How do I figure out how much amps a solenoid need?


I am new to the forum and new to everything that have to do with Arduino and electronic, so forgive my probably stupid question. I am trying to build a circuit that runs a 12V solenoid (from aliexpress) and having a hard time understanding how much amp it needs. I found another place that seems to sell the same one and it says its 5 watts. When I measure the resistance over its terminals it says 28 ohm, so then I thought it needed approx 416 milliamps.

However when I try increase the current to test it seems to open at about 220 milliamps.

So how to I figure out how much current tor run through it?

Thanks in advance for your help.


V=I * R

12V = I * 28 Ω

12/28 = I

I = 429mA

W = V * I

5W/12V = I

I = 416mA

“ it seems to open”

If you mean it ‘picks’ or ‘operates’ at 220mA that’s great.

The datasheet is the place to check for this, if one exists. Otherwise just assume it needs 12V and about 0.42A
and budget your power supply and switching device accordingly. As solenoids get hot their resistance increases so the current will fall a bit if you drive it with a high duty cycle.

Many solenoids have a maximum rated duty-cycle - they are not designed for continuous operation.

Thanks for your replies! :slight_smile:

@larryd - It is a solenoid water valve and it opens for water (or beer as it will be if I get my project to work) at 220 mA. That made me afraid that 420-ish would fry it.

@MarkT - No datasheet unfortunately. It will not be operated for long at a time, just long enough to fill 500 ml of beer.

I guess I can just give it 420 mA and it should be ok then.

Thanks a lot for your help

Regards Gunnar

Some solenoids have an ON duty cycle.

Keep an eye on the temperature.

Curious how you get the solenoid to operate at 220ma? Do you reduce the voltage to 6 volts?


@Paul_KD7HB - I might not be reading the instrument right, but I have a lab power supply. I dial the voltage to 12 and then slowly increase the current. When the current is around 210-210 the solenoid opens the valve and then the voltage read at little more than 6 Volts.

Solenoids have a must operate level, similar to relays.

Obviously this will be less than the 5W equivalent current level.

Remember to keep an eye on the solenoid temperature.

@larryd - on the temperature, my plan is to mount my whole project within a junction box. The solenoid will only be active for around 20-30 seconds and then off for about the same time.

Could this cause too high temperatures? I have done a few tests, and seems ok. Is it better to use less current?

Thanks again for all help!

15 watts isn’t much, so you most likely will be fine.

You need to keep an eye on the solenoid initially to see if it gets hot.

If it just runs warm, all is okay :slight_smile:

You can add a series power resistor to lower current at the rated voltage but then you need to worry about the temperature on the resistor.

If the solenoid picks at 6 volts, 9 volts might be a good trade off too.