PSU Capacitor Size

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PSU Capacitor Size

Postby DortoH » Sat Oct 30, 2004 11:51 am

What is the general rule of thumb for PSU capacitor sizing.

Lets say I have a design calling for 2 55uF caps, but that size is not available. Would it be better to go with 2 47uF or 2 68uF? My thought has always been bigger is better.
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Postby erichayes » Sat Oct 30, 2004 12:46 pm

Hi All,

Rod, there are a couple of things that need to be considered when sizing caps: 1. type of rectification being used, 2. type (age) of capacitor(s).

With directly heated tube rectifiers (5U4 family), you want to keep the input capacitance at or below around 40µF to keep the charge current from exceeding the tube's capability. For indirectly heated tubes such as the 5AR4, I've gone as high as 100µF (although it's not recommended), because the charge current is metered by the cathode warmup time.

Solid state rectifiers have a rule-of-thumb peak/continuous current capacity of 25/1, so you can pretty much throw anything at them without much consequence. I'm running 500µf input capacitors on my 50, 100 and 200 watt prototypes, but they're in a choke-input configuration with time delay turn-on. There isn't really any need to go over 100µF in most cases.

If you're using new stock capacitors, the ratings for both capacitance and voltage are pretty accurate (generally ±10%). NOS caps, or caps harvested from old gear, on the other hand, can have rated tolerances of up to +80/-20%. And I've seen old caps increase as much as 1800%! (Was in a Fender Princeton; output cathode bypass cap was supposed to be 25µF but measured 450µF.) So be careful, particularly when working with tube rectifiers. Direct-reading capacitance meters are not that expensive anymore, and are sometimes bundled in with other functions on some of the fancier DMMs available now. I have an LCR meter that I got on sale for 80 bucks from MCM that I've damn near worn out checking caps and chokes over the last 6 years. Definitely worth the investment.
Eric in the Jefferson State
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