UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Reply
 
Thread Tools
Old 5th Dec 2019, 5:12 pm   #1
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 797
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!
__________________
I got food in ma belly and a license for ma telly

My Blog - http://g7mrv.blogspot.com
Martin G7MRV is offline   Reply With Quote
Old 5th Dec 2019, 7:17 pm   #2
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 797
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.
__________________
I got food in ma belly and a license for ma telly

My Blog - http://g7mrv.blogspot.com
Martin G7MRV is offline   Reply With Quote
Old 5th Dec 2019, 10:29 pm   #3
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,680
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.
Argus25 is offline   Reply With Quote
Old 6th Dec 2019, 11:31 am   #4
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 797
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.
__________________
I got food in ma belly and a license for ma telly

My Blog - http://g7mrv.blogspot.com
Martin G7MRV is offline   Reply With Quote
Old 6th Dec 2019, 12:02 pm   #5
Argus25
No Longer a Member
 
Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,680
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.
Argus25 is offline   Reply With Quote
Old 6th Dec 2019, 3:23 pm   #6
Herald1360
Dekatron
 
Herald1360's Avatar
 
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 13,769
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.
__________________
....__________
....|____||__|__\_____
.=.| _---\__|__|_---_|.
.........O..Chris....O
Herald1360 is offline   Reply With Quote
Old 6th Dec 2019, 4:32 pm   #7
Martin G7MRV
Heptode
 
Join Date: May 2008
Location: Selby, North Yorkshire, UK.
Posts: 797
Default Re: 'Cancelling' part of a meter's impedance?

Quote:
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
__________________
I got food in ma belly and a license for ma telly

My Blog - http://g7mrv.blogspot.com
Martin G7MRV is offline   Reply With Quote
Reply

Thread Tools



All times are GMT +1. The time now is 4:46 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Copyright ©2002 - 2020, Paul Stenning.