Radio Shack ESD mat doesn't meet spec

[Mike Morrow-3]

Gary Hvizdak wrote:

> You can read the full text of Alan's post in the Reflector archives at
> http://mailman.qth.net/pipermail/elecraft/2008-February/083794.html

Fred wrote:

>Ingenious test set up Alan used to measure 1.0E10 ohms.

I must be missing something in the test description.  Wouldn't the
very low internal resistance of the Simpson voltmeter grossly swamp
the effects of the very high resistance of the mat?  Even the input
resistance of a VTVM would be very low compared to the resistance
that is to be measured (even if measurement is only periodically
made on the circuit).

Mike / KK5F



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[Alan Bloom]

> I must be missing something in the test description.  Wouldn't the
> very low internal resistance of the Simpson voltmeter grossly swamp
> the effects of the very high resistance of the mat?

Yes.  What is happening is that the capacitor is discharging through the
meter in a millisecond or so.  The meter needle jumps up only a small
amount because of the damping in the meter movement.

The only reason that technique works is that the absolute meter reading
is not important, only the ratio of two readings.  The meter jumped to
about 1/10 scale when the capacitor was charged to 15V.  So a few
minutes later when the meter jumped to 1/20V, I deduced that the
capacitor voltage must have been 7.5V.

Alan N1AL


On Sun, 2010-03-28 at 13:45 -0500, Mike Morrow wrote:

> Gary Hvizdak wrote:
>
> > You can read the full text of Alan's post in the Reflector archives at
> > http://mailman.qth.net/pipermail/elecraft/2008-February/083794.html
>
> Fred wrote:
>
> >Ingenious test set up Alan used to measure 1.0E10 ohms.
>
> I must be missing something in the test description.  Wouldn't the
> very low internal resistance of the Simpson voltmeter grossly swamp
> the effects of the very high resistance of the mat?  Even the input
> resistance of a VTVM would be very low compared to the resistance
> that is to be measured (even if measurement is only periodically
> made on the circuit).
>
> Mike / KK5F
>
>
>
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> Elecraft mailing list
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>
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> Please help support this email list: http://www.qsl.net/donate.html

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