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I am considering using some 4 pole caps and was wondering how the hookup would be in a voltage doubler circuit. I would be using 4 4pole 160v caps with 2 caps in series for each side of the voltage doubler. thanx for the advice. PS,Is it necessary to use bleeder resistors, too? Dak
Follow Ups:
Yes you can use them. The extra lugs ("poles") are just to get a low inductance connection to the external circuit. In switch mode power supplies switching at 100s of kHz that is very important. Can't hurt for an audio amp. I have a bag of 470uF/450V caps (inherited from the day job) with 4 terminals like these.Like any voltage doubler make sure the "top" caps do not have an exposed case connected to the -ve terminal which in a voltage doubler circuit will sit at half rail voltage, got a nasty surprise from a cheap Chinese 845 SET which had dergraded insulation on the "top" cap (Half rail was 440V).
Series connecting caps for both top and bottom positions is OK, ALWAYS use voltage share resistors across series connected caps. Make sure the voltage share resistors are appropriate value - biggest mistake I see with posted designs using series connected caps is using way too high a value share resistor value.Work out the typical leakage current from the formulae on the data sheet or if not given use the following:
Ileak = 0.006 CV
where C is the capacitance value (in Farads) and V is the applied voltage (not the caps rated voltage) and then size the the resistor such that you get at least 3 times that leakage current in the share resitor ( 5 times is better). Then do an I squared R calc to get the power dissipated and use a resitor of at least 3 times that power rating.Example Calc:
assume your caps are 1000uF 160V and you will have 130V applied
leakage current from above will be 0.8mA.
Want 5 x 0.8 = 4mA in the share resistor, 130V across it so 27K share resitor is what you need. I squared R gives you 0.43 watts, use a 2W metal film minimum.Hope this helps,
Cheers,
Ian
Edits: 07/30/12
.
What's a "4 pole cap?" One with 3 caps with a common ground? The term is could be confused with 4 terminal "feedthrough caps" (the inner and outer foils have leads attached at both ends of the cap) which are intened to be used for filering HF noise in the power source.
See link below.
I am not sure if what you are mentioning is the same as the 4 pole electrolytic caps which i am writing about. In this type there are 2 + terminals and 2 - terminals. As opposed to the standard 2 terminal electrolytic capacitor. This construction is supposed to have lower resistance and better frequency response. This type is marketed by Jensen and Mundorf amongst others. regards, Dak
You can wire all the sections together and use each can like a regular single cap. But why? Just get some Sprague Atoms and save the multi sections for restoration work.
The "Atoms" are good stuff, but the values they come in are too small for a doubler stack. Use 820 μF./250 WVDC parts, like AA sponsor Jim McShane stocks and installs in quite a few "vintage" units. NTC inrush current limiting is definitely in order.
Want a coal black background? Insert a "hash" filter made from a high current RF choke and a 1000 pF. cap. (mica or NPO ceramic) between the doubler stack and the reservoir LC section.
Eli D.
Eli,What would be the value of the choke for the hash filter in a 7189 PP stereo amp? (my current project).
Installation of the filter would be after the voltage doubler with the 1000pf cap in series, parallel, or to ground?
Choke and cap in parallel?
Inserted in series from the voltage doubler stack to the B+ power supply filters?
A choke and cap in parallel form a tuned circuit, which, if reasonably high in Q and in series with the rectified B+, will only be effective (as a filter) around the resonant frequency. I blieve what you want is a simple choke, which will have increased impedance as the frequency is increased.
I assume you are replying to Chris, since as I wrote we don't use the cap and choke in parallel.
No - the small choke goes in series between the doubler stack and the filter caps, the 1000pf goes to ground downstream of the choke and ahead of the filter caps. It's a simple LC filter.Edited to make small clarification...
Edits: 07/31/12
Also,
Treat the disease and not the symptom - use ultrafast soft recovery diodes for the restifier. They will produce 5 times less "hash" than standard diodes.
Cheers,
Ian
Even when zero switching noise Schottky diodes are employed, an issue remains to be dealt with, if high value caps. are installed in the doubler stack. Large amounts of capacitance imply a tiny conduction angle and a highly "triangular" ripple waveform. Per Fourier's Theorem, such a waveform contains high order harmonics of the ripple fundamental that extend well up into RF. The winding capacitance of a commonplace PSU filter choke is a short circuit at RF. The "hash" filter eliminates the crud that would sneak past the "routine" filter choke.
BTW, the "hash" filter also disposes of garbage riding on the AC mains.
Eli D.
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