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Old 15th November 2004, 08:25 PM   #1
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Default Transformer Damaged?

I purchased an 800VA Toroidal Transformer with 2 x 50V secondaries. When the transformer is hooked up to a 115 AC outlet and the secondaries are wired in series I get a center tap and two outputs at +58V and -58V. If those outputs are then connected to a bridge rectifier the DC outputs come to +55V and -55V. What would be causing this? Isn't the DC output after rectification supposed to be the AC voltage multiplied by 1.414? This should come out to around +-82 V. Both of the labels on the transformer are seen below and I drew out a wiring diagram of how and where I am getting the voltages.

Am I doing something wrong or could the transformer be damaged in some way? Thanks for the help

Brian
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Old 15th November 2004, 08:40 PM   #2
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hi,

you have to connect a cap after the bridge in order to get the good voltage.
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Old 15th November 2004, 09:08 PM   #3
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Are you referring to a 100V Filter capacitor? Why does that effect the output DC voltage? Thanks and sorry for the dumb questions.

Brian
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Old 15th November 2004, 09:31 PM   #4
Stocker is offline Stocker  United States
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The output of a bridge rectifier is only DC when compared to the input. It is still pretty nasty compared to a filtered DC voltage. I have a feeling the voltmeters most of us can afford aren't true RMS voltmeters and that might be your problem. Hook up a few thousand microfarads to each rail like a normal power supply and see what you get.
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Old 15th November 2004, 09:49 PM   #5
moamps is offline moamps  Croatia
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Hi Brian,

you can find some useful info about rectifiers here

http://www.tpub.com/neets/book7/27b.htm

Regards,
Milan
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Old 15th November 2004, 09:49 PM   #6
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Thanks for the help guys.

I have a Tektronix TX-1 True RMS Multimeter and it has worked perfectly on my previous projects.

I'll try and hook up some filter caps and see what happens, thanks.

Brian
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Old 15th November 2004, 11:21 PM   #7
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Well, Thanks to all the knowledgeable people around here, I now have a fully functional Power supply. I hooked up a couple of 4700uF caps to the bridge outputs and came up with a consistent 80.6 volts + and -.

I wonder why my Multimeter didn't get the correct reading as it works perfect on lower voltage toroidals. At least now I can test my completed amplifier boards and I'll be posting all the pictures of my amps construction fairly soon.



Thanks everyone.

Brian
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Old 15th November 2004, 11:46 PM   #8
Enzo is online now Enzo  United States
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Unfiltered rectified DC will measure low because it is not stable DC. It is a series of half sine waves one after another. Bumps. Your meter measures the average. 120 times a second the voltage starts at zero and moves up to 1.414 times the AC RMS voltage and back to zero. Your meter averages this out. When yu add a filter cap, the first time the voltage moves from zero to 1.414 it charges the cap to that level. Now as the rectified AC moves back towards zero, the charge in the cap keeps the DC voltage at the 1.414 level.

In fact that is a tip I give to techs I train. If your audio has a loud hum and your DC reads about half or so, then you have probably lost a filter.
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Old 16th November 2004, 04:45 AM   #9
Eva is offline Eva  Spain
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There is nothing wrong with these measurements, just a bit of misunderstanding

Multimeters in AC mode measure RMS voltage, wile in DC mode they measure average voltage. Average is not the same as RMS

If you measure in AC before the diode bridge, you get the RMS voltage from the output of the transformer

If you measure in DC after the diode bridge, without smoothing capacitors, you get the average DC of the rectified AC waveform

The peak voltage of a sine wave is equal to 1.4142 times [sqr(2)] its RMS voltage

The average voltage of a rectified sine wave [without smoothing] is equal to 0.636 times [2/pi] its peak voltage

58 * 1.4142 * 0.637 = 52 V

So things match quite reasonably, except for the fact that mains waveforms are highly distorted sine waves whose RMS-peak and peak-average coefficients are not exactly 1.4142 and 0.636 respectively

The diode bridge has no magical smoothing effect, it just flips one of the halves of the AC waveform to make both having the same sign. Then, the capacitors charge during the peak of the sine wave [typ. 30% of the time] and discharge during the rest [70%] of the time to keep the load powered

This fact is what makes 50/60Hz supplies so inefficient in terms of capacitor, transformer and mains line usage, requiring allways bulky oversized components and lines

Any 1KVA 50/60Hz transformer is only guaranteed to produce 300 Watts of rectified DC without overheating
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Old 16th November 2004, 05:01 AM   #10
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Thanks for the information everyone. I understand what’s going on now (thanks to Enzo and Eva). Now the only question is whether or not my P101 can handle a rail voltage of +-80. Rod shows the High Power version using a maximum of 70V rails with a 2 x 50V trafo. I purchased the 2 x 50V model but it seems that the measured values are higher than expected. I may have to send Rod an email and ask what he thinks. I would only be driving an 8 ohm load so it may work out.

Wouldn't want the thing to come out of the gates smoking!!! lol
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