Toroidal Voltage problem

bowdown · 12th March 2004, 04:55 AM

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 · 12th March 2004, 07:12 AM

It helps to hook up some capacitors too.

bowdown · 12th March 2004, 09:08 AM

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

kilowattski · 13th March 2004, 03:38 AM

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.

mwh-eng · 13th March 2004, 04:39 AM

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 · 13th March 2004, 04:51 AM

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 · 13th March 2004, 05:01 AM

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 · 13th March 2004, 08:19 AM

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 · 13th March 2004, 08:28 AM

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 · 13th March 2004, 08:42 AM

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?

12th March 2004, 04:55 AM	#1
bowdown diyAudio Member Join Date: Feb 2004 Location: Perth	Toroidal Voltage problem 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

13th March 2004, 03:38 AM	#4
kilowattski diyAudio Member Join Date: Aug 2003 Location: Long Island, New York	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. __________________ ----------------------------------------------- Kilowattski

13th March 2004, 08:42 AM	#10
kilowattski diyAudio Member Join Date: Aug 2003 Location: Long Island, New York	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? __________________ ----------------------------------------------- Kilowattski

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12th March 2004, 07:12 AM	#2
djk diyAudio Member Join Date: Feb 2001 Location: USA	It helps to hook up some capacitors too.

12th March 2004, 09:08 AM	#3
bowdown diyAudio Member Join Date: Feb 2004 Location: Perth	Yeah i tried that as well but still the same problem, i am using 50,000uF per rail(5 x 10,000uF caps)...

13th March 2004, 04:39 AM	#5
mwh-eng diyAudio Member Join Date: Feb 2004 Location: Livingston, Montana	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.

13th March 2004, 04:51 AM	#6
Nelson Pass The one and only Join Date: Mar 2001	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.

13th March 2004, 05:01 AM	#7
mwh-eng diyAudio Member Join Date: Feb 2004 Location: Livingston, Montana	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

13th March 2004, 08:28 AM	#9
Christer diyAudio Member Join Date: Sep 2002 Location: Sweden	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.

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