Low ohm meter problem!, please help

[Ja7me]

Hello everyone,

I have posted on here recently about making an accurate low ohm meter. I have still not achieved this and would appreciate some help.

Previously I came to the conclusion that the panel meter display itself was inaccurate. I have recently purchased a good quality one from Farnell (order number: 9932801). However I am still getting the same problems as before, the display does not show the readings I would expect.

Attached is a simple layout of the circuit, below are my experimental findings:


LOW OHM METER EXPERIMENT:

Parts:

Regulator = LM317
Ra = Adjustment resistor
Rload = Resistor to be measured
Vs = 6V

When:

Ra = 120Ohm
Rload = 2.2Ohm
Vload = 0.024
Panel meter reads 1.87

When:

Ra = 120Ohm
Rload = 4.4Ohm
Vload = 0.047
Panel meter reads 4.10

Also I tried a variable resistor for Ra, so that I could adjust the the panel meter display to read the correct resistance:

When:

Ra = Variable resistor
Rload = 2.2Ohm
Vload = 0.028V
Panel meter reads 2.20





When:

Ra = Variable resistor
Rload = 4.4Ohm
Vload = 0.054V
Panel meter reads 4.80

When:

Ra = Variable resistor
Rload = 6.8Ohm
Vload = 0.082V
Panel meter reads 7.53


As you can see from my findings, the panel meter display can be way out. I also experimented with a different type of regulator the LM234, I tried using both a 120ohm resistor and a 1KOhm resistor for Ra. I still got inaccurate results. I really would be thankful for some help on this one, I have no idea why these displays should not read an accurate reading.

 

[MikeMl]

Is the panel meter the kind that can share grounds between the voltage being measured, and the negative pin of its own power supply?

I went and looked at the data sheet. Do you have the inputs wired like this?

View attachment 67947

 

[JimB]

Mikes point is a good one.

Also be aware that the regulator may be oscillating, check using an oscilloscope if you have access to one.
A few decoupling capacitors close to the pins on the 317 are a good idea any way.

What about the supply?
Is it clean and free from noise and mains ripple?

JimB

 

[MikeMl]

Here is an LTSpice sim of what you are trying to build: I used an LTSpice trick of making a resistor R2 a function of voltage, and am plotting the voltage across R2 (the resistor to be measured) and the current through it as a function of V2, which is swept logarithmically from 1mV to 1000V. When you look at the plot, think of the X axis as being the resistance of R2 in Ohms (not Volts), varying logarithmically from 1mΩ to 1kΩ.

Note that with a 6V supply, the output voltage (which is what you are trying to read with your DMM) would go from 10uV to about 3V, where the current source no longer has enough compliance to work right. This corresponds to a load resistance of about 300Ω. By raising the input voltage, you could make the circuit work for higher values of load resistance...

Also note that you need to adjust R1 to match the ACTUAL reference voltage of the LM317 that you have in the circuit. The specified range of reference voltages is 1.200 to 1.300V, so the resistor should be somewhere between 120Ω and 130Ω. That is, if you want to get ~1% accuracy.


View attachment 67949

 

[cachehiker]

Shouldn't you be making a 4 wire measurement with separate force and sense lines?

I've found it necessary for 1% accuracy below about 30Ω and mandatory for any sort of accuracy below 3Ω.

 

[MikeMl]

Shouldn't you be making a 4 wire measurement with separate force and sense lines?

I've found it necessary for 1% accuracy below about 30Ω and mandatory for any sort of accuracy below 3Ω.

First things first. The OP has a first order problem with how to connect the current source, or how to connect the meter to the device under test, or a common grounding problem between the current source and meter?

 

[cachehiker]

Grateful somebody could point out something is likely amiss with the way his components are interconnected.



The picture shows a 4 wire connection. The schematic makes it clear as well. The OP's does not.

 

[audioguru]

The original project uses an LM317L and the resistor is about 12.5 ohms. But the OP is using an ordinary LM317 and a resistor is 120 ohms. A completely different circuit.

 

[MikeMl]

Same principle, but forcing 10mA instead of 100mA.

 

[chemelec]

WHY do you have TWO SEPERATE Postings for this?

 

[Ja7me]

Thankyou all for your comments, they have been much help. My main issue seemed to be my breadboard connections, once a made fever and more secure connection it seemed to solve the problem..