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I am trying to wire an electromagnetic lock. The lock needs 24VDC at about 25mA. Transformers seem to be rated in volts and watts, so what should I buy?
To calculate power, the formula is P = I X E
Convert Milliamps to amps --- 25 / 1000 = .025
24 X .025 = .6 watts
Look for a very small or even a transformer that can be adjusted.
Handy Andy in Mt Airy
If you need 24 VAC you can pick up one at Homedepot http://www.homedepot.com/p/Packard-2-ft-Mount-Transformer-PF42440/203565058?MERCH=REC-_-PIPHorizontal1_rr-_-202264717-_-203565058-_-N as long as the transformer is rated higher than 25 ma you can use it.
If you actually need 24 VDC, I would recommend a wall wart other wise you will need a 24 volt transformer and a rectifier to convert to dc and probably a filter capacitor. As long as the output is greater than 25ma you would be OK.
Probably have models available at most hardware or big box stores also.
Check out local computer repair shops (not the chain stores) and have them look through their bins of spare parts. There's a good chance they will have a power transformer that will do just what you need, and they probably won't charge you too much!
The voltage rating of the transformer must be the same as the lock requires. The current rating of the transformer must be equal to or greater than the current requirements of the lock (25mA).
Be sure you know whether you need AC or DC supplying the lock. They are not interchangeable.