[Test-Equipment] Question about series/parallel capacitors

[Richard Knoppow]

----- Original Message ----- 
From: "Barry" <n4buq at knology.net>
To: <test-equipment at mailman.qth.net>
Sent: Sunday, May 15, 2011 2:21 PM
Subject: Re: [Test-Equipment] Question about series/parallel 
capacitors


>> I really think the use of balancing or equalizing
>> resistors is a non-issue here. These are very low leakage
>> film caps, mostly equalizing resistors are used with
>> seriesed electrolytics to prevent excessive voltage 
>> across
>> one _when they charge up_, after that the current is 
>> fairly
>> low. Film caps have such low leakage that I don't think
>> charging voltage or static voltage can cause any problems 
>> at
>> all.
>> When putting capacitors in either series of parallel the
>> errors in their values are averaged, however, that 
>> assumes a
>> normal distribution of error and my not be true of a 
>> small
>> sample, especially if they are all from the same
>> manufacturing lot. You must really measure them to see at
>> least in what direction the error is. If you have a known
>> precise capacitor of the right value, no matter what its
>> voltage rating, you can measure to a very high degree of
>> accuracy by the substitution method where the cap becomes
>> the standard. The overall accuracy depends on the 
>> precision
>> of the measuring instrument, its accuracy is of no
>> importance since that is established at the desired value 
>> by
>> the comparison standard.
>> I think in practice there is no significant difference
>> between the two series-parallel arrangements. As a side
>> note, it would be interesting to know what is in the
>> original GR standard cap. The catalogue shows caps up to 
>> 500
>> volts and some to 700 for much lower values of 
>> capacitance
>> than the one in the bridge. It may be that the bridge
>> standard is a specially made single stack cap or its
>> possible its made up of two seriesed catalogue standard 
>> caps
>> in a single case. Since the cap is bad you could open the
>> case to find out what is in it. The fact that its shorted
>> suggests its a single cap since its unlikely that both 
>> caps
>> of a series would short (but possible).
>> The accuracy of the bridge with the replacement cap can
>> be established by measuring a known accurate cap. If the
>> standard cap is off value the error will be the same on 
>> all
>> ranges using that cap. The _precision_ of the bridge will
>> not be affected.
>>
>>
>> --
>> Richard Knoppow
>
> This along the line of what I was thinking.  If I can get 
> the 1409-X, I can ensure my home-made replacement is at 
> least accurate.
>
> According to Henry (the designer of the 1617A), the 
> original cap was a mix of polyster film and mica (at least 
> I think that's correct) the reason being that the TCs of 
> both cancelled each other out.  I plan to open the 
> original.  It would be nice if something internal besides 
> the caps shorted and I could fix it but I doubt that's the 
> case.  I assume there aren't individual caps with leads, 
> etc., inside so repairing it is probably not feasible but 
> I'll know more once I open it.
>
> The replacements I had planned to user are polyester film. 
> While there could be some TC issues, hopefully it would be 
> good enough for hobby work; if not, then I can rely on the 
> 1409-X and just remember not to turn the bias up over 
> 500V.  Yes, it would probably take it, but I'd hate to 
> ruin a 1409 finding out...
>
> Thanks!
> Barry - N4BUQ
>
     Henry Hall is a jewell and I hope he is able to say 
around a long time.
     The 1617-A is a high-precision insrument but much of 
its use in evaluating capacitors will remain even if the 
accuracy of the standard is not quite on. I certainly am 
interested in what is in the original standard cap. From the 
descriptions of the standard caps in the catalogue its a 
fairly complex mounting with shock absorbing material, 
dessicants, and other things to maintain its performance.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com