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MrAdel1984 12th April 2012 11:45 PM

CT transformer Amperage Question
Hi Guys

I bought a 28 0 28 / 4 amp CT transformer to use it to build my first Gainclone
I suspect the seller has given me a lower than 4 amp trafo , so I wanted to test the amperage myself , the only way I found is to connect the trafo's secondary to an ATX ps 12v , I did this after I measured the seconary resistance which was 3 0 3 ohm, so 12/3=4 amp ,anyway I got only 2.5 dc amp running throug the secondary , the transformer gradually heats up and took about 25 min to be so hot that I can barely touch it , so can this trafo handle say, 3~4 amp ac current.

Thanks in Advance

godfrey 13th April 2012 04:32 AM

There really isn't any non-destructive way to test the amperage. What you did sounds like quite a torture test; overheating the trafo isn't good for it's health.

I would probably just try it in the gainclone and see how hot it get's playing music. Unless you're playing continuously at high levels, it should (hopefully) be OK.

The voltage sounds a bit high for a gainclone, though. After rectification you'll have close to +-40V. The 3ohm resistances could be a blessing in disguise as they'll drop some voltage when the amp's drawing current.

DF96 13th April 2012 11:18 AM

Under normal (AC) use there will be heating in the transformer primary too, and maybe some in the core. If 2.5A DC in the secondary alone is too much then it might not be a 4A secondary.

AndrewT 13th April 2012 12:29 PM

What weight is the transformer?
What type is it, EI, Toroid, Rcore, Ccore?
What diameter wire is used to wind the secondary?
Can you see the diameter of the primary winding?
What temperature was the insulation in the interior of the transformer when the surface proved to be too hot to touch?
Have you blown up your transformer?

KatieandDad 13th April 2012 12:33 PM

Best way is a LOAD TEST.

Most toroidals will lose about 8-10% of their output voltage at full current.

If this is a 56V 4A transformer, try loading it with 15R 240W of resistors, it should still output about 51V.

You'll need to make up the huge resistor from many smaller ones.

KatieandDad 13th April 2012 12:36 PM

If it doesn't hold that voltage you can use the same test to evaluate at which current the voltage does drop by 8-10%.

AndrewT 13th April 2012 12:43 PM


Originally Posted by KatieandDad (
Most toroidals will lose about 8-10% of their output voltage at full current.


Most toroid transformers have a regulation between 3% and 30%.

KatieandDad 13th April 2012 12:46 PM

That why I quoted 10% - its approximate.

AndrewT 13th April 2012 01:47 PM

It's still wrong.
Try to pull sufficient current that the output voltage drops by 8% to 10%, from a 5% regulation transformer will prove nothing about the VA capability of the transformer.

KatieandDad 13th April 2012 01:58 PM

If you've got nothing else to work with loading it to its rated output will still prove that it is good for that amount of load.

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