# Transformer Current Output

Discussion in 'Power Electronics' started by Electromotive, May 6, 2020.

1. ### Electromotive

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4
Feb 14, 2018
Can anyone advise how I can determine (measure or calculate) the current output (in Amps) of a small 120vac/12vac transformer?

2. ### Bluejets

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Oct 5, 2014
Does it have a va rating anywhere marked..?

3. ### Electromotive

38
4
Feb 14, 2018
No markings... That's the issue. thanks...Tom

4. ### Harald KappModeratorModerator

10,614
2,372
Nov 17, 2011
Not easy to do. Try the steps described here.

Edit:
An idea for a more pragmatic approach:
It seems that the standard EN 61558-1 allows a max. temperature of the transformer windings of 100 °C for material of class A1, more for higher quality materials. Since we do not know what materials your transformer is made of, 100 °C (~210 °F) seems to be a safe limit.
You could determine the max. current by:
- apply a reasonable load current (depending on the size of the transformer use 50 % of the rated current of a similarly sized transformer), by adding power resistors to the secondary side.
- wait for e.g. 30 minutes and measure the temperature of the coil (measure at a few different points to get a valid reading).
- If the temperature of the transformer is below 100 °C, increase the load current in increments of e.g. 10 % (or more if the transformer is really cold) by adding more resistors (in parallel). Again wait, then measure. Continue until you reach a winding temperature of ~100 °C. That should set the max. output current for reliable operation.
If the temperature is above 100 °C, decrease the load and perform essentially the same steps as described before in the reverse direction (lower current).

Add a fuse on the primary side with a current rating 1/10 of the load current (or a bit above) for overload protection.
Be aware of the temperature of the environment while doing these measurements. If you want to operate the transformer in a closed housing, possibly badly vented, the environmental temperature of the transformer will be higher than during your initial measurements. Accordingly the winding temperature will also rise to a higher value. To avoid exceeding 100 °C, take this effect into account and lower the current accordingly,

Last edited: May 6, 2020
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6. ### Electromotive

38
4
Feb 14, 2018
Some very helpful information here. If the below doesn't mean anything to anyone here, I will try the Power resistor technique..Thank you.
However: Removed the transformer from the enclosure and found this: LEI Logo in a circle, and 60LEIB-002?
Anyone?

7. ### Bluejets

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Oct 5, 2014
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8. ### Electromotive

38
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Feb 14, 2018
Bluejets: Thank you. This is great information. Except, mine doesn't have all that rust on it.

9. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Another technique is to weigh the transformer. Generally speaking, transformers of a similar weight have a similar VA rating. This can vary for all sorts of reasons, but likely puts you in the ballpark. At the very least it provides a starting point for determining the load resistor(s) to try.