Author Topic: coin cell powered R4  (Read 11428 times)

TomWS

  • Hero Member
  • *****
  • Posts: 1883
Re: coin cell powered R4
« Reply #15 on: April 25, 2016, 05:43:59 PM »
Well in that case you can just switch the booster itself via the en pin. No load switch necessary. One thing to keep an eye on is input current. Coin cells are not good at providing larger currents for long periods of time. You'll need more than the specified current of the humidity sensor because you're converting the voltage up. Probably still ok - in particular with a cr2450 - but it's a pretty big load for a coin cell.
I'd be concerned about the quiescent current using the EN pin.  A single P FET would have much lower leakage.  No need for the N FET.

Also, what type of soil and depth are you planning to use the VH400.  In my experience this device was totally useless in North Carolina clay.  It seems to work reasonably well in potting soil, however.

Tom

perky

  • Hero Member
  • *****
  • Posts: 871
  • Country: gb
Re: coin cell powered R4
« Reply #16 on: April 25, 2016, 05:45:49 PM »
Bear in mind there's probably also a DC path through the booster from Vin to Vout via the inductor and diode, this means you'll have at least Vin minus a diode drop on Vout regardless of whether you use an enable pin. You may need a high side switch on the input to the booster if you want to isolate completely.
Mark.
Edit: You might want to slew rate control that high side switch (just a P channel FET, no need for the botton N channel FET). One way to do that is put a resistor in series with the enable voltage and the gate, and have a capacitor from the switched output to the gate. Here's a useful app note: www.onsemi.com/pub/Collateral/AND9093-D.PDF
« Last Edit: April 25, 2016, 06:24:01 PM by perky »

davegravy

  • Jr. Member
  • **
  • Posts: 56
  • Country: ca
Re: coin cell powered R4
« Reply #17 on: April 25, 2016, 08:39:45 PM »
Also, what type of soil and depth are you planning to use the VH400.  In my experience this device was totally useless in North Carolina clay.  It seems to work reasonably well in potting soil, however.
Tom

Was planning for about 4" down, and the soil around here is pretty sandy. Testing in my local soil so far it's performed reasonably well.

But I was hoping the sensor would work well in other soils too so I could build these for some interested friends. How did it behave badly in clay soil? Did a tiny drop of rain send it straight to full scale output? Did you find another sensor that worked better in clay?


TomWS

  • Hero Member
  • *****
  • Posts: 1883
Re: coin cell powered R4
« Reply #18 on: April 25, 2016, 09:28:54 PM »
Also, what type of soil and depth are you planning to use the VH400.  In my experience this device was totally useless in North Carolina clay.  It seems to work reasonably well in potting soil, however.
Tom

Was planning for about 4" down, and the soil around here is pretty sandy. Testing in my local soil so far it's performed reasonably well.

But I was hoping the sensor would work well in other soils too so I could build these for some interested friends. How did it behave badly in clay soil? Did a tiny drop of rain send it straight to full scale output? Did you find another sensor that worked better in clay?
I started a test in October where I planted it in the vicinity of a Watermark sensor that has been working well for about a year; just to compare results. 

The area was a landscaped section (shrubs & perenials) next to a strip of lawn and the Watermark is buried about 8".  The VH400 could only go from the surface to about 5-6" deep.  It was a wet fall and winter so both devices were reading 'wet' for most of the time (the VH400 ALL of the time). 

Finally, this Spring, the Watermark started to show signs of water uptake by the surrounding plants and eventually read "It's time to water." but the VH400 barely waivered from reading 'wet'. 

I finally gave up on it and moved it to a pot where we've planted some Basil in potting soil just to see if it worked at all and it seems to be behaving in an expected, albeit uncalibrated, way.  Sandy, loamy soil where the roots are shallow (ie no more than 6" deep) is probably ok.  I wouldn't use this in denser soil or for shrubs/deeply rooted plants, and, frankly, won't buy another one.

