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Old 1st March 2011, 09:06 PM   #1
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Default PSUDII Question

I'm using PSU2 to size a transformer for a rectified 24 volt supply (30 before the CRC) . I'm using full wave solid state rectification.

The program suggests a 35 volt transformer. That means 70CT, correct? 35-0-35?
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Old 1st March 2011, 09:51 PM   #2
SY is offline SY  United States
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Depends on the rectification. For full wave, yes, 35-0-35. For full wave bridge, just a single 35V secondary, no CT needed. Or you could use an 18-0-18 and ignore the CT.
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Old 1st March 2011, 09:51 PM   #3
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Quote:
Originally Posted by Captn Dave View Post
I'm using PSU2 to size a transformer for a rectified 24 volt supply (30 before the CRC) . I'm using full wave solid state rectification.

The program suggests a 35 volt transformer. That means 70CT, correct? 35-0-35?
Not necessarily, depends on whether you are planning on using full wave center tap rectification (two diodes) or a bridge rectifier.

For this application I would probably use a bridge rectifier and a single 35V winding. Might want to work on that design a bit more, a 35V secondary isn't right for a 30V raw supply prior to the CRC filter. (Consider CLC as well or a regulator..)

I assume also that the program you are using is PSUD2..?
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Old 1st March 2011, 09:53 PM   #4
SY is offline SY  United States
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Kevin, how spot on and perceptive!
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Old 1st March 2011, 09:56 PM   #5
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by SY View Post
Kevin, how spot on and perceptive!
That's me, perceptive to a fault... Except when it really counts..

Edit: We must have been tapped into the same cosmic vein of thought, and I see we posted simultaneously.. I sort of missed that when I first commented..
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Last edited by kevinkr; 1st March 2011 at 09:59 PM.
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Old 1st March 2011, 11:10 PM   #6
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Yes I'm using PSUD2; I just noticed the typo in the header. I was using the center tapped with a pair of diodes. Thanks for the replys, I'm thinking correctly about it now.

I'm looking at a Fisher design where they provide 600ma of filament current at 24 VDC via center tapped windings rectified by a pair of diodes to make 30 vdc, followed by a 1000uf and 10R CRC filter. The end result, after the filter, is 24vdc with a volt or so of ripple for the twin triodes ganged up in series parallel.

The transformer is toast and so I'm asking myself what winding voltages I should specify. PSUD2 predicts 35 RMS. I'm just about to fire up the Variac with a bridge rectifier and varify that.

Sy, do you make it out to the SXSW events later in the month?

Last edited by Captn Dave; 1st March 2011 at 11:12 PM.
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Old 2nd March 2011, 12:31 AM   #7
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One more question.

PSUD2 is good. The results are very close. Ripple is about 1.4 p-p where PSU2 predicts .48 RMS, but that probably is more to do with 50 year old electrolytics than the program. Voltages very close to are as predicted, especially after running capacitance values down to recreate in the simulation the actual ripple.

So, I note that the unloaded transformer secondaries measure 32.2 RMS. They drop to 27.8 RMS under load. I suppose one would then specify the 32 RMS secondaries together with the winding resistance used in PSUD2. But, looking at Hammond's and other transformer catalogs, the winding resistance is not given. Is there a standard or assumed "slippage" with the transformer at rated load?
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Old 2nd March 2011, 11:31 AM   #8
DF96 is offline DF96  England
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Some catalogues or datasheets will specify winding resistance, but coded in the form of "regulation" (typically 5-20%, depending on transformer size). Assume a transformer is rated at 30VA, 15V 2A secondary, and 10% regulation. This means that the effective winding resistance is around 0.75ohms.

Note that effective winding resistance includes both secondary resistance and reflected primary resistance. It could easily be double the measured secondary DC resistance.
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Old 2nd March 2011, 02:21 PM   #9
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OK, thanks. I follow you. 15V/2A*.1=R winding. Knowing that, I see I was using the wrong winding resistance in the PSUD2 simulation, off by a factor of 10. That explains a lot.

I just located a SAMS on a similar model with winding resistance marked on the drawing. That drawing indicated 30v at 430ma with just 2.6R winding resistance. A higher rated transformer than I used yesterday (and that one did get hot).

So, if I back out the transformer rating from those figures: 30V/2.6R and 10% regulation would suggest 1.15A. That has the ring of truth. I suppose that spec (30V or 15-0-15 at 1.15A w/10% regulation) would work for my purposes.

Looking back at the PSUD2 file that produced the original 35V result, that transformer with a 7R winding resistance was significantly under rated at just 500ma for a 580ma load. GIGO
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Old 2nd March 2011, 04:09 PM   #10
DF96 is offline DF96  England
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Yes. Don't forget that there are two issues when sizing transformers. One is voltage droop under load. The other is heat under constant load. You can solve the first one by having a higher secondary voltage. You can solve the second by having a bigger transformer, or convincing yourself that the average load (averaged over a couple of minutes) will be much smaller than the peak load.
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