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

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 5th Jul 2021, 3:47 pm   #1
Bazz4CQJ
Dekatron
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,923
Default Simple Low Resistance Measurement

I'd like to be able measure from around 0.1R to let's say 0.5R. My various DMM's are 'marginal' down there.
Searching Google yields the very simple idea shown in the attached, which I found here http://diyaudioprojects.blogspot.com...asurement.html.

Wonder if anyone has any comments, good or bad, or other simple ideas to do this? I could pull out my Wayne Kerr bridge and set it up in Kelvin mode, but this project needs something a bit more "field" orientated.

Thanks
B
Attached Files
File Type: pdf Document11.pdf (31.7 KB, 238 views)
__________________
Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.
Bazz4CQJ is offline  
Old 5th Jul 2021, 4:40 pm   #2
GMB
Dekatron
 
GMB's Avatar
 
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
Default Re: Simple Low Resistance Measurement

Yes, that's the idea. But I think the LM317 needs some decoupling to avoid oscillation.

I have a professional meter that works like this, which I bought very cheaply.
The nice feature of a proper one if you can find it is that they have special dual connection croc clips. So each half of the "mouth" are separate. One does the current feed and the other is the voltage measuring contact - a very neat arrangement.

The 4-wire aspect is key to getting the right answer!
GMB is offline  
Old 5th Jul 2021, 4:50 pm   #3
Bazz4CQJ
Dekatron
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,923
Default Re: Simple Low Resistance Measurement

Thanks for that info. I think I'll start by trying to get a good measurement of the resistance of the two leads, see how the corrected results come out, and think about 4 leads if that seems to be necessary.

B
__________________
Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.
Bazz4CQJ is offline  
Old 5th Jul 2021, 4:58 pm   #4
Goldie99
Heptode
 
Join Date: Jul 2014
Location: Boston, Lincolnshire, UK.
Posts: 992
Default Re: Simple Low Resistance Measurement

Here's one I knocked up in 'a local field' a few weeks ago - I was going to tidy it up.... but you know how these things go. It looks pretty 'rough', but it's 5 - 10 minutes work, and it did the job perfectly - checking some low value emitter resistors.

Click image for larger version

Name:	field.jpg
Views:	257
Size:	63.8 KB
ID:	237131
Goldie99 is online now  
Old 5th Jul 2021, 5:17 pm   #5
GMB
Dekatron
 
GMB's Avatar
 
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
Default Re: Simple Low Resistance Measurement

You really do need to 4 wire system if you want the reading to be meaningful.

The trouble is that contact resistance will be of the same order as the resistor unless you are very careful.
GMB is offline  
Old 5th Jul 2021, 6:04 pm   #6
Goldie99
Heptode
 
Join Date: Jul 2014
Location: Boston, Lincolnshire, UK.
Posts: 992
Default Re: Simple Low Resistance Measurement

Quote:
Originally Posted by GMB View Post
You really do need to 4 wire system if you want the reading to be meaningful.

The trouble is that contact resistance will be of the same order as the resistor unless you are very careful.
I agree - I just double checked the 'field' board - running it to a low value resistor with a dmm in circuit (for mA), and then using a second dmm for mV drop across the resistor, I'm getting a steady 19.35mV drop at 88.0mA current = 0.21988 Ohms. The resistor is a brand new 0.22R, 1% metal film from Mouser. That's close enough for me.
Goldie99 is online now  
Old 5th Jul 2021, 8:27 pm   #7
Radio1950
Hexode
 
Join Date: Apr 2017
Location: Buderim, Queensland, Australia.
Posts: 428
Default Re: Simple Low Resistance Measurement

Best to avoid chrome plated crocodile clips as the plating can sometimes interfere with low resistance measurement.
Tinned clips are better. Screwed connection to brass connectors better still.
Radio1950 is offline  
Old 6th Jul 2021, 1:02 am   #8
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Simple Low Resistance Measurement

Quote:
Originally Posted by Goldie99 View Post
Quote:
Originally Posted by GMB View Post
You really do need to 4 wire system if you want the reading to be meaningful.

