# A robust way to supply power to a solenoid

Solenoids typically heat up very quickly and it seems a few seconds of power supply is already adequate. Does anyone know of a robust/established way for supplying just a few seconds of power to a 12V/1A solenoid? How about cap discharge?

Thanks

EDIT 1 The solenoid pushes and pulls a lock for my rabbit house, so continuous operation isn't needed. In fact, my solenoid needs a fair bit of cooling time.

EDIT 2 My current plan is to drive it with 8051 and a transistor, but I'm open to alternatives. I could get the 8051 to give the pulses, but I fear that my code is buggy and frying my rabbits as a result. So I was hoping to come up with a relatively fail-safe method.

The solenoid is spring loaded, so that the pin retracts when the solenoid is engaged (spring is compressed) and releases when it's not (spring is released). With this motion, I'm planning to get it to unblock and block the gate (thus, a lock).

The rabbits will be locked up at night.

Yes. This is easily achieved with a switch and a battery. No need for a capacitor.

simulate this circuit – Schematic created using CircuitLab

Operation

• Press the button.
• Hold for required number of seconds.
• Release the button.

As you suspected, a capacitor could be used to provide a pulse to operate the solenoid.

simulate this circuit

Here we'll charge up the capacitor through a resistor. If the switches are left on the resistor will limit the current to a safe value that won't overheat the solenoid. Let's figure out some values. First the solenoid.

$$R = \frac{V}{I} = \frac{12}{1} = 12 Ω$$

The power (heat) in the solenoid is given by

$$P = V·I = 12·1 = 12 W$$

Let's limit the current to 0.25 A. This will require a total resistance of 48 Ω (12 V / 0.25 A) so we'll need a 36 Ω resistor. We'll use a 39 Ω as this is a standard value.

Let's say we need a 0.5 s pulse to move the solenoid. We don't have a figure for the inductance of the solenoid so we'll just treat it as a resistor for now.

The time constant for an R-C circuit is simply R·C. Therefor

$$C = \frac{t}{R} = \frac{0.5}{12} = 0.041 F = 41,000 uF$$

This is a lot but not impossible. 4,700 uF capacitors are readily available. Assuming your rabbit is on a tight budget you might find capacitors in old power-supplies, etc. You need to make sure that they are rated for 12V or higher. Simply parallel all the capacitors making sure to keep the polarity correct.

I'm suggesting a 3-way, centre-off switch. This way the solenoid can be reversed for open and closed.

If finding the capacitors proves to be a problem we'll have to add some electronics to do the timing.

Edit after Update 2.

The requirements are simpler now because the solenoid is known to be spring return.

simulate this circuit

I tried to come up with an alternative but it's hard to beat the old 555 timer. This configuration is a monostable. When Q1 turns on it triggers the 555 which turns on Q2 and the relay. Time delay is set by $R4 \cdot C2$.

• I'm not sure if it'd be wise to trust the user. A solenoid can heat up rapidly, so surely something more robust than a switch may be desirable. Perhaps a cap, at least? – WKleinberg Jan 3 '16 at 14:27
• Many solenoids are designed for continuous operation. What sort of solenoid are you talking about? What is the application? Please post the additional details in your question and not here in the comments. – Transistor Jan 3 '16 at 14:35
• That's a three carrot answer if ever I saw one. – Brian Drummond Jan 3 '16 at 15:52
• @BrianDrummond: Ho, ho. Very bunny. – Transistor Jan 3 '16 at 15:56
• Thanks for the great answer. 41,000uF, that's very high indeed. I don't think I can get my hands on something that size. Perhaps, cap discharge won't fit my case so well... Voted up for the great write-up though! – WKleinberg Jan 3 '16 at 20:00

If you want something really simple you could consider a PTC .This is a resistor that increases its resistance greatly when it heats up.Place the PTC in series with your solenoid .Select a PTC rated at 12V or better that has a cold resistance of less than say 5% of the DCR of your solenoid.This means that the solenoid will pull in normally.You can tape the PTC around the solenoid winding to get good thermal contact .The PTCs that are designed for loudspeaker protection would be a good starting point to make your selection .PTCs are sometimes marketed under different names like "PolySwitch"You could place a 680 ohm resistor and a red led across the PTC to indicate that the system is in a high resistance state due to getting hot .