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Old 12th April 2012, 10:45 PM   #1
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Default 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
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Old 13th April 2012, 03:32 AM   #2
godfrey is online now godfrey  South Africa
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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.
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Old 13th April 2012, 10:18 AM   #3
DF96 is offline DF96  England
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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.
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Old 13th April 2012, 11:29 AM   #4
AndrewT is offline AndrewT  Scotland
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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?
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Old 13th April 2012, 11:33 AM   #5
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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.
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Old 13th April 2012, 11:36 AM   #6
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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%.
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Old 13th April 2012, 11:43 AM   #7
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by KatieandDad View Post
Most toroidals will lose about 8-10% of their output voltage at full current.
Wrong.

Most toroid transformers have a regulation between 3% and 30%.
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Old 13th April 2012, 11:46 AM   #8
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That why I quoted 10% - its approximate.
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Old 13th April 2012, 12:47 PM   #9
AndrewT is offline AndrewT  Scotland
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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.
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Old 13th April 2012, 12:58 PM   #10
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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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