Heat from a transformer

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#21
DNic said:
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 ........
Click to expand...
Pkease post WHICH Preamp it is and a link to its specs, specially DC voltage it needs.
DC is NOT AC
 
#22
Hi, It is not item specific. This is the 2nd time I have tried using a Trans for a P/S which needs to be above 35VAC. Both times I have not been able to find a suitable trans & there seems to be no high S/Q adjustable regulator available.
This is what I am after, I found while searching the net. But I do not understand the conversion formula.
https://www.google.com/search?clien...UKEwja6ve1_aOBAxWlcWwGHQVXDDUQkeECKAB6BAgIEAE

But for reference this is what I need to achieve
34 volts into Raw Power Supply (I currently have 42V)
44 volts out of Raw Power Supply

Cheers
 

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#24
DNic said:
no high S/Q adjustable regulator available.
Click to expand...
There are circuits that allow an ordinary LM317 (style) regulator to drop much more voltage, by putting a transistor in front of it. The transistor takes most of the voltage drop and current, the LM317 takes the rest of the voltage drop and current. So the transistor takes the big beating, while the '317 operates normally.

Similar to what's shown in https://www.diyaudio.com/community/threads/using-the-lm317-with-higher-voltages.344416/ which I found by google search "high voltage lm317 circuit"
 
#25
Well Mandu, I have run into this problem 3 times in the last 10 years & still not got the answer. Manufacturers advise the raw voltage & most of the time the trans puts out 3 to 5v more, then sometimes like the 30v trans I got, it was spot on. Do you know any manufacturer that will guarantee a 33v trans.
-JJ : I know I can put a resistor like these https://campaign.aliexpress.com/wow...738&OLP=1085800308_f_group2&o_s_id=1085800308
in from of the regulator, what I do not know is what ohms it should be. Also sadly the 317 is not good enough for this application.

Cheers
 
#26
DNic said:
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.
Click to expand...

Do not use a dimmer. That device will phase-control the mains voltage sinewave. The result is that the sinewave will suddenly disappear twice within the 50 (60) Hz period (positive and negative sides are affected). What happens when you suddenly remove the voltage from the coil? A back EMF of substantial proportion (amplitude) is generated, which will cause a lot of heat. You are risking the wire lacquer burn-out and eventual shorting of the primary coil. Dimmers should never be used with an inductive load.
 
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#27
The effect you describe in your original mail is called transformer regulation. A transformer is specified at full load. So a 30VA transformer will give the correct AC output voltage when the secondary is loaded by 30 watts. That you get more voltage is because it is lightly loaded in your application. This effect is called transformer regulation - have a look at https://www.electronics-tutorials.ws/transformer/voltage-regulation.html .

A typical 30VA toroid has a regulation of about 13%. You seem to be getting 25%

What is the make of the transformer you are using?
 
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#28
You have a 30 VA transformer and the output is 36 volts as specified.
A 30 VA transformer will draw approximately 130 ma in the primary.
Get a small transformer say 15 volts rated at 200 ma, (about 3 VA) and connect in a buck configuration with the 30VA transformer.
The 30 VA primary will be supplied with 215 volts instead of 230 volts, which will reduce the secondary voltage as per the turns ratio.
Your buck transformer secondary voltage depends on how much you want to reduce. I have shown an example.
My house supply sometimes reach 250 volts and I have a 100 VA 250/20 volts transformer to reduce the power supply to 230 volts powering the computers and monitors.

Regards.
 
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