Determining the amperage of an unknown transformer

On a sunny day (Tue, 10 Aug 2010 22:05:49 -0700) it happened Robert Baer wrote in :

This is OK for TOTAL power. But a transformer may well have several windings for different purposes. For example his 48 V may be designed to carry only 1 A, while the 18-0-18 was designed for 6A.. This is even mroe common in TV transformers, many windings, some with very thin wire for low power signal circuits. So the only reliable way is to measure the wire diameter for a windiong, that gives you an allowed maximum current, and as you know the voltage, multiplcation of all that gives the total power. Because you MUST know what you can load each winding with, else the power figure is useless. For example I could wind 100 turns wih .01 mm. You could not load that to the full power of the core.

I explained in another post how to do this with a simple resistance measurement and knowledge of the output voltage of the two windings.

Still nothing to say about transformers? You bever talk electronics any more.

Did you run the ball-knocker thing yet? Any data?

Your setup, with the three single strings, looks unstable to me, sort of a chaotic oscillator. The commercial units have two strings per ball, to keep the balls centered.

I was looking forward to data. My guess is in the 100 us range, from pulse leading edges. But the physics is messy.

John

Thank you for that, only thing left is to guess at the capacity (VA) rating of unknown transformer? Assuming the thing is stable temp no load, one could load it until a drop of what 5%, 10% no load to full load voltage?

Then monitor loaded temperature rise to make sure transformer is not going to cook with that load. What sort of temperature rise on load to use? I'd guess at max 40'C about ambient? I've seen many have a

Grant.

Ah. Ohm's Law. I think I've seen that somewhere before.

John

Or, as another poster suggested, find a similar size transformer in a catalog and use its power rating as a starting point.

Transformers designed for general purpose use are usually Class A; see:

This class allows a hot spot temperature of 105' C. If room temperature is about 20' C, then you can have a 65' C rise.

Have a look at post #14 in this thread:

There is a discussion of how to determine temperature rise by measuring the change in winding resistance after several hours on load.

Only a little bit ;)

Grant.

Makes no sense? Take a 30V secondary, DCR is 0.21, open circuit V is

Okay, if I tip the percentage other way, I get 7.2A and 8.7A, which would produce a ~300VA transformer? Hmm, which looks about twice as large as my estimate? So if I halve the currents, that might be fair?

I can see the 9V winding, looks about 4 to 5A copper (1.0, 1.1mm diam?), can see the mains at maybe .4mm diam, can't see the 30V wire.

This transformer so old is doesn't use a split bobbin, goes 9V sec., then shield, 240V pri, then the 30V secondary. Copper and iron shields on the outside.

E+I core measurements, give or take a millimetre or so:

______________________________________ ________ / / / / 38mm / / Stack / / -------------------------------------- -------- | | | | | ----- ------ | ---- | | | | | | | | | | | | | | | | | | | | | | | | 48mm | | | | | | 80mm | | | | | | | | | | | | | | | | | | | ----- ------ | ---- | | | | -------------------------------------- --------

| | | | | | | | | 31mm | | | | | | | | | 63mm | | | | | 95mm |

Loaded Measurements (A, B switching until repeatable, mains V moving about).

For the 30V secondary, with a pair of 10 Ohm 80W resistors:

open 31.0V, 10.25R 29.8V

Anyone want to do the sums, or estimate xfmr VA rating?

Cheers, Grant.

...

Easier to attach a 10k thermistor and lookup the temp from R ;)

Grant.

That will get a rather robust rating. Some transformers go as high as

Who are you waiting for?