have an updated library part for the Moteino? I have the one you pointed me to previously but it seems like the one you are using now is more polished.

My latest library is attached below...

Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: EloyP on April 26, 2015, 05:05:08 PM
Hi Tom!

You've made me realize that there is a flaw in my measurements!  I had been thinking that this solenoid valve is merely a low cost version of what I've been using, but your comment about testing without water reminded me that there is a FUNDAMENTAL difference between these two solenoids!  The Hunter solenoid, like virtually all commercial solenoids have a diaphram/gasket, that isolates the plunger from the water.  The Orbit plunger is in contact with the water and, as a result, the timing on the plunger is going to be affected by the viscosity of the water and any crud that's built up over the years.   If you do use this valve, I wouldn't use it at anything less than 24V and 2000uF.  I'm beginning to ask myself if I want to bother with the Orbit...

I don't have experience with other solenoid valves but I understand the Orbit valves are low cost, perhaps not the best quality. The old Orbit valves (the ones with two solenoids) lasted a few years. I don't know about these new ones because this would be the first season that I use them. I did have one go bad after almost no use, but Orbit replaced it promptly. As always, we get what we pay for, i.e. cheap probably means not very good quality. I like that I can go to Lowe's and find them there and that they come with a 5-year warranty.

(Yes, I am planning to keep the 2,200 uF capacitor and I'll drive the solenoids with at least 22 volts, just like the Orbit timer does.)

Quote
The resistors on the gate are really to current limit the gate voltage when On, and bias off the transistor when Off - not really a voltage divider (hence the large difference in resistance.

Hhhhmmm. I'll have to think about this some more. I thought we want to keep the VGS voltage under the maximum specified in the data sheets to prevent damage to the transistor, and the "voltage divider" was a way to accomplish that. I have not (perhaps incorrectly) worried about current into the gate because of the very high impedance at the gate. But I have worried a lot about VGS voltage, as you've probably noticed by now.

Quote
Mouser PN: 781-SI4564DY-T1-GE3, http://www.mouser.com/ProductDetail/Vishay/SI4564DY-T1-GE3

I am sorry, what I meant is that when I use their tool to search for transistors matching certain criteria (like dual transistors, VDS > 30V, SOIC-8, etc.), this specific dual N-channel/P-channel transistor does not show up in the search results. I do see several options (about 40 in total) but not the Vishay that you are looking at. It's possible that a classification field is missing and that causes this part to not be included in the results.

By the way, that dual transistor in SOIC-8 that you are looking at looks great. I see the low VGS threshold as well as some key data sheet graphs that you have pointed out before and can see how one should be able to drive them with 3.3V. Nice find; thanks!

Quote
LOL on the Lowes 'digital rain/freeze sensor'!  Did you ever take one of those apart?  They have a fiber washer that expands when its wet and contracts when it dries and probably expands when it freezes.  The expanding washer pushes the conductive washer on the top of it to close a circuit. You adjust the 'rainfall amount' by mechanically changing the gap between the contacts...

I haven't taken it apart but a friend once did. I did suspect that its operation is very "mechanical" and simple. I think soil moisture is a better way to determine when to water but the rain sensor is relatively cheap so I installed one.

Quote
If you're nice to me I just might send the other page of the schematic...  :D 

I'll get you a bottle of wine sometime ;-)

Quote
My latest library is attached below...

Wonderful; thanks a lot!

Cheers,

Eloy Paris.-
Title: Re: Driving watering electronic timer with Moteino
Post by: TomWS on April 26, 2015, 05:31:04 PM
<snip>

Quote
The resistors on the gate are really to current limit the gate voltage when On, and bias off the transistor when Off - not really a voltage divider (hence the large difference in resistance.

Hhhhmmm. I'll have to think about this some more. I thought we want to keep the VGS voltage under the maximum specified in the data sheets to prevent damage to the transistor, and the "voltage divider" was a way to accomplish that. I have not (perhaps incorrectly) worried about current into the gate because of the very high impedance at the gate. But I have worried a lot about VGS voltage, as you've probably noticed by now.
Well, it IS a Vgs problem.  Once you go past the breakdown voltage, the junction will act like (IS) a zener diode.  The 10K resistors simply keep the current, at breakdown, to a safe level, yet supplies enough current into the junction capacitance to turn on the transistor with sufficient speed.

