# How to match solenoid and transformer amperage (does it matter??)

Hi, I'm trying to use my first solenoid with a relay. It's a 12V solenoid, 131 ohm 92 mA and the transformer is 12V 1A. Is there a problem if the solenoid and the transformer have different amperage? (The relay has a 10A writing on it, but I'm assuming that just indicates the maximum it can handle) If so, can I do something about it? I searched but I'm quite confused, so any input would be appreciated.

Are you using a transformer to drive a 12 volt solenoid? I tend to think that the solenoid is rated for 12 Volts DC, not AC. There may be too much inductive reactance for that solenoid to work on 12 Volts AC.

The 1A power supply will work with the 91mA solenoid.

The current depends on the voltage and the resistance of the solenoid coil. ([u]Ohm's Law[/u] describes the relationship between voltage, resistance, and current.)

Transformers and power supplies are approximately "constant voltage", and they are rated for a maximum current. If you "draw" more than 1A, your transformer might burn-up.

(The relay has a 10A writing on it, but I'm assuming that just indicates the maximum it can handle)

That is correct. Your relay is OK at 10A or less.

A relay has two voltage & current ratings. The coil ("input") ratings are fixed... A relay coil rated at 12V and 100mA must be operated at 12V and it will draw 100mA. But, the contact ("output") voltage & current ratings are the maximum it can handle.

and the transformer is 12V 1A

I assume that's actually a DC power supply? A "transformer" is AC and it's one component of a power supply.

OK, thanks guys. Embarrassingly enough I just discovered I was thinking AC when it has to be DC. Did the search and found out I have to build a small circuit to do that. How to Convert AC to DC: 11 Steps (with Pictures) - wikiHow

There is a little secret lurking in the AC power systems. When you read the AC Voltage with a meter it will tell you the RMS Voltage, which, in a nutshell, is the equivalent DC voltage that will provide the same heating in a resistive load. The actual peak AC voltage is actually 1.414 times the RMS voltage, so a 12 Volt transformer output after rectifying and filtering will come very close to 17 Volts. If you include a regulator circuit to bring that back down to 12 Volts be sure to include a kickback diode across the solenoid to protect the regulator from the very high reverse Voltage spike when the solenoid is de-energized. That will also prolong the life of your relay contacts.