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ocool_15 1st December 2004 06:44 AM

Transformer Current output
I have a couple pulled transformers that are physically bigger than I need and put out the appropriate voltage. The problem lies in that the transformers have multiple outputs. The one transformer in particular puts out 30VAC and 10VAC at both at no load. Is there a way to appoximate the amount of current handling it has on each output?

If measure the voltage drop on a couple different dummy loads(100ohm, 60 ohm ect) would that give me an idea? or by measuring the DC resistance of the coils of the transformer?
Line voltage is 120V here in canada. I don't remember what i pulled some of the stuff out of but that one was something to do with those old big satelite dishes people used to have.

gmphadte 2nd December 2004 05:05 AM

There was a thread on the same subject here

The final verdict of that thread is that there is no other means except size to judge the VA.

Having 2 secondaries, the case is hopeless. Best bet is to get the current consumption of the original eq on each secondary.

Gajanan Phadte

richie00boy 2nd December 2004 09:07 AM

You can look at the thickness of the winding wire and correlate it with the standard wire gauges and current capacities, but still you are just guessing in reality as it depends on how much the manufacturer decided to 'squeeze out' of the wire.

Sch3mat1c 2nd December 2004 03:22 PM


Originally posted by richie00boy
as it depends on how much the manufacturer decided to 'squeeze out' of the wire.
Pffbt, I'm the manufacturer, I'll decide for myself. :D

But really, such a thought WILL work... I mean, it's just wire around some iron. Big deal.

If you know the cross section of the core (the part where the turns are wound around) you can find the VA capacity *based on how warm it can run*. If you want to run it hot as hell, you can easily double the current output. Resistance will then be a bigger factor and you'll have worse regulation, and gets hotter... but if it doesn't smoke at that, who's to say whether it's right or wrong? See what I mean?

If you know the VA and the voltage of the two windings, you can find the current of both. (Two givens, two unknowns, simple algebra solves.) Three windings will need a guess, however.

Hm. Either way will need a guess. 'Cuz, what if V1=V2, then I1=I2, *NOT necessarily*.. one could be wound for 1mA and the other the remainder of the VA. Nevermind, there's a few other unknowns so it cannot be purely calculated from VA and volts.


AndrewT 3rd December 2004 07:19 AM

test each winding individually.
Find what current causes a 5% volts drop from the open circuit voltage.
Then test with all windings driving their own load. Now measure the new volts drops probably between 5% & 10% below open circuit voltages. If your outputs are unbalanced adjust the load on the worst winding and test again.
now leave it on long term test with all loads connected, monitoring the transformer temperature. If it runs hot then reduce your loads, if it is cold increase load.
this should get you into ballpark figures.
Q Any thoughts anyone????
regards Andrew T.

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