Re: HV Measurement - The Divider Problem (fwd)

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---------- Forwarded message ----------
Date: Thu, 23 Sep 2004 10:56:25 -0500
From: Dr. Resonance <resonance@xxxxxxxxxx>
To: High Voltage list <hvlist@xxxxxxxxxx>
Subject: Re: HV Measurement - The Divider Problem (fwd)



When working with high potential circuits always use a cheap surplus analog 
meter for all measurements.  We have toasted a few over the years but a 
surplus $10 meter is cheap compared to the 27XT.   We NEVER use our 27XT in 
Tesla coil or HV circuits of any type --- too many things can go wrong as 
you discovered.

We only do power off measurements such as inductance, etc. with the Wavetek.

Dr. Resonance



> According to the manual on my Wavetek 27XT the input impedance is 10
> megohms ( This is typical of most DMMs).  Therefore when you connect the
> meter across a 33 meg resistor its effective value becomes:  R =
> (33*10)/(33+10) =  7.67 meg.  The division ratio is then 39.7 :1 instead
> of 10 to one.  Sorry about the fried meter, I just cooked one myself.
> </blockquote>
>
> Aargh!  Yep, you've hit the nail on the head.  Thanks to Bert as well for
> spotting what I missed.
>
> I've (foolishly) regarded meters of "having such a high input impedance I 
> don't
> need to worry about it".  In future, I will!
>
> The upshot of this is that:
> a) My divider is fine.
> b) I have a 37XR on the way which has the added advantage of an inductance 
> range
> up to 40H.  I approximate transformer ratios using SQRT(Lp/Ls) and have 
> found a
> lot of MOTs seemed to give me "OL" (out of range) for Ls with the 20H 
> range of
> the 27XT.
> c) I have proven conclusively that DMMs are fragile beasts and don't like 
> 3.4kV
> put on their 1kV range!
>
> The irony is that I actually built the divider to extend the range of my 
> 4kV
> electrostatic meter.  (Made in the early 1940's and, by all indications, 
> still
> just as good and accurate as it was sixty odd years ago.)  I guess that
> electrostatic devices have an input impedance as "near to infinity as 
> makes no
> odds" (to quote the Hitchhiker's Guide).
>
> Cheers
>
> M
>
>
> -- 
> Matthew Smith
> Kadina Business Consultancy
> South Australia
>
>
>