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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?

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closed as unclear what you're asking by Chris Stratton, Bimpelrekkie, Enric Blanco, Voltage Spike, Dmitry Grigoryev Jul 11 '17 at 17:01

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ Solenoids like motors have a DC current to accelerate and then a holding current depending on pressure or vibration. Check specs for holding current and see if an RC filter can be used to supply the switched current and a higher R or lower V for holding current. ALso heat sources that are enclosed will rise in temp needs to be part of the design specs. \$\endgroup\$ – Sunnyskyguy EE75 Jul 8 '17 at 19:13
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    \$\begingroup\$ You're going to have to use the Edit button to largely re-write this into clearer form. Be specific about if your concern is when the solenoid is activated or when it is not, and if it is the solenoid that is getting hot or the switching circuitry which controls the solenoid. Finally, how did you decide that the solenoid and the supply voltage are a match? Tony has a point that you'll need more "kick" to move it than to hold it, though there are probably better ways of achieving that. \$\endgroup\$ – Chris Stratton Jul 8 '17 at 19:19
  • \$\begingroup\$ rather than just say you have a thermal problem, define the overall specs: Inputs, Outputs (values), characteristics, power limitations, budget then concerns: Heat, EMI , ratings. \$\endgroup\$ – Sunnyskyguy EE75 Jul 8 '17 at 19:23
  • \$\begingroup\$ Cars have 12V solenoids but they are not rated for continuous use to start a car. But then neither is the starter. \$\endgroup\$ – Sunnyskyguy EE75 Jul 8 '17 at 19:24
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    \$\begingroup\$ You wouldn't have the solenoid connected to a power source by a closed circuit except when you want to actually actuate it. Yes you would need a circuit which can deliver a brief pulse upon the loss of a different signal, but this needs its own power source which endures that loss, at least temporarily. A microcontroller is an option, and a common choice as it allows implementing rules like rate limits - but somehing like 555 timer can do the basic version, too. Either driving a big FET or a smaller and relay. Beware its hard to make a reliable failsafe - you want a mechanical backup! \$\endgroup\$ – Chris Stratton Jul 8 '17 at 23:07
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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:-

schematic

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).

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  • \$\begingroup\$ Would the solenoid in the RLY1 12 heat up if the circuit was closed for days or weeks? Or is the work to pull the switch closed so small that it's negligible? \$\endgroup\$ – Joshua Park Jul 9 '17 at 16:43
  • \$\begingroup\$ No. Look at the circuit again. The solenoid is never connected directly to the power supply. The solenoid is always off except for the moment the power is switch off. Then the relay contact falls back to the position shown and the charge on the capacitor pulses the solenoid. \$\endgroup\$ – Transistor Jul 9 '17 at 16:49
  • \$\begingroup\$ @Transistor the springy thing (inductor?) inside RLY1 12V \$\endgroup\$ – Joshua Park Jul 9 '17 at 17:05
  • \$\begingroup\$ Sorry, I missed that. The relay will be continuously rated so heat won't be a problem. The coil resistance will be high relative to your solenoid and will run much cooler. \$\endgroup\$ – Transistor Jul 9 '17 at 17:20
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I'm not exactly sure what you want to do, but I will suppose you want a solenoid which works the opposite of the usual way: instead of "doing something" when it is powered, it should "do something" when the power is cut off.

Suggestion: use a return spring. The solenoid pulls against the spring, and when power is lost, the spring does the action you want. A spring doesn't have software bugs. It's also cheap. It may still fail mechanically after lots of actuations, though.

You can also use gravity. Solenoid holds something up, and when power is lost, it falls down. Pretty foolproof, but the device will have to be installed the right way up, and acceleration (like inside a vehicle) could be a problem.

Well, 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. I don't have any specs at the moment because I just want to know how everyone solves this problem first.

I'll continue with the spring. You can design the mechanical contraption so that the force required from the solenoid to hold the thing in place is much lower than the actual spring force required to do what you want (hint: use a lever). So you need a small solenoid, which hopefully won't overheat.

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  • \$\begingroup\$ Are there any mechanical components that can work quickly to actuate a spring, the instant the circuit is disconnected? For the mechanical solution, how would you actuate the lever to release the spring the moment the switch is open? For the electrical solution, would using a capacitor be the only way to avoid overheating and to power the solenoid, while it's still connected to the power source? \$\endgroup\$ – Joshua Park Jul 9 '17 at 16:55

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