Quote

I am sorry, what I meant is that when I use their tool to search for transistors matching certain criteria (like dual transistors, VDS > 30V, SOIC-8, etc.), this specific dual N-channel/P-channel transistor does not show up in the search results. I do see several options (about 40 in total) but not the Vishay that you are looking at. It's possible that a classification field is missing and that causes this part to not be included in the results.

By the way, that dual transistor in SOIC-8 that you are looking at looks great. I see the low VGS threshold as well as some key data sheet graphs that you have pointed out before and can see how one should be able to drive them with 3.3V. Nice find; thanks!
Ok, yeah, I understand.  I didn't find it on the Mouser site using their tool either.  I went to the Vishay site and used their selection tool (which isn't great, but at least usable).
I like the part too, it's just more expensive than I'd prefer.


Quote

I'll get you a bottle of wine sometime ;-)


For a bottle of wine you might even get a circuit board!!!   8)

Tom
Title: Re: Driving watering electronic timer with Moteino
Post by: EloyP on May 02, 2015, 10:22:24 AM
Hi Tom,


[...]

Now, how does that affect your circuit?  In the latest one and the previous one you're driving the N transistors from the processor and, as I say, I'm not sure that's a good thing.  One thing to notice on the FDS8858 datasheet, however, is that they explicitly show the 'zener' diode between the gate and source pins.  This implies that you can reliably overdrive if you current limit and I think that's a safe bet, especially given that the Vgs breakdown is 20V for the N device.

This is the other thing that has kept me thinking all weekend -- I did see the clamping zener diodes in the FDS8858CZ data sheet but the data sheet also says that the +/- 20V VGS is a maximum. I suspect the transistor might get destroyed if the VGS voltage exceeds the maximum indicated in the data sheet. I have an open query with the manufacturer to see what they say regarding exceeding that voltage even with the clamping diodes there but I think they'll say the value is what the data sheet says it is, i.e. the maximum value, after which the transistor can be destroyed.

I finally heard back from Fairchild Semiconductor regarding this so I thought I'd share. My question was:

Quote
Would it be okay to drive the N-Channel MOSFET of the FDS8858CZ with                                 
 a VGS voltage of around 25 VDC? Data sheet says the max. VGS is +/-                                 
 20V but the pinout also shows Zener diodes between gate and source.                                 
 What would happen if VGS exceeds 20V? Would a resistor in series with                               
 the gate allow to exceed 20V? Thanks in advance for the                                             
 clarification.

Their response was:

Quote
Not recommended to exceed gate input specification even with gate                                   
zener. Zener can easily be damaged with any current. Zener is more of an esd                               
protection device than something to carry continuous current.

This is not a problem anymore because I am now going to use different transistors that I will drive directly from a 3.3V ATmega I/O pin (the same transistors and driver circuit you came up with for your design).

Cheers,

Eloy Paris.-
Title: Re: Driving watering electronic timer with Moteino
Post by: Lukapple on May 21, 2015, 11:34:02 AM
I finally got all components for sprinkler controller (by TomWS) and it works great! Thanks Tom!

(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/controller_s.jpg)

(https://dl.dropboxusercontent.com/u/2261256/forums/watertimer/sprinkler_controller_pulse_zoomed.png)
Title: Re: Driving watering electronic timer with Moteino
Post by: Felix on May 21, 2015, 01:24:49 PM
Nice.
Did you make the Moteino yourself?
Title: Re: Driving watering electronic timer with Moteino
Post by: Lukapple on May 21, 2015, 01:33:27 PM
Nice.
Did you make the Moteino yourself?

Yes. I also ordered some Moteinos from your online store, but problem is that shipping and import taxes are not so cheap (I'm from Europe), so I made few Moteinos by myself (already talked with you about that on PM :) ).