The trouble is that contact resistance will be of the same order as the resistor unless you are very careful.
I agree - I just double checked the 'field' board - running it to a low value resistor with a dmm in circuit (for mA), and then using a second dmm for mV drop across the resistor, I'm getting a steady 19.35mV drop at 88.0mA current = 0.21988 Ohms. The resistor is a brand new 0.22R, 1% metal film from Mouser. That's close enough for me.
It's worth appreciating how much test current can be safely handled by the "DUT". Not a problem for measuring the resistance of a bolted contact, but could be a concern for an smt resistor. I have a specialist milliohmeter bridge (Pontavi Th2) that applies 1.3A to the DUT, which is too much for some transformer windings and small parts.

Another comment is that 0.21988 is not the way to present a measured value when the value is a multiplication of two 3-4 digit values that are likely measuring down in the weeds already. Perhaps 0.22 absolute is a likely value, although doing comparative measurements can allow a better discrimination of the difference in resistance of samples.
trobbins is online now  
Old 6th Jul 2021, 8:46 am   #9
Goldie99
Heptode
 
Join Date: Jul 2014
Location: Boston, Lincolnshire, UK.
Posts: 992
Default Re: Simple Low Resistance Measurement

I stand corrected on the R value to 5 decimal places, that was sloppy, and I didn't intend to imply that level of precision. The DMM's I used specify resolutions of +/-0.1mA and +/-0.01mV at the values I cited, so technically I should have rounded the result to 0.22R. My point was more to illustrate that using the indirect method the OP had asked about, it is possible to get a much more meaningful measurement of a low resistance value than it is by using a direct resistance measurement. The same DMMs I used would have had a resolution of only +/- 0.1R at low resistance, so not too useful if you're trying to measure at the same level (and that's not even considering meter lead resistance).

We could also go into the 'accuracy' of each measurement - it's clearly a lot better using this type of indirect method, but frankly, if particularly high accuracy is needed, then we're back to using the right equipment etc., that the OP was looking to avoid in the first place.
Goldie99 is online now  
Old 6th Jul 2021, 10:51 am   #10
Julesomega
Nonode
 
Julesomega's Avatar
 
Join Date: Dec 2012
Location: Stockport, Greater Manchester, UK.
Posts: 2,085
Default Re: Simple Low Resistance Measurement

I rely on the calibration of a PSU to set a defined current through a suitable resistor, which is applied to the DUT, and measure the resultant mV with a DMM (4-wire method)
__________________
- Julian

It's good here
Julesomega is offline  
Old 6th Jul 2021, 12:23 pm   #11
Guest
Guest
 
Posts: n/a
Default Re: Simple Low Resistance Measurement

Quote:
Best to avoid chrome plated crocodile clips
I thought that the idea of four wire measurement was to remove the problem of contact resistance.
 
Old 6th Jul 2021, 2:33 pm   #12
GrimJosef
Dekatron
 
Join Date: Sep 2007
Location: Oxfordshire, UK.
Posts: 4,310
Default Re: Simple Low Resistance Measurement

It is, but as well as having four separate wires you also need four separate contact clips to the DUT. In that case it shouldn't matter if the clip contacts to the metering wires have a bit of resistance. But you may have to worry about thermocouple voltages at the contact points if the metals are different and the local environment is hot.

Cheers,

GJ
__________________
http://www.ampregen.com
GrimJosef is offline  
Old 6th Jul 2021, 8:35 pm   #13
trsomian
Hexode
 
Join Date: Oct 2010
Location: Box End, Beds. UK.
Posts: 271
Default Re: Simple Low Resistance Measurement

Since thermocouples fundamentally measure temperature difference, providing everything is at the same temperature I don't think that the thermocouple effect matters - the law of intermediate junctions will be the saviour. The measurement may need to be done fairly quickly if somat is likely to get warm during the testing though
trsomian is offline  
Old 7th Jul 2021, 1:23 pm   #14
Station X
Moderator
 
Station X's Avatar
 
Join Date: Jan 2003
Location: Ipswich, Suffolk, IP4, UK.
Posts: 21,192
Default Re: Simple Low Resistance Measurement

Has anyone tried using an ESR Meter to measure low resistance? I don't see why it shouldn't work.
__________________
Graham. Forum Moderator

Reach for your meter before you reach for your soldering iron.
Station X is online now  
Old 7th Jul 2021, 3:37 pm   #15
GMB
Dekatron
 
