Heat from a transformer

Hi, I have been trying to build up a 240VAC transformer unit to run a phono stage. The trans is rated at 36V & 300W, but when connected to the mains it is massively putting out 48V.
So to reduce the voltage to the required 36V, I inserted a 240V voltage reducer, before the phono trans mains lead. (I bought for another project into the mains circuit, but did not end up using it) To get the Trans output down to 36V I had to reduce the mains down to 180VAC.

I have tried the phono amp out in this format I it works very well, however the trans gets very, very hot (still touchable, but only just) so I have only used it for short periods of time because I do not want to burn the unit out!!

So if I can ask for guidance,
  • How hot should a trans get.
  • Is there a better way to get the trans to operate, I have read inserting Schottky diodes in the output line can do it. But do not know how to configure this.

Cheers for any help, suggestions.


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Joined 2011
I would not use that device at all, it is dangerous.
It's intended for use as a light dimmer, not for audio use,
and also will cause lots of noise, since it uses triacs.

How much current at 36 VDC does your circuit need?
Seems like your transformer is oversize, so it's lightly loaded and outputs too much voltage.
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Hi Guys, Sorry my bad it is a 30W trans, not 300!!:mad:

Anyway using the unit is not noisy at all, using it with my SPU through a FR Sut to MM phono amp, it is very, very quite. Very pleased with the sound but it's the heat I am worried about. Recommendation is to use a 30W with the phono amp.
So if the dimmer is not recommended, then how do I reduce voltage. I Need to drop 12V
Joined 2012
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The three pin regulator suggested by rayma is an easy way to drop the voltage.

Another easy option is a zener shunt regulator controlling a transistor or mosfet voltage follower. That would require understanding the circuit in order to determine the zener diode (or multiple diodes) and voltage dropping resistor values.

Zener regulator
The trans is rated at 36V & 300W, but when connected to the mains it is massively putting out 48V.
I bet your transformer is rated 36V AC and your preamo needs 36V DC

You can´t receive or profit from useful answers unless you understand the difference.

By the way, a 36VAC transformer will supply roughly 50VDC (the 48VDC you are measuring) so it´s just working as expected.

You bought the wrong transformer; instead of putting band aids on it (and each making the unsolved problem worse), get a 24/25V AC rated one, period.
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Why not getting the proper transformer, which will be cheaper and better?
I keep making the mistake to think that other people are like me and quite possibly has this stuff just lying around. Of course two transformers is a bit ridiculous, unless you can solve it for free (via lying-around stock) and put a cover over them.

Hence some of my suggestions. I actually happen to have a 12V transformer on a shelf, US voltage primary though...
Hi, Yes I had this one laying around, that is why I am trying to use it. Measuring the output from the trans powered at 240, I get 48VAC. So I only need to reduce the unit by 6V, I have tried a 30V trans in the unit, but did not work properly. I am not (as you may gather) very experienced with dabbling in AC.
But with every trans I have ever used it seems impossible to get the correct output voltage. If the circuit required a raw voltage of under 30V I could use the trusty LM317 and then reduce. But that is not what I have here & the LM317 is not good enough for the Phono amp.

On reading through P/S blogs I have found this, but not sure how to implement it. Same with the Schottky, which I have read is a better option.

''Raw PSU voltage is about 50V. I read somewhere that a 20omh resistor in series with the PSU output could bring the voltage down to a save level. So that is implemented, and the Voltage is now about 42V.''

I am anticipating that using those options will allow the Trans to operate at 240V & therefore not get to HOT?

Cheers & thanks for all help
Firstly, try powering the transformer from 240v mains without anything else connected to it. If the transformer gets hot, it is faulty and probably has shorted turns causing it to overheat. If it remains cool we can eliminate that and move on to the next step.
You mentioned that the Phono Amp has a power lead. What voltage is the amp designed for and is it AC or DC? More photos and a clear description would help to diagnose your problem. At the moment it's mainly guess work :unsure:
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