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Old 31st March 2005, 01:00 AM   #1
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Default Kofi Annan in: "B+... PLUS!!!"

Same Kofi, new jam.

In the spirit of frugalphilia, I decided to use an old transformer I had laying around for Thorsten's phono pre project. I used a VOM to find the secondaries and the center tap, plugged it in and measure 290VAC. "Perfect!", sez Kofi. According to Duncan Amps PSU designer (see attached), that will work out to the 250VDC required for the project.

So, I assemble the power supply and I get 385 - 400VDC... and I say, "well, there's no load on the power supply right now, Kofi. You're scared over nothing. Just assemble it and I'm sure eveything will be fine."

Can you guess the rest?

Not fine. Not at all. I'm still getting about 385VDC fully loaded. So here's my question:

What the hell did I do wrong this time?

Please respond soon and help a brother out. Mrs. Annan is waling on eggshells and the dogs are hiding from me. I'm a peaceful man, really, but there are these times....

Help. Please help.

Yours in relentless ignorance,
Kofi
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Old 31st March 2005, 01:18 AM   #2
SY is offline SY  United States
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Either your resistors are all off, your load is not 30 mA, your transformer is not the voltage you think it is, or you have found an exception to Ohm's Law. Now I know that you've been distracted recently with the Cotecna matter, but if you can get away from the press for a few minutes, I'd suspect the second- make sure you're actually drawing current. You should be dropping something like 35V across each of the resistors, which you can verify (carefully!) with a voltmeter.
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Old 31st March 2005, 02:36 AM   #3
rdf is offline rdf  Canada
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Hi Kofi, how does the voltage across C1 compare to the PSUD2 prediction? That'll tell you pretty fast if the secondary was measured correctly. BTW, you may already realize this but, unless you're designing a fireworks display for the next UN jamboree, PSUD is predicting more than 4 amps of peak current through those nominally 1 amp rated 1n4007 diodes. Wouldn't want to see an international incident over it.
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Old 31st March 2005, 02:45 AM   #4
Sherman is offline Sherman  United States
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Having just made pretty extensive use of PSUDII when designing the PS for my new monoblocks I would have to say that it is pretty accurate. I modeled several different PS designs and actually built two. The end result was within a few percent of the predicted value.

How about taking out the constant current in your PSUD simulation and replacing it with a fixed resistor of a value you have on hand. See what PSUD says the voltage at that resistor should be then in your actual circuit put in that value resistor and check the voltage. If they are close then I think Sy is correct. Somehow you aren't drawing 30mA in your circuit.

BTW, did you ever get the electrician out to check out your other issue? Maybe you were checking the voltage on the trafo when your neighbor was running her blender!
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Old 31st March 2005, 05:11 AM   #5
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Yipe! WTH's the 5 x fake transmission line for?

Two caps and a single resistor, or better yet inductor, works better than anything.

I would say your transformer and rectifier are obeying the laws of physics quite amicably. So the question remains, what did you screw up in PSUD?

Tim
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Old 31st March 2005, 10:49 AM   #6
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Is it possible that I measured the AC wrong? I got about 285VAC in the VOM, but is could it be that I needed to measure peak voltage and not RMS?

According to the designer of the circuit, 30mA is about right for current consumption.

Also, I'm using FREDs for rectification rated at 6A, so the curent shouldn't be a problem.

Thanks for the advice. Any additional shoves in the right direction would be much appreciated. I'll run tests on the circuit tonight.

Kofi
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Old 31st March 2005, 11:22 AM   #7
SY is offline SY  United States
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Tim: there actually is a good reason for splitting up the RCs. If you've got a simulator and a few minutes, you can convince yourself.
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Old 31st March 2005, 11:38 AM   #8
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Default Re: Kofi Annan in: "B+... PLUS!!!"

Konnichiwa,

Quote:
Originally posted by Kofi Annan
So, I assemble the power supply and I get 385 - 400VDC... and I say, "well, there's no load on the power supply right now, Kofi. You're scared over nothing. Just assemble it and I'm sure eveything will be fine."

Can you guess the rest?

Not fine. Not at all. I'm still getting about 385VDC fully loaded. So here's my question:

What the hell did I do wrong this time?
If your resistors are truely 1.2KOhm and not of a wrong value (eg 1R2 instead of 1K2) and your load is attached (a 5 - 8 Watt 240V incandescent lightbulb or two 110V <4 W ones in series may be employed as convenient test load, as might be a 8K2 resistor with at least 20W actual dissipation rating without heatsink) and confirmed to draw current, the voltage should pretty reliably fall into pace.

Sayonara
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Old 31st March 2005, 11:55 AM   #9
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Konnichiwa,

Quote:
Originally posted by Sch3mat1c
Yipe! WTH's the 5 x fake transmission line for?
Reduction of ripple to absolutely miniscule levels using inexpensive and readily available, small value inexpensive capacitors.

Quote:
Originally posted by Sch3mat1c
Two caps and a single resistor, or better yet inductor, works better than anything.
I would suggest that you actually compare "lumping" the RC values with a multiple stage approach and you may to your surprise find that two caps and resistor or choke work MUCH WORSE.

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Old 31st March 2005, 01:11 PM   #10
Sherman is offline Sherman  United States
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Quote:
Originally posted by Kofi Annan
... could it be that I needed to measure peak voltage and not RMS?
...

PSUD expects the trafo voltage as an RMS value. When you enter the voltage into the simulator it assumes it is RMS. You may have also entered an incorrect resistance value for the trafo but I don't think that would account for the amount of difference you are finding.

Also the RMS voltage of the trafo would be lower than the peak so if you used the peak value your resulting B+ would be lower than predicted by PSUD.
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