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Old 24th July 2012, 06:26 PM   #271
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In the 4A 48-0-48 PSU that I described previously, I used 2mH and concentrated on the ability of the chokes to handle 4A continuously. (Air cored to prevent saturation)

As I said, It was a CLCC, CLCC, CLCC, CLCC supply. 2 x 600VA split secondary transformers feeding 4 x 50A Standard Bridges then 33000uF 2mH 33000uF 33000uF four times over for four separate supplies. The 33000uF were Cornellier Dubillier.

I also added 4.7uF Audio Grade Polyprops across each bank but it didn't seem to make too much difference.

The Pass Aleph 4 does not have any decouplers on the PCB board so all the decoupling is done at the PSU end.

The PSU cabling and PCB tracks are monsterous though (32A Auto Cable to the PCB).

Last edited by KatieandDad; 24th July 2012 at 06:29 PM.
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Old 24th July 2012, 06:37 PM   #272
DF96 is offline DF96  England
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Quote:
Originally Posted by forr
I wonder what is the cause of this "distorsion".
Stray inductance resonating with the other cap? One cap means inductance is in series with the cap. Two caps means each inductance is almost in parallel with the other cap. The inductance is not so much caused by the length of wire but the area of the loop formed by the caps and the wire.

Alternative explanation: more stored charge in the rectifier junction? Bigger caps means shorter but larger charging pulse, hence more stored charge? Stored charge means the rectifier stays switched on when it should have switched off so having charged the caps up it can then drag them down again for a brief while until all stored charge has been removed from the junction. Having said that, I'm not sure I believe it myself but who knows?
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Old 24th July 2012, 07:00 PM   #273
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I'm leaning towards rectifier drop as well, but when I was simming a PSU buffer chain and playing with values I recreated the same sort of distortion by giving the buffercaps a noticable ESR. I doubt that in reality ESR would go up with more caps in parallel so I'm considering the rectifier drop the purpetrator: At the peak the supply has to recharge both the cap and keep supplying the load as well. Less ESR means more current together with that what the load is demanding; depending on the bridge I could see this be a cause for the momentary drop at the peaks. Or, what DF96 says. A bigger, badder rectifier could possibly give an answer to that hypothesis.
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Old 24th July 2012, 07:13 PM   #274
PChi is offline PChi  United Kingdom
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Quote:
Originally Posted by forr View Post
PChi,


I've done this experience with different caps (I checked them first with a tan-delta meter), transformers, rectifiers and connections of the probes.
Each time, the phenomenon happened for high values of capacitor.
It has been repeatable for me but I do not exclude an error in my procedure.
Maybe somebody could do the same test and show his findings,
I still think that the distortion is due to high ESR capacitors or measurement technique.
Is the power supply '0V' connection connected to mains earth?
The Oscilloscope ground is conected to mains earth. It might be worth probing the '0V' connection with the probe tip.
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Old 24th July 2012, 07:44 PM   #275
forr is offline forr  France
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The following test was done with a single rail power supply, loaded by a 39 Ohm power resistor. The transformer came from an old B&O 2*20 W receiver where the reservoir cap was 3300 F.

The scope probe was connected at the diode bridge + and - terminals.
The ground reference was given by the shield of the probe cable, the scope mains earth terminal was not connected to a real earth.

Click the image to open in full size.
4400 F

Click the image to open in full size.
6600 F

Click the image to open in full size.
17000 F
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Old 24th July 2012, 07:50 PM   #276
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Are those 3 different caps? Or you adding more parallel capacitance?

Either way, reading the voltage scale of 200mV/Div I'm guessing diode drop by the momentary charge current.
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Last edited by MagicBox; 24th July 2012 at 07:54 PM.
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Old 24th July 2012, 08:20 PM   #277
forr is offline forr  France
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Quote:
Originally Posted by MagicBox View Post
Are those 3 different caps? Or you adding more parallel capacitance?
4400 F : two 2200 F 63 V CMF-FP SIC SAFCO in parallel.
6600 F : three 2200 F 63 V CMF-FP SIC SAFCO in parallel.
17000 F : one 2200 F 63 V CMF-FP SIC SAFCO in parallel with one 15000 F 50 V MARCON.
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Old 24th July 2012, 09:14 PM   #278
tsiros is offline tsiros  Greece
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can you repeat the test with one big versus two small in parallel that add up to the big one?
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Old 24th July 2012, 09:46 PM   #279
DF96 is offline DF96  England
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Yes, maybe it is ESR. Two caps might mean half as much ESR, but twice the size of charging pulse, so the ESR related voltage is about the same. It only looks bigger because the two caps have half as much ripple so the scope is twice as sensitive.
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Old 24th July 2012, 10:16 PM   #280
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Given the caps mentioned and looking at the sensitivity of the last two scope images they are both 200mV/Div, only the first image is 500mV/Div.

The difference between B and C is the 3 small caps vs 1 small and one big one. Going from 3 to 2 caps increases ESR relatively, while the much increased capacitance increases the charge current notably for the big cap.

Adding up all of this I think it's safe to say it's an ESR issue, where the ESR performance of the 15mF cap seems quite bad in relation to the smaller caps.
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