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Transformer Estimations

Many people recycle components from old electric or electronic equipment. Most components are quite easy to identify and find information about, even datasheets for, but transformers aren't one of them. This page is a recap of the information found in the links below. I recommend using caution when using this information, mains voltage can kill you.

Question

How much current can I draw from this transformer?

Answer

It depends on how long you want to do it.

Temperature

What ultimately kills a transformer is temperature. The temperature rises with load depending on the resistance of the winding. The temperature of the transformer will rise with the current consumption. R.G. notes that "A transformer will work fine at temperatures hot enough to fry eggs on the case", theoretically up to the iron looses its magnetic abilities (700F/370C). What fails is the insulation, and the real limit is totally dependent on what material is used as insulation.

Estimation by loading

This is what PRR has to say about testing by loading a transformer (edited slightly).

PRR wrote:
Transformer sag is a direct linear function of current, this can be used to estimate it's current limit. In small transformers the rating is often 10% sag. In very-small, often 20%.

Eyeball the iron to estimate a VA, then aim low. You think it may be 12V 12VA (1A)? Then figure a 0.1A load. 12V/0.1A is 120 ohms. Power is 1.2 Watts. A couple 270 ohm 1/2W in parallel is a suitable short-time load.

Say that you measure 14.0V no-load and 13.5V with 135 ohm (0.1A) load. 13.5/14.0= 0.964, or 96.4%. You have 3.6% sag. You can probably go to 20% sag. So you can suck 20%/3.6% or 5.5 times more current. 5.5*0.1 is 0.55A probably-safe load.

Estimation by software

There are programs that will do current limit estimations based on a few measured values. See the links section for a download link.
Transformer Estimation Thread - This is the thread that spurred this page and from which most information is taken.
Transformer estimation software

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