[AMPS] Cap leakage

Tue, 18 Jan 2000 09:55:30 -0500

Jim, you can't go wrong if you follow Rich's suggestion of replacing
anything with more than 250 ua leakage, but you may end up replacing
certain caps prematurely with that rule-of-thumb.

My review of the Mallory catalogs shows that a leakage current formula is
given for each series of capacitor.  So satisfactory leakage current for a
Mallory type CG won't necessarily be the same as you'd find on a type CGS.
Here's an example :

For Mallory type CG, dc leakage current = (6 x C x V)/ 10^6, after 30
minutes at full rated voltage, maximum not to exceed 4.0 ma.   C is uf and
V is max. rated working voltage.

For Mallory type CGHand CGS, dc leakage current = 0.006 x sq.root(C x V),
after 5 minutes at full rated voltage, not to exceed 6.0 ma.

Mallory don't give a leakage formula for the FP series.  The FP's were
originally designed for non-critical radio/TV service . Ripple current
rating for these things would only be perhaps 100 - 200 ma.  

The CD multisection caps appear to be a down and dirty quick fix.

The 0.5 ma may or may not be OK - again depends on the original spec.  My
experience with reforming caps (using a machine similar to yours) is that
almost any good cap ends up with a fairly low leakage current, almost
regardless of the spec. For low, I would have to agree with Rich's 250 ua
number in general.  I have several 300 uf/450V caps with leakages well
below 100 ua.

 Suspect units show leakage that tends to bounce around, and don't approach
the low end.  They're still probably useable, but for how long ?

At 09:08 PM 1/17/2000 -0600, you wrote:
>
>I have am going thru the process of reforming the PS caps in a ML-2500 amp.
>I am using a Eico 950B resistance capacitance comparator bridge for a
>variable PS and am measuring the ma by monitoring the voltage across a 20K
>resistor in series with the cap.  The general technique is to rise the
>supply voltage until I see about 1 ma of current (20 volts across the 20K
>resistor. .  I let the voltage set until the indicated current quits
>dropping and then rise the voltage until I see about 1 ma of current again
>and then repeat the cycle.
>
>This schematic on this amp shows 6 FP electrolytics 150 mf a @450 Vdc.  The
>amp has 3 spragues that match the skematic and 3 CD caps that are mutli
>section that are hooked in parallel.  So far I have reformed the 3 spragues
>which had a leakage of about .5 ma at 450 Vdc.  I will have to see how the
>CDs look.
>
>This subject has be covered before, but what is a good leakage limit guide
>line for reuse or replace in this application and if replacement is
>warranted, what is a good source for replacement caps?
>
>The amp uses a 100K ohm carbon comp resistor across each cap.  I plan to
>replace these  3 watt MOF resistors.
>
>73,
>
>Jim Zellmer, KA0VSL
>
>
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>
>

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