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Sub ohm resistance measurement.

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Buk

Active Member
Is there any simple way to measure resistances less than 1Ω to 2 or 3 digits of accuracy using a cheap multimeter?
 

ronsimpson

Well-Known Member
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No, ..... maybe.
Do you have a way to measure (and make) current? Put current through the resistor and measure the voltage drop.
You will need to measure current (1A) and voltage (1V) at the same time.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, put a constant current through it, and measure the voltage drop across the resistor - usually done with a four wire probe, but easy to do with separate wires. Ensure the meter connections are actually on the resistor, in order to eliminate the resistance of the wires carrying the constant current.

You simply choose the current (and can switch it for different ranges) based on the required resistance range, and the FSD of your meter. You can also use an opamp to increase the sensitivity of your meter.
 

Musicmanager

Well-Known Member
Some members will remember Chemelec - Gary, who passed away, seems like a few months ago, but I bet it's a couple of years .. .. he was a great bloke !

He showed me this and I had intended to build it but not yet got around to it; Knowing Gary's designs I'm sure it will work well .. .. .

Low Ohm Meter Schematic.png


MM
 

audioguru

Well-Known Member
Most Helpful Member
I agree that Gary was a nice guy and provided us with many of his excellent circuits.
 

danadak

Well-Known Member
Most Helpful Member



Possibilities.


Regards, Dana.
 

Pommie

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Some members will remember Chemelec - Gary, who passed away, seems like a few months ago, but I bet it's a couple of years .. .. he was a great bloke !
Agree completely and was surprised when I discovered that his site is still available. http://chemelec.com/

Someone should make a copy of this before it disappears.

Mike.
Edit, seems it was Nov 7th 2019. Over two years ago.
 
Last edited:

Nigel Goodwin

Super Moderator
Most Helpful Member
Some members will remember Chemelec - Gary, who passed away, seems like a few months ago, but I bet it's a couple of years .. .. he was a great bloke !

He showed me this and I had intended to build it but not yet got around to it; Knowing Gary's designs I'm sure it will work well .. .. .

View attachment 135202

MM

A friend of mine designed and built one the other year, very similar to that - he'd been wanting to built one for many years, but couldn't find a reasonably priced four point probe, then he discovered cheap parts from China :D

I was going to built one for work, but then I just ordered a complete meter from China.
 

Buk

Active Member



Possibilities.


Regards, Dana.
Thanks.

Any thoughts on the comparative efficacy of this one?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
For info and depending on the accuracy you need, the Peak ESR70 meter reads low value resistance to two decimal places, eg. 10 mOhm increments - and it's a very useful device for checking capacitors, the main reason I got one.


And are made not far from me, in fact I visit a friend on the same Industrial Estate as Peak are - we sub-contract work to my friend.

The Peak instruments are also PIC based, but pretty expensive, I have two of them - where my friend has the full range, given to him for evaluation purposes as he's very near to them.

I actually have an ESR70 (old version) but there's no mention of using them as a low resistance meter, only for ESR - as it uses 100KHz for it's readings it's accuracy as a resistance meter would be suspect (such as trying to read wirewound resistors). Great little instruments though.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Looks like a great device; just more pennies than I can justify for something that might get used twice a year.

If you're repairing switch-mode PSU's (in TV's, set-top boxes etc.) then you use them multiple times per day for checking ESR of the electrolytics - otherwise you're unlikely to use it as much as twice a year, I've not used mine once in the three years or so since I stopped TV etc. repairs.

As I mentioned above, it's not designed for resistance measurements.
 

rjenkinsgb

Well-Known Member
Most Helpful Member
does it work OK then on wirewound resistors?
Interesting point!

I don't have a large selection of low value wirewounds, but I've found three:
0.22 10% = 0.28
1R0, no tolerance = 1.34
1R5 10% = 1.82

As a comparison, a one ohm metal film 1% reads 1.00

It does appear to read somewhat high on wirerwounds.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Interesting point!

I don't have a large selection of low value wirewounds, but I've found three:
0.22 10% = 0.28
1R0, no tolerance = 1.34
1R5 10% = 1.82

As a comparison, a one ohm metal film 1% reads 1.00

It does appear to read somewhat high on wirerwounds.

Surprising on the 0.22 ohm, as when you look at them there's VERY few windings, so presumably not much inductance?.

However, it's always been suggested that you don't use wirewounds for emitter resistors or zobel networks in high quality audio amplifiers, because of their inductive nature.
 
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