Tom

TomWS

  • Hero Member
  • *****
  • Posts: 1883
Re: coin cell powered R4
« Reply #19 on: April 25, 2016, 09:33:12 PM »
Bear in mind there's probably also a DC path through the booster from Vin to Vout via the inductor and diode, this means you'll have at least Vin minus a diode drop on Vout regardless of whether you use an enable pin.
LOL!  Oh yeah, that would be a bit more 'leakage' than I had suggested!  Good call!
Quote
Edit: You might want to slew rate control that high side switch (just a P channel FET, no need for the botton N channel FET). One way to do that is put a resistor in series with the enable voltage and the gate, and have a capacitor from the switched output to the gate. Here's a useful app note: www.onsemi.com/pub/Collateral/AND9093-D.PDF
Excellent suggestion!  The 10uF cap will represent a very hefty 'short' to the high ESR battery and may actually reset your processor without some slew rate control.

Tom

davegravy

  • Jr. Member
  • **
  • Posts: 56
  • Country: ca
Re: coin cell powered R4
« Reply #20 on: April 25, 2016, 11:08:59 PM »

I started a test in October where I planted it in the vicinity of a Watermark sensor that has been working well for about a year; just to compare results. 

The area was a landscaped section (shrubs & perenials) next to a strip of lawn and the Watermark is buried about 8".  The VH400 could only go from the surface to about 5-6" deep.  It was a wet fall and winter so both devices were reading 'wet' for most of the time (the VH400 ALL of the time). 

Finally, this Spring, the Watermark started to show signs of water uptake by the surrounding plants and eventually read "It's time to water." but the VH400 barely waivered from reading 'wet'. 

I finally gave up on it and moved it to a pot where we've planted some Basil in potting soil just to see if it worked at all and it seems to be behaving in an expected, albeit uncalibrated, way.  Sandy, loamy soil where the roots are shallow (ie no more than 6" deep) is probably ok.  I wouldn't use this in denser soil or for shrubs/deeply rooted plants, and, frankly, won't buy another one.

Tom

The first VH400 I had somehow lost the bottom half of it's dynamic range, so wouldn't read below 1.5V even when dry and sitting in air. Vegetronix is shipping me a replacement at no cost. I wonder if something similar might have happened to yours?

But you've been happy with the Watermark? Hadn't heard of it - might have to pick one of those up to test. This is for lawn watering alerts, so shallow roots. 

joelucid

  • Hero Member
  • *****
  • Posts: 869
Re: coin cell powered R4
« Reply #21 on: April 26, 2016, 12:35:25 AM »
Quote
Bear in mind there's probably also a DC path through the booster from Vin to Vout via the inductor and diode, this means you'll have at least Vin minus a diode drop on Vout regardless of whether you use an enable pin. You may need a high side switch on the input to the booster if you want to isolate completely.

 :D - good catch, I missed that, thanks.

perky

  • Hero Member
  • *****
  • Posts: 871
  • Country: gb
Re: coin cell powered R4
« Reply #22 on: April 26, 2016, 04:19:33 AM »
Thinking about this further, high current pulses from coin cells can significantly reduce their capacity so I would probably do the following:

1) Use a high side switch with aggressive slew rate control
2) Use the smallest input and output bulk capacitors that the boost regulator needs (10uF is probably too high)
3) Use a boost regulator with a very slow soft start, preferably one I that can be programmed with an external component like a capacitor, and
4) Enable the boost regulator some time later once the input has reached working voltage of the regulator

Mark.
 

SadE54

  • Newbie
  • *
  • Posts: 36
Re: coin cell powered R4
« Reply #23 on: April 26, 2016, 05:40:34 AM »
Maybe it could be a solution in this case :
https://lowpowerlab.com/forum/index.php/topic,1831.0.html

davegravy

  • Jr. Member
  • **
  • Posts: 56
  • Country: ca
Re: coin cell powered R4
« Reply #24 on: April 26, 2016, 08:15:02 AM »
Thinking about this further, high current pulses from coin cells can significantly reduce their capacity so I would probably do the following:

1) Use a high side switch with aggressive slew rate control
2) Use the smallest input and output bulk capacitors that the boost regulator needs (10uF is probably too high)
3) Use a boost regulator with a very slow soft start, preferably one I that can be programmed with an external component like a capacitor, and
4) Enable the boost regulator some time later once the input has reached working voltage of the regulator

Mark.

Thanks for the discussion guys, this is all stuff I'd have to learn the hard way otherwise...

