Generating pulses of opposite polarity

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dviktor
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Posted by dviktor: Mon. Feb 22, 2021 - 10:19 PM
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Hello, dear freaks!

I'm planning a project for programmable garden irrigation. The idea is to have huge containers with water and several bistable electromagnetic valves connected in parallel to them so I'll be able to drive them with my AVR. The valves require positive pulse (3.6 .. 6 V, >= 30 ms) to open and negative pulse to close. I've already experimented with them by shortly connecting supply wires to different battery leads and all seems fine. The most obvious solution that came into my head is to use single H-bridge per valve to implement it. But I wonder if there some more elegant ways exist and pros/cons of them, if any.

Viktor Drobot
A.N. Belozersky Research Institute Of Physico-Chemical Biology MSU

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Learning and Information, General Electronics
Last Edited: Mon. Feb 22, 2021 - 10:30 PM
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ka7ehk
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Posted by ka7ehk: Mon. Feb 22, 2021 - 10:39 PM
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I think that you are stuck with H-bridge because the "negative pulse" has to appear to idle at zero volts and go negative for the prescribed time. I've had to deal with latching valves like that and it is a pain but H-bridge is what I ended up with.

 

Jim

 

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ron_sutherland
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Posted by ron_sutherland: Mon. Feb 22, 2021 - 10:43 PM
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Perhaps the H-bridges can have one side that is common if you don't need to operate multiple simultaneously.

 

my projects: https://github.com/epccs

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dviktor
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Posted by dviktor: Mon. Feb 22, 2021 - 10:54 PM
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I've found suitable IC for that task (I think so, but I may be mistaken, correct me please) - SN754410. Benefits: 2 group of independent outputs; protection flyback diodes are already included in package.

Negative pulse in that case means just polarity inversion, I think. So if we're considering only one group of outputs (for example), then, according to the datasheet, to generate positive pulse we must set 1,2EN pin to high, 1A pin to high and 2A pin to low. And to generate negative pulse we must set 1,2EN pin to high, 1A pin to low and 2A pin to high. At the idle we must set 1,2EN pin to low. Did I understand that correctly?

Viktor Drobot
A.N. Belozersky Research Institute Of Physico-Chemical Biology MSU

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ron_sutherland
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Posted by ron_sutherland: Tue. Feb 23, 2021 - 03:23 AM
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Sounds right, only one way to know for sure, build and test. The bistable solenoid I was controlling needed about 5 Amp for 30mSec; it would also smoke after about 500mSec.

my projects: https://github.com/epccs

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awneil
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Posted by awneil: Tue. Feb 23, 2021 - 09:05 AM
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ron_sutherland wrote:
The bistable solenoid I was controlling needed about 5 Amp for 30mSec; it would also smoke after about 500mSec

Discharging  a capacitor into it can be good here ...

 

Maybe a changeover relay to make the positive/negative ?

 

So the sequence could be:

 

  1. Charge capacitor
  2. Set positive/negative on the relay
  3. discharge capacitor

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dviktor
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Posted by dviktor: Tue. Feb 23, 2021 - 09:55 AM
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@ron_sutherland

Thank you for provided schematics! It's a good point for me to start (I had no experience with H-bridges). According to the specs my valves consume about 200-300 mA at 3.6 V so I think I should be on safe side with 30 mS pulses

Viktor Drobot
A.N. Belozersky Research Institute Of Physico-Chemical Biology MSU

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awneil
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Posted by awneil: Tue. Feb 23, 2021 - 10:21 AM (Reply to #7)
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dviktor wrote:
30 mS pulses

note that it's lowercase 's' for seconds;

 

uppercase 'S' is for siemens (conductance; reciprocal of ohms)

 

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DocJC
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Posted by DocJC: Tue. Feb 23, 2021 - 02:46 PM
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Mission critical or not?

i.e.  What is the crop worth, how much water is in the supply tank, and what happens if a valve sticks in the open position?

 

One might consider putting a mechanical open/close valve in series with the automated valves as cheap insurance, so that one could manually turn off the water flow, just in case something ever went wrong with the controller.

 

JC 

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dviktor
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Posted by dviktor: Tue. Feb 23, 2021 - 02:53 PM
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I have two tanks of 1 m^3 each. Every tank has its own main valve at the bottom and I can easily open/close it. There will be no problem if valve will stuck in open position, it's even better situation - I live on territory with a lot of sand without any clay so water diffuses through it quite easily.

Viktor Drobot
A.N. Belozersky Research Institute Of Physico-Chemical Biology MSU

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