GMB's Avatar
 
Join Date: Aug 2003
Location: near Reading (and sometimes Torquay)
Posts: 3,086
Default Re: Simple Low Resistance Measurement

ESR meters are usually 2 terminal and do not go down all that low.
GMB is offline  
Old 7th Jul 2021, 4:36 pm   #16
Goldie99
Heptode
 
Join Date: Jul 2014
Location: Boston, Lincolnshire, UK.
Posts: 992
Default Re: Simple Low Resistance Measurement

Quote:
Originally Posted by Station X View Post
Has anyone tried using an ESR Meter to measure low resistance? I don't see why it shouldn't work.
For the sake of comparison, I just measured the exact same 0.22R resistor on an ESR meter set up for low resistance measurements as well (a Russian 'ESR-Micro 4.0s'). With the fixed probes freshly zeroed, the measured resistance was exactly 0.22 Ohms with a cited tolerance of +/- 0.02 Ohms in the 0 to 1 Ohm range.
Goldie99 is online now  
Old 7th Jul 2021, 5:54 pm   #17
Bazz4CQJ
Dekatron
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,923
Default Re: Simple Low Resistance Measurement

Thanks for your various inputs. I don't have an ESR unit.
I think I need to measure somewhere in the range 0.08 to 0.12 ohm (expected) , ideally say within +-10% (or 20%) but I may find something nasty like, 0.2 ohm!

Waiting for the 317's to arrive.

B
__________________
Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.
Bazz4CQJ is offline  
Old 8th Jul 2021, 2:01 am   #18
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Simple Low Resistance Measurement

I found that an ESR based measurement technique is just as accurate and sensitive as a vintage Kelvin double-bridge that I have. I made up a diy ESR meter circuit from eevblog forum (below link) that gave me 0.1mohm sensitivity and 1 ohm full scale, and confirmed accurate at 47mohm - but yes that ESR meter needs 4-terminal connections as well. One main advantage of a modern ESR meter is the relatively low signal level applied to the DUT.

https://www.eevblog.com/forum/projec...-construction/

I have two ANENG 8009 cheap 4-digit meters for quick bench use. As an example, they provide a 1A FS with 0.1mA resolution, and accuracy hearsay is better than 0.5% at 1/3 FS from those with quality comparison meters. So with 100mA test current the accuracy is likely about 0.5%. A 200mohm resistor would read 20mV across it. The voltage accuracy on that 8009 meter is much better than for current by hearsay, but even at 0.5% accuracy the power measurement is about 1% accurate, so well within your aim even with cheap (but accurate) meters when using a 4-terminal setup with a steady dc test current.
trobbins is online now  
Old 10th Jul 2021, 6:43 am   #19
Bazz4CQJ
Dekatron
 
Bazz4CQJ's Avatar
 
Join Date: Apr 2009
Location: Oxfordshire, UK.
Posts: 4,923
Default Re: Simple Low Resistance Measurement

I stumbled upon this paper on the web by complete accident. https://www.homemade-circuits.com/si...ester-circuit/. I think it contains some interesting ideas which could be "bolted on" to a 317-based tester.

B
__________________
Saturn V had 6 million pounds of fuel. It would take thirty thousand strong men to lift it an inch.
Bazz4CQJ is offline  
Old 11th Jul 2021, 1:09 am   #20
trobbins
Heptode
 
trobbins's Avatar
 
Join Date: Oct 2009
Location: Melbourne Australia
Posts: 898
Default Re: Simple Low Resistance Measurement

The 317 method identified in the first post is so simple I'd suggest keeping a focus on it first, and letting any DMM do all the range switching. The main issue to confirm is that accuracy is acceptable and verifiable - otherwise you may mislead yourself right from the start.

Initially that means appreciating what meters you have, and what resolution they have when measuring low (MV) DC voltages, and whether any can measure low mA DC currents. Then checking their stated accuracy down at those low levels, and then cross comparing their measurements to see if they are all the same (given that none are new or in recent calibration).

If you have a sensitive and reasonably accurate mVdc meter, then setting the 317 for 10mA rather than 100mA may provide a better outcome for a few reasons, including the type of battery you need to use and any need for heatsinking the 317 (eg. if you just use a 12V batt or wall-wart or a 317L and say a 5V supply such as a USB supply wall-wart).
trobbins is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:35 am.


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 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.