In terms of an optimal rise time for the boost regulator supply to prevent processor restarts, I expect this is something that'll have to be determined experimentally, but if there's any reasonably simple analytical method you're aware of then please do share.

As I understand, another means (perhaps good to be used in tandem with the above methods) to mitigate voltage drops from load switching is bulk caps right after the battery.
« Last Edit: April 26, 2016, 08:16:48 AM by davegravy »

davegravy

  • Jr. Member
  • **
  • Posts: 56
  • Country: ca
Re: coin cell powered R4
« Reply #25 on: May 01, 2016, 10:10:03 PM »
I started to prototype the boost converter on breadboard and when it didn't work I remembered that high freq circuits and breadboards don't get along. Going straight to PCB without bread-boarding first makes me really nervous.

Is there something like the TPS61222 but all in one package (i.e not requiring discrete capacitors/inductor) I can use on a breadboard. TI has an evaluation board for this boost converter but it' >$50 which breaks my hobby budget.

Otherwise I guess I'll have to go straight to PCB and tweak component values by soldering/desoldering.

TomWS

  • Hero Member
  • *****
  • Posts: 1883
Re: coin cell powered R4
« Reply #26 on: May 01, 2016, 11:48:13 PM »
I started to prototype the boost converter on breadboard and when it didn't work I remembered that high freq circuits and breadboards don't get along. Going straight to PCB without bread-boarding first makes me really nervous.

Is there something like the TPS61222 but all in one package (i.e not requiring discrete capacitors/inductor) I can use on a breadboard. TI has an evaluation board for this boost converter but it' >$50 which breaks my hobby budget.

Otherwise I guess I'll have to go straight to PCB and tweak component values by soldering/desoldering.
I think you should do the two battery approach or go with a battery that's high enough voltage to operate the VH400 (LiPo probably will).  Or dump that VH400 and use a different moisture sensor (Chirp I2C for instance) which will work within your voltage budget.  The boost circuit is an 'optimizer' solution (and, as you've observed, requires fab skills that may be challenging) and you should probably get your 'feet wet' with a 'good enough' solution.

If you REAAAALLLLY want to try the boost circuit, LowPower Labs and others sell a SOT-23 breakout board that will work with the TPS61222 (SC-70 package I believe) but that could be unstable due to poor layout compromises.

Tom

joelucid

  • Hero Member
  • *****
  • Posts: 869
Re: coin cell powered R4
« Reply #27 on: May 02, 2016, 02:48:11 AM »
Quote
The boost circuit is an 'optimizer' solution (and, as you've observed, requires fab skills that may be challenging) and you should probably get your 'feet wet' with a 'good enough' solution.

Also boosters are inherently noisy and might well increase your noise floor, with a negative impact to radio sensitivity. I never fully realized how important that is until my recent noise tests.

perky

  • Hero Member
  • *****
  • Posts: 871
  • Country: gb
Re: coin cell powered R4
« Reply #28 on: May 02, 2016, 04:48:29 AM »
They can be, but you need to follow careful guidelines when laying them out e.g. reducing the two current loops, minimizing stray inductance, using snubbers to remove ringing, proper post filtering and pre-filtering, careful placement, using shielded inductors. You can filter practically all conducted noise with adequate power supply filtering and what's left is E and H fields which can be minimized. Trying to put a switching regulator on a piece of veroboard is not a good idea for noise, you really need proper PCB layout.
Mark.

damonb

  • Newbie
  • *
  • Posts: 26
Re: coin cell powered R4
« Reply #29 on: May 02, 2016, 04:59:50 AM »
@davegravy, I don't know how attached you are to your design goals, but for comparison I power my soil nodes from 3 x AA alkaline cells. The VH400s are switched from a JeeNode's regulated 3.3V rail via a P-MOSFET high-side switch. I get about 2 years battery life sampling soil and temperature (DS18B20) every 10 minutes. I chose to use the 3.3V regulated instead of the battery rail as the VH400 output is somewhat proportional to the supply voltage, and hence the calibration varied as the alkalines ran down. This way i can run them down to just under 1.2V per cell.
It would save a lot of R&D time messing with boost regulators.