Toroidal Voltage problem

[bowdown]

Hey there evryone, im hoping sombody out there will be able to help me sove a problem i am having.Well this is how it goes....
I have a 1500VA Toroidal With primary of 240V and secondaries of 66V, theoretically once it is rectified i should be getting approximately +/- 92V, well this is not the case..... i measured the secondary ac voltage which is actually correct but once it goes through the bridge rectifier i am only getting +/-65V and i have tried 2 rectifiers that were bought at the same time and the same place!!!
these are all no load voltages, i have checked high and low on the net and the forums but cant find anything that can help me solve the problem...Pleeeaaase help if you can as i am desparete.
If you need more info please let me know.

Regards

Bowdown

 

[djk]

It helps to hook up some capacitors too.

 

[bowdown]

Yeah i tried that as well but still the same problem, i am using 50,000uF per rail(5 x 10,000uF caps)...

 

[kilowattski]

I do not see how it is possible to get your readings with the capacitors attached. With no capacitors attached you are actually reading the correct voltage. You are actually reading the RMS value of the ripple with your voltmeter which is about 65 Vdc (reference the top figure). With the capacitors connected and no load you should read about 91 Vdc. The capacitors charge to the peak value and there is no (relatively little) resistance to discharge them (see the bottom figure). You can easily verify this with an oscilloscope.

An externally hosted image should be here but it was not working when we last tested it.

An externally hosted image should be here but it was not working when we last tested it.

 

[mwh-eng]

I think most DVMs will read the average voltage when set for DC volts. RMS is an AC voltage reading, right? I agree, with the circuit properly wired with capacitors and no load, a DVM should read the peak voltage less rectifier(s) voltage drop.

 

[Nelson Pass]

Well, something is wrong. It might be that the transformer
is mis-labeled or mis-represented. It happens to even the
most reputable companies. Talk to the vendor about your
problem. If he is any good, he will help you. If not.... don't
despair, I wouldn't want 92 volt rails myself anyway. There
are lots of other deserving projects. Maybe this is a blessing
in disguise. 😎

 

[mwh-eng]

Make sure the bridge rectifier or discrete rectifier rating is 400 PIV. A 200 PIV is marginal and a 100 PIV may cause your problem if they are breaking down (conducting in reverse bias) between 100 and 200 volts. Just a wild guess.

I agree that + and - 90 VDC rails is awful high!

Good Luck

 

[lucpes]

If you have everything wired as in the image below there is something wrong with the bridge rectifiers/transformer. I'd go with two 400V/25A or 400V/35A bridges if you haven't done so yet...

some 'good' SS power supply 'literature':

http://www.tnt-audio.com/clinica/ssps1_e.html
http://sound.westhost.com/power-supplies.htm
http://sound.westhost.com/psu-wiring.htm

 

[Christer]

Bowdown,

I am sure you have done it right, but just in case, make sure that
you have the meter set to DC when you are measuring
the voltage over the caps. I think we had a similar case recently
where someone turned out to have the meter set to AC, which
gives the wrong result for DC.

 

[kilowattski]

mwh-eng

Wow, I stand corrected.....I can't believe I wrote RMS, and I wasn't even drunk at the time. I don't know what I was thinking?

 

[sss]

Make sure the bridge rectifier or discrete rectifier rating is 400 PIV. A 200 PIV is marginal and a 100 PIV may cause your problem if they are breaking down (conducting in reverse bias) between 100 and 200 volts. Just a wild guess.

thats would be my guess too

plus your primary is rated at 240V so the secondary will be 66V AC (or 92V DC like u calculated) only when the primary is 240V AC , what is the voltage in your country??? i dont know many countries that using 240V ,i got 220V 🙂

 

[emuman100]

the 65V secondary is an rms value. The peak value is 91.93Vp. This is what you will see with the capacitors filtering the pulsating DC. Without the capacitors, you are measuring the half sine wave. So instead of taking the rms value of it, your meter probobly measures the average of the pulsating DC. You can calculate this by taking the peak value and multiplying it by .318. This comes to 29.23V. I'm almost certain my power supply theory is correct, but correct me if I'm wrong.

 

[bowdown]

Thank you very very much everybody, you guys are great tried everything you guys said and i finally found the culprit, it was the bridge rectifiers!!!!!😡 but i got a new one yesterday and its a ripper 🙂 oh and as for the voltage railings, i know that they are a little high but its for a leach super amp had it going and i tested it just before clipping and its producing just under 400wrms @ 8ohms.... and just under 600wrms @ 4 ohms!! not too bad hey??
Again i thank you all for your help and support.

Regards
BowDown