# Electrical – How to power a circuit that has a solenoid without heating the solenoid up?

solenoid

I am building a project that requires a solenoid, but after searching the web I realized that the most underlying problem with solenoids is that it can heat up very quickly. The basic principle of my project is that a solenoid has to actuate when it is cut off but the problem is that the solenoid can heat up if it's connected for too long. I looked around and people seem to be using relays, transistors, FETs, or capacitors to actuate the solenoid.

Is there a way to keep the solenoid in a "sleeping state", meanwhile still supplying current to other components, and then actuate it when the circuit is shut off?

I'm not quite familiar with electronics so I'll just try to guess the solutions.
If I use a capacitor, would I basically be making a open/short circuit (sorry, I'm confused with this one, too) to keep the solenoid from being actuated?

What about transistors? How exactly do they serve as switches? If I want the solenoid to be actuated when the circuit is disconnected, or the voltage supply is cut off, how exactly do I make the transistor actuate the solenoid?

the only use of the solenoid in the project is to "kick" the plunger when the circuit is cut off. So, essentially, I don't need it for a continuous use; just for the moment when the circuit is switched off.

If no other power source is available to operate the solenoid when the circuit is switched off, you will need to store enough energy to 'kick' it when power is removed. You can do this with a capacitor which is charged up when the circuit is connected, and discharged into the solenoid when the circuit is disconnected. Here's one way to do it:-

simulate this circuit – Schematic created using CircuitLab

When S1 is closed (ie. circuit is switched on) relay RLY1 operates and connects capacitor C1 to 12V via current-limiting resistor R1 and diode D1. C1 then takes about 2 seconds to charge up to +12V. When SW1 opens (ie. circuit is switched off) RLY1 releases and connect C1 to the solenoid. C1 then discharges its energy into the solenoid, causing it to 'kick' until the capacitor has discharged.

D1 prevents the voltage on the capacitor from keeping the relay operated when power is off.

The value of C1 required depends on the solenoid's resistance and how long it needs to be operated for. With the values shown it should operate for ~150ms (assuming that once operated it holds in until voltage drops to 3V).