[Collins] 75A-4 Recap

[Eugene Hertz]

Thank you for a really superb explanation. Granted, it will take me sometime to fully digest it all, but superb nontheless!  

Eugene

>-----Original Message-----
>From: Dr. Gerald N. Johnson [mailto:geraldj at ispwest.com]
>Sent: Tuesday, May 23, 2006 03:20 PM
>To: collins at mailman.qth.net
>Subject: Re: [Collins] 75A-4 Recap
>
>On Tue, 2006-05-23 at 01:35 +0000, Eugene Hertz wrote: 
>> On the subject of recapping and testing. I have a basic question. 
>> I have a cap tester that will show leakage in uA for caps tested 
>> up to 600v (sencore lc53). I have heard that ESR is what one 
>> should be looking for, somehow its a more accurate or telling measurement. 
>> 
>My test with power supply and VTVM shows full scale for 0.15 microamps 
>leakage. Is yours that sensitive? Just one microamp leakage from audio 
>driver to audio output grid with a 1 megohm grid resistor (uncommon, but 
>possible, more commonly 1/2 megohm) shifts the bias one volt. On a tube 
>with 30 volts grid bias that's not much but on a tube with only 3 volts 
>grid bias, that's unacceptable unless you have a bucket of tubes and 
>output transformers because that bias shift will move the operating 
>point of the tube to more plate current than needed. So the parts run 
>hot. 
> 
>> My question is, can't leakage and ESR be equated? For example R=V/I. 
>> Couldn't ESR be somehow derived from the leakage? Could ESR=600v/uA ? 
> 
>Leakage and ESR are different phenomena. Bridges sometimes try to equate 
>the two as a function of capacitor Q or dissipation factor. Dissipation 
>factor is 1 / Q. And in an AC bridge circuit with dissipation factor 
>that might show relative equivalence. The ideal bridge would have three 
>balance adjusments, one for series resistance, one for capacitance, and 
>one for leakage resistance. The bridges in common use (yesteryear) had 
>only two balance adjustments allowing the series resistance arm to be 
>the total Q adjustment. Even then there can be interaction between the 
>two making the bridge null hard to reach. I'm not sure one could 
>separate series and shunt resistance (and that shunt equivalent resistor 
>might need a range into gigohms which is hard to do with a high quality 
>precision wire wound rheostat) which may be why bridges tend to go for 
>equivalent series or parallel resistance only. 
> 
>Trouble is sometimes DC leakage is voltage sensitive and the bridge 
>operating voltage is probably only a volt or two on the capacitor while 
>the DC leakage may get significant only above 150 volts (or some 
>threshold). DC leakage need not be linear either, it can have a 
>threshold and essentially be a breakdown phenomena, or arcing in the 
>dielectric. 
> 
>On the other hand, ESR is the series resistance of the capacitor. A 
>capacitor can be leaky but have low series resistance. In some circuits 
>(like that grid coupling capacitor above) a bit of series resistance is 
>of no consequence, like what's a 1k series resistance feeding a half meg 
>load? A voltage drop from 500 parts to 499 parts. On the other hand in a 
>by passing circuit or a power supply filter circuit, a 1K series 
>resistance allows an AC current on the supply line to generate a volt of 
>ripple for each milliamp of ripple current. In the filter capacitor 
>situation a few ohms of series resistance is a sign of a failed 
>capacitor while the equivalent as a leakage resistance based on 
>capacitor Q or dissipation factor would not have a significant effect on 
>radio operation. 
> 
>A capacitor truly has both a series resistance and a parallel or leakage 
>resistance. The third element of the capacitor is the capacitance. They 
>can be lumped together but as I've said, have different circuit effects. 
>For convenience some AC bridges lump them together measuring only the 
>equivalent of one or the other and some bridges aren't adequately 
>sensitive to leakage for my tastes for audio coupling capacitors and AGC 
>bypass and time constant capacitors. 
> 
>A bit of leakage in screen bypass capacitors when in every capacitor in 
>the radio tends to make the total gain of the radio a bit low and the 
>performance a bit punk which gets worse with time as the added heat from 
>the extra current in the screen dropping resistor tends to make it drift 
>high in value while the added heat in the bypass capacitor makes it leak 
>more. 
> 
>Leakage comes from defects in the dielectric media. Series resistance 
>comes from poor connections to the metal plates (or the electrolyte in 
>an electrolytic capacitor). They can't be equated. 
> 
> 
>> 
>> Anyone shed some light on this subject? 
>> thanks 
>> Eugene 
>> 
>> 
> 
>-- 
>73, Jerry, K0CQ, Technical Advisor to the CRA 
>All content copyright Dr. Gerald N. Johnson, electrical engineer 
> 
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