# Making your own ohm meter  help needed!

Discussion in 'The Projects Forum' started by ja7me, Sep 17, 2012.

1. ### ja7me Thread Starter New Member

May 24, 2012
18
1
Hello everyone,

I am attempting to make my own low ohm meter in the range of 0  100 Ohms.
I have based my circuit on the design shown in the link below:
http://www.electronics-diy.com/electronic_schematic.php?id=951
The circuit kind of works ok, however I would like more stability in the readings I get. I measured across a 2.2ohm resistor and was getting fluctuating readings between 1.8ohm and 3ohms on my panel readout display.
When I use my digital multi-meter in replace of the panel meter I get a stable reading close to 2.2ohms.

Is there anything I can do to make this circuit more stable?
Do I need a different panel meter voltage readout display?
I would be very grateful if anyone can help me with this project.

Regards,

Jamie.

2. ### Dodgydave AAC Fanatic!

Jun 22, 2012
5,131
767
can you post some pictures of your circuits?

i would use a constant current source to put through the "unknown" resistor and use your meter to measure the voltage across it.

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3. ### wayneh Expert

Sep 9, 2010
12,368
3,223
100mA is enough current to change the temperature and thus resistance of the resistor under test as well as the ones in your circuit. I wouldn't expect the big change you reported, though.

Is your panel meter stable on its own?

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4. ### ScottWang Moderator

Aug 23, 2012
4,924
776
1. The first, you have to make sure that your lowest Vout of LM317 is 1.25V, and very precisely.
2. LM317 used a heat sink, the size close or over 5x5 com, over is better.
3. calculating Limited resistor : (for Cement Resistor)
Limited_R = Vref/Iout
Limited_R = 1.25V / 0.1A
Limited_R = 12.5 Ω
How to get a 12.5 Ω, you can use 10Ω+2Ω+0.5Ω

4. calculating Walt of Limited resistor :
Walt = I x R
Walt = 0.1A x 12.5 Ω
Walt = 1.25W
You better choose a resistor for over 20 times, exmp : 1.25W x 20 = 25W

You can use (10Ω+2Ω+0.5Ω)/30W for Cement Resistor, or use adjustable(variable) Winding resistance, 12.5 Ω/30W or more, if you can add a small fan to blow the resistor is better.

5. You can use two kinds of Limited resistor:
b) Cement Resistor.
Of course a) is better.

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5. ### ja7me Thread Starter New Member

May 24, 2012
18
1
Hi wayneh and dodgydave, thanks for your rapid response. I have attached a schematic of the circuit.

When I apply zero power to the resistor I am measuring, the panel digital readout does infact fluctuate, therefore I dont think it is stable on its own, is this a problem?.

Accuracy = +-(0.1%rdg.+2 dgt).
Com mode voltage = +- 1V max
Full scale range = 200mV
Conversion rate = 2.5 per second

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6. ### ja7me Thread Starter New Member

May 24, 2012
18
1
Hi Scottwang, thanks for your input. So do you think the temperature is the main contributor to the fluctuations I am getting?. What I dont understand is why i recieve a stable reading when i measure with my Fluke multimeter. It leads me to beleive my panel meter is the main issue.

7. ### ScottWang Moderator

Aug 23, 2012
4,924
776
Hi Jamie:
OK, now we have two places probably have some problem, one is the limited resistor, the second is the meter.
1. Check the panel meter, what's main IC inside of the meter, is ICL7107?
2. Some multimeter(your fluke) have the key called hold, if you have that key, when measure the testing resistor, press and toggle the key, and see what happen? (check scan and catch time)
3. If you have another multimeter, then you can do the test as 2.
4. Use any fan to blow the limited resistor, and measure the testing resister, and see how the value of panel meter.

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8. ### ja7me Thread Starter New Member

May 24, 2012
18
1
Ok, I have been playing around with my circuit more. I think I accidently wired something up wrong, my display seems to fluctuate like crazy now. I am going to try and order a different, better quality display to see if that makes a difference. To be honest it was the cheapest one i could find, could be why it was so unstable.

Oh and to answer your question scottwang, the only IC I could see is one labelled 'L74H0D417'

I tried toggled the sample and hold on the Fluke multimeter, still gives a stable reading. I have come to conclusion that the panel meter is rubbish for this type of application.

Thanks for everyones help, I will update you once i have recieved my new meter.

9. ### Wendy Moderator

Mar 24, 2008
20,772
2,540
Just to test the basic circuit you could use an off the shelf DVM in voltmeter mode. This will allow you to eliminate some variables.

