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PWM solenoid driver

Discussion in 'General Electronics Discussion' started by timsrv, Apr 15, 2015.

  1. timsrv


    Jun 22, 2014
    Hi all. I'm an RV repair tech and I have a customer who does a lot of dry camping. He travels with his dog and is worried about leaving it alone in the coach with the LP gas and the furnace on. Of course he's also worried about the dog getting cold, so as a precaution he had an LP gas detector/shut-off system installed. Basically this is an LP gas and CO detector that controls solenoid valve at the tank. When the detector is happy it supplies 12VDC to the solenoid via a built-in relay. If the detector unit detects CO or LP gas inside the coach it sounds an alarm and turns off power to the solenoid valve. He is happy with the concept, and likes knowing his pet is safe while away.

    The problem with the system is the solenoid operates at approx 1 amp continuous and this puts a drain on the battery, (therefore shortening the time he can camp before it goes dead). When evaluating the system I noticed the solenoid valve gets quite hot & think it's sucking up a lot more power than is needed to do the job. I was wondering if installing a pulse width modulated solenoid driver between the detector head & the solenoid could reduce draw. Unfortunately I have a very limited knowledge of such devices. In my mind I think the ideal driver should initially apply full trigger voltage to pull the solenoid plunger, but then use PWM to drop current (after a second or so) to a point barely sufficient to maintain the seated position. Perhaps having a pot to adjust draw would be helpful so it could be dialed into a sweet spot.

    My question is whether or not this would significantly reduce current draw. If this sounds like a good idea, where would I find such a unit (if one exists). If not, what might you suggest as to building such a device? I think it will be easy to find a unit to supply a constant PWM (without the initial second or so of full trigger voltage). Was wondering if I could incorporate a large capacitor or perhaps something like a PTCR device. Any suggestions or observations would be helpful and much appreciated. Tim
  2. Wesley Weaver

    Wesley Weaver

    Apr 15, 2015
    Not likely no. It sounds like it's a fail shut device and the solenoid is holding it open and it's going to require a pretty steady amount of current to keep it open against spring pressure.

    So current is going to be proportional to the strength of the spring which is going to be proportional to the size of the valve.

    My recommendation would be to find a more suitable solenoid for the job. The gas flow should be VERY minimal so it should only require a tiny needle valve to provide enough gas flow which you should be able to match up with a solenoid that can be held open for days on something like a 9V battery. Barring that you could change out the loading spring to something the requires less current to hold open and using a trim pot like you mentioned to dial back to something just above where it closes, effectively doing the same thing as getting the right solenoid BUT you're then fighting the fact that the solenoid is way overpowered for the spring and you're performing a balancing act.

    All that being said, the NORMAL way to combat a battery going dead is to install a low battery trigger to start the generator (or in this case the engine). There are a number of remote start/alarm combinations that can be made to start the engine at intervals to keep the battery charged and can usually be coupled with a low voltage relay to start the engine. THAT would be the CORRECT way to go about solving the problem along with making things safer for the dog and the camper being left alone.

    I like autopage systems. Equivalent performance to Viper systems at a lower cost AND are easier to interface other controls into the module.

    I hope that helps.
  3. BobK


    Jan 5, 2010
    Tha fios agaibh and hevans1944 like this.
  4. timsrv


    Jun 22, 2014
    Yes, thank you for the suggestions. Generator operation in this case is not an option. I had thought about a latching solenoid valve, but discounted that as not suitable because if the battery were dead it could get stuck in the open position. Of course there's also the compatibility issue with the head unit. At this point I'm leaning towards a different fail shut solenoid. The one he has is mounted on the high pressure side of the system which requires it to open/close at pressures up to 312 psi. This means higher spring pressures and higher current. If I were to mount on the output side of the regulator, pressures are much less (around 1/2 psi here). So I'm going to start looking for a 12V continuous duty solenoid that's rated for around 1 psi of LP gas & suitable for an outside environment. If I find one I think we should be able to drop current draw in half.........maybe? Tim
    Last edited: Apr 15, 2015
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