I have a circuit with a large capacitor, a solenoid and an N-channel FET fed by a 60VDC supply. When I trigger the FET the capacitor discharge activates the solenoid. This all works as expected (there are additional components -- resistors and diodes and such).

Now I need to add a safety circuit such that if the power is turned off and the supply line quickly falls towards ground potential the capacitor will quickly (e.g. <2sec) discharge to prevent accidental firing of the solenoid. Due to various constraints a bleeder resistor will not work.

I have done a pretty thorough online search and haven't come across a simple circuit that does this. It seems that a P-channel "safety" FET connected across the capacitor in series with a bleeder resistor, with the gate connected to the power supply would do the trick -- when the supply is high the FET is off, and vice versa.

Is this a reasonable approach? Some of my concerns: What happens when the supply is turning on? Will the safety FET also be on causing a large load on the supply? What about limiting the source-gate voltage to something reasonable (or is there a FET that can do the job and handle 60V)? I'm trying to get away with as few components as possible due to severe space constraints.

Here is what I had in mind (solenoid and trigger FET not shown). A further constraint of the problem is that each of the 64 solenoid circuit "units" (the dashed box) is independent and, because of cabling/connectors and such, additional wires to it are not allowed. So, there can be no separate "discharge" signal to each unit -- the only notice a given unit gets is that power has dropped.


simulate this circuit – Schematic created using CircuitLab

  • 2
    \$\begingroup\$ The gate-source voltage ratings of most power MOSFETs are usually in the +/- 20 V range, so most likely the MOSFETs would immediately die after applying power since the source is initially held low by the charging cap while the gate is immediately brought to 60V. \$\endgroup\$
    – jms
    Jan 28, 2016 at 2:31
  • \$\begingroup\$ I would use N-channel MOSFETs in series with power resistors, with the gates connected in parallel. Then you would only need a single loss of power detector that would turn on all MOSFETs at once. Of course said circuit would require a capacitor of its own in order to ride trough the power loss. \$\endgroup\$
    – jms
    Jan 28, 2016 at 2:36
  • \$\begingroup\$ Could you please edit you schematic to show the position of all the capacitors on the power line? \$\endgroup\$
    – Dave
    Jan 28, 2016 at 4:18
  • \$\begingroup\$ Thanks for the feedback. A further constraint of the problem is that each solenoid circuit "unit" is independent and because of cabling/connectors and such, additional wires are not allowed. So, there can be no separate "discharge" signal to each of the units. The only notice a given unit gets is that power has dropped. \$\endgroup\$
    – JonB
    Jan 28, 2016 at 4:46
  • \$\begingroup\$ On the schematic, the dashed box is an independent unit, of which there are 64 total -- each with a capacitor, etc. \$\endgroup\$
    – JonB
    Jan 28, 2016 at 4:47

1 Answer 1


Tried the circuit, with 300K resistor from 60V to gate, and 100K from gate to source, using an IRF9540 P-channel MOSFET. It worked but not as well as I'd liked. I changed the 60V to gate resistor to 100K, and replaced the gate-source resistor with back-to-back 5.1V zeners. This worked much better because it tried to maintain a high gate-source voltage even when the supply voltage bled down.

BTW: Disconnecting the supply output from the solenoid unit worked well, with fast discharge of the cap, but disconnecting the input side did not. I lowered the supply bleeder resistor on the solenoid unit from 10K to 1K, and that worked better for supply input disconnect, but also boosted the supply dummy load to a few watts for each solenoid unit. Something in between works fine for my needs.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.