10. ### wayneh Expert

Sep 9, 2010
12,368
3,223
I think this is the second time in a week we've seen this. You're not alone.

FWIW, as suggested, you can buy a meter for less than \$5 that is quite acceptable for most routine work, including measuring in the 200mV range.

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11. ### ja7me Thread Starter New Member

May 24, 2012
18
1
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
Rload = Resistor to be measured
Vs = 6V

When:

Ra = 120Ohm

When:

Ra = 120Ohm

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

When:

Ra = Variable resistor

When:

Ra = Variable resistor

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.

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12. ### ScottWang Moderator

Aug 23, 2012
4,924
776
Q:
When -
Ra = 120Ohm

A:
Iout=Vref/Ra = 1.25V/Ra
Iout = 1.25V/1.2K=1.042mA
Vload = 1.042mA x 2.2Ω = 2.29mV

So you have to measure, does Iout = 1.042mA ?

Q:
When -
Ra = 120Ohm

A:
Iout=Vref/Ra = 1.25V/Ra
Iout = 4.1mV / 4.4Ω = 0.93mA
Ra = 1.25V / Iout = 1.25V / 0.93mA = 1.344K
Vload = 0.93mA x 4.4Ω = 4.09mV

So you have to measure, does Iout = 0.93mA ?
Also you can measure, does VR = 1.344K?

Q:
When -
Ra = Variable resistor

A:
Iout=Vref/Ra = 1.25V/Ra
Iout = 2.2mV / 2.2Ω = 1mA
Ra = 1.25V / Iout = 1.25V / 1mA = 1.25K
Vload = 1mA x 2.2Ω = 2.2mV

So you have to measure, does Iout = 1mA ?
Also you can measure, does VR = 1.25K?

Q:
When -
Ra = Variable resistor

A:
Iout=Vref/Ra = 1.25V/Ra
Iout = 4.8mV / 4.4Ω = 1.091mA
R1 = 1.25V / Iout = 1.25V / 1.091mA = 1.146K
Vload = 1.091mA x 4.4Ω = 4.8mV

So you have to measure, does Iout = 1.091mA ?
Also you can measure, does Ra = 1.146K?

Q:
When -
Ra = Variable resistor

A:
Iout=Vref/Ra = 1.25V/Ra
Iout = 7.53mV / 6.8Ω = 1.107mA
R1 = 1.25V / Iout = 1.25V / 1.107mA = 1.13K
Vload = 1.107mA x 6.8Ω = 7.53mV

So you have to measure, does Iout = 1.107mA ?
Also you can measure, does Ra = 1.13K?

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Feb 5, 2010
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14. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
PowerI*R. Power=I*I*R. This makes Walt=125mW.

What does Walt stand for?

15. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Ja7me, how do you know the actual values of Rload?
What types of resistors are you using for Rload?

16. ### ja7me Thread Starter New Member

May 24, 2012
18
1
Hi ron, i measured them values using my fluke multimeter, 10watt resistors are used for rload

17. ### ja7me Thread Starter New Member

May 24, 2012
18
1
ScottWang

i could do the measurements you suggested but don't think i will gain much by doin it. What really confuses me is that when the panel meter display receives a voltage signal in the mv range it does not display the exact voltage it is receiving. Why is this do you think?.

18. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Be aware that your multimeter leads probably have several tenths of an ohm resistance. Does your Fluke have a Rel Δ mode? If so, are you using it to measure your resistors? If not, the probe lead resistance will be included in your measurements. Note also that the resistance of the connection of the alligator clip to the resistor lead can change as you wiggle it.

Also, have you tried this?
1. Measure your current source (Isrc) with the Fluke.
3. Measure Vload with the Fluke.
5. Compare this with the results of your panel meter.

19. ### ja7me Thread Starter New Member

May 24, 2012
18
1
I have already measured v load with my multi-meter. And i do usually get a reading i expect. The main issue is when v load is connected to my voltage panel display meter. For example when a 2.2 ohm resistor is rload, vload is 0.024 volts, When Ra is 120 ohm. 0.024 volts is the kind of voltage i expect, i just don't understand why the panel never display reads it 1.87. Also i would expect the meter to double this figure when rload is doubled to 4.4 ohm. However it does not, and reads 4.1 instead

20. ### ScottWang Moderator

Aug 23, 2012
4,924
776
Thanks for point out my error.
That's really terrible, I wrote P = I x R, but actually I want to write V = I x R, and then calculate P = V x I.