leakage of caps...

[René Schmitz]

At 15:16 27.04.00 +0200, Roel Das wrote:

>this gives: tgd = 1/wRC
>
>tgd for metalized polycarbonate film caps is (lower than) 30 *10^-4 at 1
>KHz.
>
>Solving the equation for R, and using these values gives:
>
>R = 1 / 30 * 10^-4 * 2pi * 10^3 * 22 * 10^-9 = 2,4 MOhm
>
>This is the what my teacher told me. Is this be right? Seems quite a low
>resistance...

Mmh, you're interested in the DC resistance anyways. This equation explodes
for w->0.
So I'd say no, this is leading in the wrong direction. Might be that at
1kHz the cap behaves as if theres a 2.4Meg across it, where the caps
impedance is just -j7.2k. But this is not what you were asking. (I think
its due to dielectric absorption, which is another error you have to take
into account, apart from charge transfer out of the multiplexers. Chances
are you won't notice.)

>Lower on the page of the caps databook it says:
>R between leads, for C <= 0.33µF :     >30000 MOhm
>
>This value suits me alot better. But is this the one to use?

Right. This corresponds to the figure of 100000 Megohm-µF given in Art Of
Electronics in Chapter 7 (2nd ed. p. 396). 

Bye,
 René



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