Capacitors in a solenoid circuit?

Discussion in 'The Projects Forum' started by Beecher, Apr 10, 2010.

  1. Beecher

    Thread Starter New Member

    Jun 14, 2009

    I've been handed a legacy design to replicate and improve upon and I'm looking for some insight on a couple of questions. This isn't really my field, but I once installed a dimmer switch in the company break room so I get the duty. Any help greatly appreciated.

    The application is a controller board that switches seven solenoid valves via low-side MOSFETs under the control of a PIC chip (a diagram is attached). Five of the solenoids switch at very low frequencies (once every ten seconds, etc.). The other two occasionally cycle at around 10Hz for about a second. Precision timing of the solenoids is not critical to this application. The valves actuate motions that take much longer to complete than the response time of the solenoids themselves.


    1. What are the capacitors doing here? I assume these are just RF filters but I've never seen them used in a solenoid circuit like this. Wouldn't a flyback diode eat up any RF ringing nearly as well? Is there any other reason for having the caps in there? The capacitors are obviously not a big deal cost wise but those 1000uF cans aren't small and space is a consideration here. Replacing the caps with a single 1N4004 or such around the valve would save a lot of hassle in other areas.

    2. What's with R1? I'm going to assume that it affects the operation of the MOSFET, but I don't know why or see the point if that's what it does. I'm wondering why it's included on only one switch circuit and not the other six.

    3. Finally, any issues with adding an indicator LED and resistor in parallel to the solenoid so we can see when it's open?

  2. Bychon


    Mar 12, 2010
    Those capacitors are not across the coils. They are across the power supply. They supply current surges and generally keep the 12 volts from being noisy as the solenoids are operated. However...capacitors have improved muchly in the last few years and you can probably buy the same amount of capacitance in a much smaller package now. Adding diodes is not a bad idea, only put them across the coils, not where the capacitors are.

    R1 is a pull up resistor. Q1 would fail in the "on" condition if the PIC stopped working. I don't know why one solenoid has it and the others don't. I'd have to know about the PIC...which outputs it's using, why it needs a pull-up resistor on one and not the others, why anybody would want L1 to fail in the on condition, and I'm not that smart about PIC's.

    I expect there will be no problem with adding LED's.