Transformer Testing

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Good day all! I have an old (cheap) receiver that passed away a few years ago and want to use the power supply, heatsink and case in a new project. The problem I have is with the power transformer. The rear panel on the receiver stated that the total power consumption is 380VA (is was a 100W/ch stereo receiver). The transformer is a multi/center tapped trans and measurements have shown +/- 40V AC and +/- 17.5V AC taps.

With this information can I safely assume that the 40V taps are capable of 4.375A total (350VA/80V : I used 350VA to be a bit conservative). If so are the 17.5V AC taps capable of the same current since they are tapped off the same guage wrapping wire?

If I cannot assume this, how can I test my trans to give me something to design around?

The sticker on the rear panel may not be a good guideline. Better to work from the power output. Is the 100W/ch an RMS specification? If so, you can calculate what DC power you'd need to get the 100W/ch. 2x40VAC should give you around +/-52VDC.
Note that AC Volt-amps for a transformer aren't the same as DC watts. Generally you divide by a factor of, say, 1.5 or so.
Do you plan to use the 17.5V taps to run a Class-A amp, so need the higher current rating? If you're sure they are indeed taps, and not separate windings, you should be safe pulling the same current (whatever that is) off the lower voltage taps.
transformer testing


Transformers will often have about a 5% voltage drop at the secondary under full load. So, if you have a large variable resistor, or a bunch of resistors that you can put in parralell, you can load the secodary(ies) until you get a 5% drop of the secondary voltage. Be sure to (carefully) monitor the primary voltage so as not to include line cord drop in your calculations. When you know what resistance you have across the secondary(ies) you can figure how much current you can draw.

If you don't have suitable resistors, you might build a power supply (rectifiers and capacitors) and use some transistors on heatsinks to load down the transformer. With this scheme you will need an ammeter to moniter the current being drawn, or insert a resister in series with one lead of your transister load and calculate the current based on the voltage across this resister. Hope this helps.
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