CT transformer Amperage Question

MrAdel1984 · 12th April 2012, 10:45 PM

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, 03:32 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, 10:18 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, 11:29 AM

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, 11:33 AM

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, 11:36 AM

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, 11:43 AM

Quote:

Originally Posted by KatieandDad

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%.

KatieandDad · 13th April 2012, 11:46 AM

That why I quoted 10% - its approximate.

AndrewT · 13th April 2012, 12:47 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, 12:58 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.

12th April 2012, 10:45 PM	#1
MrAdel1984 diyAudio Member Join Date: Mar 2012 Location: Cairo,Egypt	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

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Linear PS CT transformer question	empaist	Power Supplies	6	14th January 2012 04:16 PM
Power transformer amperage	slideman82	Power Supplies	7	1st May 2010 06:54 AM
Question about CT transformer and Brian's Rev C power supply board	maurycy	Chip Amps	9	8th December 2009 09:21 AM
transformer with ct or non ct?	galih	Chip Amps	7	7th December 2009 05:08 PM
Using a transformer without CT?	Serow	Pass Labs	3	19th November 2003 09:50 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

13th April 2012, 03:32 AM	#2
godfrey diyAudio Member Join Date: Nov 2009 Location: Cape Town	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.

13th April 2012, 10:18 AM	#3
DF96 diyAudio Member Join Date: May 2007	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.

13th April 2012, 11:29 AM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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?

13th April 2012, 11:33 AM	#5
KatieandDad diyAudio Member Join Date: Nov 2011 Location: UK	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.

13th April 2012, 11:36 AM	#6
KatieandDad diyAudio Member Join Date: Nov 2011 Location: UK	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%.

13th April 2012, 11:46 AM	#8
KatieandDad diyAudio Member Join Date: Nov 2011 Location: UK	That why I quoted 10% - its approximate.

13th April 2012, 12:47 PM	#9
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	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.

13th April 2012, 12:58 PM	#10
KatieandDad diyAudio Member Join Date: Nov 2011 Location: UK	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.