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How hot is too hot for a power supply transformer?

darklife

darklife

Modulating Madness
This is a bit off topic but I built an RF tube amplifier (not for audio use). Because it's still a type of amplifier the subject is still the same I guess.
The power supply transformer I am using is an old gray plated 720v center tapped transformer that was used in a piece of broadcast gear. A Gatesway broadcast consoles power supply if you wanted to know.
I am running this transformer in a homemade amplifier and am drawing the amount of current it's capable of producing. I am right at the edge of the specs on current draw.
When I fire up the amp it stays cool until about 10 minutes then there is some noticable heat from the transformer.
Around an hour it is very hot. I can put my fingers on it for about 5 seconds before it's uncomfortable.
However the power supply transformer shows no signs of being under stress. No smoke, no fumes. Just really freakin hot!
Is it okay to run a transformer like this? Should I fan it down or maybe even heatsink it to be safe?
I know some transformers normally run hot but I'd like to get some opinions on just how hot and maybe personal experiences with hot iron :)
Thanks.
 
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Most well designed transformers are intended to have no more than about 40 degrees C temperature rise. The hotter it runs, the sooner it will fail, other things remaining equal.
 
I think that you should not be drawing more than 80% of the transformers rating.

"Freakin hot" is a sign of stress on the transformer with smoke and fumes right around the corner.
 
You can run a ST-70 and the stock PT gets hot enough to make coffee. I'd run it 'till it smoked, which it might never do, then get a beefy modern replacement. Antek makes nice overrated stuff for cheap if you have the space for an ugly torroid. I'd not worry too much about it. (I probably wouldn't leave it unattended while I went shopping or something, tho!)
 
EICO (which ran its hifi power transformers hard) notes in its manuals...

The temperature of the transformers used in the [in this case] HF-81 run at less than 195 F, despite the fact that the safety limit is a much higher 221 F. Although 195 F is cool for a transformer, it is very hot to the touch.
Click to expand...
 
darklife said:
I am running this transformer in a homemade amplifier and am drawing the amount of current it's capable of producing. I am right at the edge of the specs on current draw.
Click to expand...

Xfmr secondary current is greater than DC output current with most rectifier/filter arrangements. What circuit are you using?
 
darklife said:
This is a bit off topic but I built an RF tube amplifier (not for audio use). Because it's still a type of amplifier the subject is still the same I guess.
The power supply transformer I am using is an old gray plated 720v center tapped transformer that was used in a piece of broadcast gear. A Gatesway broadcast consoles power supply if you wanted to know.
I am running this transformer in a homemade amplifier and am drawing the amount of current it's capable of producing. I am right at the edge of the specs on current draw.
When I fire up the amp it stays cool until about 10 minutes then there is some noticable heat from the transformer.
Around an hour it is very hot. I can put my fingers on it for about 5 seconds before it's uncomfortable.
However the power supply transformer shows no signs of being under stress. No smoke, no fumes. Just really freakin hot!
Is it okay to run a transformer like this? Should I fan it down or maybe even heatsink it to be safe?
I know some transformers normally run hot but I'd like to get some opinions on just how hot and maybe personal experiences with hot iron :)
Thanks.
Click to expand...
you have done the old-timer electrician temperature test. If you can put your finger on it of 3 seconds then its less than 50C (128F). Electronic power supply transformers do not have a specific temperature range rating what they do specify is the Temperature Rise (45C MAX allowed is typical) above the ambient temperature. if your room is at 30C (84F) and you measure the surface temp of the transformer as being 50C (128f) you are well within the 45C rise. I assume that you are building the final for a transmitter. you may want to check and see if there is any RF o the DC rail. That can cause the transformer to heat up. If there is need you to clean that up.
 
First step, for me, would be to go to Home Depot or the like and buy an infrared temperature gun. With that knowledge come back and share.
 
sae-7125 said:
temp guns are so "last decade", perhaps consider a thermal camera. worth every penny when a cost effective + feature rich solution is found.

just a thought.
Click to expand...
Maybe great for audio gear but for kitchen use (the other use I mentioned) where one stirs things, the gun still works fine and isn’t as affected by the spills and splatters like a camera would be.

Now which one to get the one on sale for 99 which reads to 1112°F or the Fluke at $4346 that only goes to 1020°F? We all like Fluke.

I didn’t know these were as cheap as a c-note already.
 
Back in my repair days anything more than 'slightly warm to the touch' would be a red flag. And once I worked for "the others" it would be completly unacceptable.
That being said I remember trannies being real hot and all voltage measurements, etc. coming out fairly normal. Cant recall a customer ever coming back after bringing it to their attention.
But transformers did STB alot , esp when they brought in a repair with the output tubes glowing cherry red LOL.
 
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