Strange voltages from a toroid transformer

[johnheath]

Hi all…
This might be a stupid question but here is the story.

I bought a toroid transformer and it has three secondary windings: 250vac c.t, 48vac and 6,3vac c.t

By accident I coupled all three secondaries on to a rectifier bridge and coupled the center taps to the chassi. I did this in a hast being a bit stressed up.

I turned on the amp I very quickly measured the voltages to see if it was somewhat ok, but as you already figured out it was not.

I quickly turned of the amp and realized my mistake. I felt that the transformer was a bit warm… not hot but rather warm… ok.

I then coupled the two center tapped windings according to the attached schematic and the voltages are as mentioned in the schematic… and this is where I am puzzled. It can be said that I had the amp running for about 20-25seconds before I turned it off.

It must be said they the 48vac windings seems to be in order… even though the 73vdc seems a bit high from 48vac but not alarming.

Is it my coupling or is it something wrong with the transformer… or anything else?


Help and advise needed.


Best regards

/John

 

[RuudNL]

The voltages in the drawing don't look wrong to me.
Voltages will be higher, because there isn't any load.
Only the 3.15 V winding looks a bit low, but at such low voltages the losses over the rectifier diodes (0.7 V) play a significant role, so this may not be so strange.

 

[johnheath]

Thank you sir for the reply.

Ok… but I could not draw the entire schematic but these are the voltages with load in full circuit.

So if I have 6,3vac for the heaters and would like to rectify it to get 6,3vdc would require a different rectification to get a higher vdc value?


Best regards

/John

 

[ruffrecords]

The 2.9V is not surprising if it is full load. What is the load?

The 73V from the 48VAC is not too surprising. If this is powering a regulator for phantom power then the load will be light and the dc voltage across the reservoir cap will be near to the peak value of 48V which is just under 68V. A small current phantom winding is likely to have a regulation of about 10% so at no load this voltage could be 10% higher i.e  over 74V. So 73V is possible.

However, the 204V looks too high to me. Even no load with 10% regulation it should only be 194 V. If it has a 230V primary and your local mains is high you might get that high.

Transformers are quite tough. It looks to me that this one is OK. If a winding had burnt out you would get a very low voltage somewhere. The only possible damage that might explain these results is if a few turns of the primary got shorted. I don't know if that ever happens though. CJ would know better.

Cheers

Ian

 

[johnheath]

Thank you Ian

One of the problems that is included here is that I in wrong way calculated the resistors from a 1,4 x NNvac (rectifier bridge) and I would have expected a rise in voltage after the rectifier… but with the two-phase rectifiers it should be lower as RuudNL suggested.

However the 2,9vdc after the rectifier see a load of 3R9 and it was based on the calculated rise to roughly 8,8 (6,3 vdc x 1,4) and a current of 640mA which was the current before I changed the transformer. The former transformer had a 12vac winding but reducing the voltage down to 6,3vdc put a lot heat on the resistors so I figured that a 6,3vac winding would ease it a lot.

Even the measured 7,52vac across the heater winding would not get me near the desired 6,3vdc using a two-phase rectifier so another question that appears to me is if it would be possible to use a bridge for rectifying even if the transformer have a c.t?… and if so just shorting the c.t? (maybe another stupid question)

The measured 204vdc after the two-phase rectifier is still workable so the problem here is the heater arrangement because I do not want to use ac for the heaters… haven't tried it yet though


Best regards

/John

 

[ruffrecords]

Yes, you can use a bridge rectifier even if the transformer has a centre tap. Just do not connect the centre tap.

Cheers

Ian

 

[johnheath]

Thank you sir

That will make things easy

Best regards

/John

 

[joaquins]

  You are confused about rectifiers, take a quick read at something about half-wave rectifier, ful-wave rectifier and bridge rectifier and look at the numbers there, that will help you with what you are doing and for the rest of your life if you still deal with electronics. Is one of the basis you need to know, it's not called two-phase but-full wave rectifier.

JS

 

[ruffrecords]

  You are confused about rectifiers, take a quick read at something about half-wave rectifier, ful-wave rectifier and bridge rectifier and look at the numbers there, that will help you with what you are doing and for the rest of your life if you still deal with electronics. Is one of the basis you need to know, it's not called two-phase but-full wave rectifier.

JS

Sowter has a page all about this:

http://www.sowter.co.uk/rectifier-transformer-calculation.php

Cheers

Ian

 

[johnheath]

JS - Thank you sir… I was reading about it, but sadly I could not get any information about wether it was a proper way to use a full-wave bridge if the transformer had a center tap or not… now I know that it is ok if not connecting the c.t. And I do not claim that I know about the correct names but when reading on the internet about rectifiers (Valve-wizard) he says the two diodes are in a two-phase rectifier… that is why I used the expression  :-\

http://www.valvewizard.co.uk/bridge.html



Ian - Thank you again for the input on the center tap. The values seem normal again and the circuit works as predicted.

But, when reading the sowter file I get a bit puzzled… When using a (full-wave) bridge and the dc sees a cap it says that the vdc is 0,71 x vac, but that was my problem when using the two diodes… the voltage was way below my calculated voltage and it was in fact roughly 0,7 x vac. But when I used the (full-wave) bridge (not using the c.t ) the voltage is roughly 1,4 x vac.

What am I missing here?


Best regards

/John

 

[PRR]

> reading the sowter file ...  it says that the vdc is 0,71 x vac

[quote author=Sowter]Vac = Vdc x 0.71[/quote]

Maybe new reading glasses? (Not laughing; been having trouble myself.)

 

[johnheath]

Hahaha - yes, of course… my girlfriend tells me to get glasses and use them when soldering because she thinks that I am having my face into the chassi… I bought a pair of these…  :

 

[joaquins]

... And I do not claim that I know about the correct names but when reading on the internet about rectifiers (Valve-wizard) he says the two diodes are in a two-phase rectifier… that is why I used the expression  :-\

http://www.valvewizard.co.uk/bridge.html
...
/John

I'm just telling you the names so google search gives you better results, also using the usual names helps a lot to understand each other. Both, full wave rectifiers and bridge rectifiers use both half cycles of the wave, half wave doesn't. The wizard claims there are two phases because it uses two windings of the transformer with different polarity, can of worms discussing terminology, if you look for full-wave or bridge rectifiers you will find the info you need to understand what's going on, that's all that matters.

JS