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).
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 :-)
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.-