Question: How can I safely control a homemade solenoid that draws around 90A at 12v with a relay that has a hard limit of 10 amps?

Background: First of all let me start off by saying that I know enough to be dangerous and try to approach things carefully. I try to research as much as possible, but in this circumstance I cannot figure it out on my own and would like some guidance.

I have seen many small solenoids that I can buy and use much less current to drive the solenoid. However, I generally like to try to build things first before usually being forced to break down and buy it anyway. I guess I should state that it is my assumption that current (amps) is the driving force that draws the bolt into the solenoid and that the larger the weight of the bolt in the solenoid = higher amperage required.

I took my solenoid to a local battery supply store and I got to talking with the service tech. I convinced him to attach my solenoid to a battery and some kind of large expensive looking tester that is able to variably apply current to the attached item. My thought was to find out at 12V what amperage will the bolt be drawn into the solenoid. Then with this information I will at least know what I am dealing with. Well it turns out that magical number was 90~ amps at 12V. The battery service tech told me that even if I bought a 5Ah battery it will still produce several hundred amps upon shorting it as I am doing.

I have been reading online and I know that I need a diode to handle the inductive load. However, where I get confused is possibly adding in a resistor to handle the difference between what is required to draw the solenoid and what my relay can handle. Is there a way that I can place resistors into my circuit to make it work?

What role does the gauge of the wire play in making a solenoid?

Background on the control circuit: I have an arduino attached to a relay board, the relays are rated 10A 28-30V DC... up to 250V 10A. I have no issues programming the arduino or interfacing the relay board. My only question is how can I safely control the solenoid and still be able to use my homemade solenoid.

Here is the relay board: http://www.amazon.com/SainSmart-8-CH-8-Channel-Relay-Module/dp/B0057OC5WK/ref=sr_1_1?ie=UTF8&qid=1442459317&sr=8-1&keywords=8+channel+relay+card


1 Answer 1


The simple answer is indeed simple - you can't do it. If you really want to continue on this path you'll have to do one of two things.

1) Use the relay to energize the coil of a much beefier relay, called a contactor, which is rated for at least 100 amps. A note of caution: they're expensive.

2) Take your solenoid, and rewind it so as to use 10 times as much wire, which will give you (more or less) 10 times as many turns. With 10 times the wire length the current at 12 volts will be 1/10 what you're drawing now, but the magnet will be just as strong. Now you can use your 10 amp relays to energize the coil.

Whichever you do, you'll need to be careful about running your coil for any length of time. The power dissipated in the solenoid is volts times current, and in your case that's 90 amps times 12 volts, or 1080 watts. The internal windings of the solenoid will get very hot, very quickly. Since I assume you're using fairly cheap wire, you'll melt the insulation in minutes (or maybe less).

  • \$\begingroup\$ Awesome! Thank you very much! Understood on the heat aspect, I only need to energize the solenoid for a second and only once every 4 days. \$\endgroup\$
    – Cory
    Sep 18, 2015 at 4:00

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