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I need a 120 to 24 volt transformer. The one I bought says 24 Vac secondary Just below that it say 40VA output.

Is this a 24 volt transformer or a 40 volt output

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    \$\begingroup\$ A 24V 1.6A RMS (40VA/24V) transformer. 24V is at full load- you will measure somewhat more voltage open-circuit. \$\endgroup\$ – Spehro Pefhany Dec 24 '14 at 0:24
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With transformers, you have Primary and Secondary voltages. You can think of them as Input and Output, although it actually works in either direction.

So, you are looking for a transformer with a 120V Primary and a 24V Secondary.

The primary-to-secondary ratio is a fixed property of the transformer. In this case, 120V:24V is a 5:1 transformer. If you hook up 240V to the primary, you'll get 48 on the secondary, and so on.

(There are practical reasons not to do this. One example: perhaps the windings were only specified to handle 120V, and would fail at a higher voltage.)

As far as the 40VA, that is a power rating, not a voltage rating. In a DC circuit, this would be given as 40W (Watts). However, with AC, the voltage and current waveforms can be shifted in relation to each other. This causes a difference between the "real power" and the "apparent power". It is customary to give the apparent power rating of a transformer in VA (Volt-Amps)

Here is a Wikipedia article on the VA.

Even better, here is a good question/answer on EE.SE :)

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It is a 24Vac (rms) output transformer. It has a power capability of 40VA (volt-amps)

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The 40VA means "volt-amps" and is a measure of power handling. It means that you can draw 40VA / 24V = 1.67A from the secondary. Why don't they just put 24V at 1.67A? Not sure. I guess even though a transformer is listed to have a 120V primary and 24V secondary, it could actually take other voltages. For example, you could probably put 240V into it and it would work just fine and the secondary would put out 48V. But the power would stay the same so you could only draw 40VA / 48V = 0.83A. If you draw more power than that, it would get too hot and maybe start to melt or maybe even burst into flames (seriously).

Note that the 24V secondary means RMS. Meaning if you used an oscilloscope to look at the peak-to-peak swing it would be more like 30-something volts. But if this transformer is for a power supply, after the AC is rectified and filtered, it will be a little above 24V such that it is a good voltage to be regulated down to 24V. The point is that if you need a DC power supply that puts out 24V regulated, you actually need a 24V transformer and not 26V or whatever.

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