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Old 5th Dec 2019, 5:12 pm   #1
Martin G7MRV
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Default 'Cancelling' part of a meter's impedance?

This is a follow on from my post previously checking on my maths regarding measuring high voltages,

My an unlucky coincidence, my DMM has an impedance of 11Mohm, rather than the expected 10Mohm. With my first built 10:1 resistor chain, I could select resistors to allow me to include a series preset to adjust for exactly 10:1 readings,

Ive now obtained a 1Gohm resistor for 100:1, but of course the numbers are not coming out exactly 100:1 due to the meters rather odd extra Mohm!

Im wondering if I can 'cancel' this extra M of the meters impedance using a parallel resistor? This would need around an extra 100M which I could make up using low voltage resistors from stock, including a preset to trim the readings to an exact 100:1.

Ive never tried this method with a meter, and dont seem to be able to find any info on whether it works. I'd make up a parallel resistance and try it, but im out of 10M units at the moment (on order) and its a bit much to mock up 110M from smaller values!
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Old 5th Dec 2019, 7:17 pm   #2
Martin G7MRV
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Default Re: 'Cancelling' part of a meter's impedance?

Well, thats what comes of looking at a problem from the wrong angle!

My maths scholar son took one look and told me to up the series resistance! Problem solved.
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Old 5th Dec 2019, 10:29 pm   #3
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Default Re: 'Cancelling' part of a meter's impedance?

You might want to consider the % error for your 100:1 divider, with your meter at 11M its nearly ideal for the division ratio which is 11/(1000 +11) 0.0108803, very close to the required 0.01. Think of the % error of this and your meter's error by comparison. But its nice to have it exactly right.

Many scope probes are designed for 1M so typically for a 10:1 probe you have a 9M in the probe head.
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Old 6th Dec 2019, 11:31 am   #4
Martin G7MRV
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Default Re: 'Cancelling' part of a meter's impedance?

That is indeed where I started - pondering the 1G resistors tolerance - and managed to sidetrack myself off into a siding!

What Ive actually done is add the 1G ahead of the little 100M chain I built earlier! Thats got it reasonably close, enough to not worry about it for the present.
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Old 6th Dec 2019, 12:02 pm   #5
Argus25
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Default Re: 'Cancelling' part of a meter's impedance?

Quote:
Originally Posted by Martin G7MRV View Post
That is indeed where I started - pondering the 1G resistors tolerance - and managed to sidetrack myself off into a siding!

What Ive actually done is add the 1G ahead of the little 100M chain I built earlier! Thats got it reasonably close, enough to not worry about it for the present.
That will do nicely, the exact value you need would be 89M added to the 1G, but the 100M is more than close enough.
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Old 6th Dec 2019, 3:23 pm   #6
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Default Re: 'Cancelling' part of a meter's impedance?

To answer the original question, yes. You need to shunt the 11meg of the meter down to 10.1meg.This will need a parallel 123meg resistor.

The meter will then see 1/100 of the total voltage and will display it regardless.
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Old 6th Dec 2019, 4:32 pm   #7
Martin G7MRV
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Default Re: 'Cancelling' part of a meter's impedance?

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Originally Posted by Herald1360 View Post
To answer the original question, yes. You need to shunt the 11meg of the meter down to 10.1meg.This will need a parallel 123meg resistor.

The meter will then see 1/100 of the total voltage and will display it regardless.
Yes, that was my original thought. Glad I wasnt totally out with the maths. When the 10M resistors ive got on order arrive, i'll be able to build the necessary resistance up.

So it seems either parallel or series will do the job. My worry was that parallel would have odd effects on the DMM itself

Cheers
Martin
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