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

    Distinguished Member

    Jun 22, 2012
    4,974
    744
    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
    12,101
    3,033
    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
    4,853
    767
    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:
    a) adjustable(variable) Winding resistance.
    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?.

    Here are some of the panel meter specs to help you:
    Accuracy = +-(0.1%rdg.+2 dgt).
    Com mode voltage = +- 1V max
    Full scale range = 200mV
    Conversion rate = 2.5 per second
     
  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
    4,853
    767
    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
    20,765
    2,536
    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
    12,101
    3,033
    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.

    [​IMG]
     
  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
    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.
     
  12. ScottWang

    Moderator

    Aug 23, 2012
    4,853
    767
    Q:
    When -
    Ra = 120Ohm
    Rload = 2.2Ohm
    Vload = 0.024
    Panel meter reads 1.87

    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
    Rload = 4.4Ohm
    Vload = 0.047
    Panel meter reads 4.10

    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
    Rload = 2.2Ohm
    Vload = 0.028V
    Panel meter reads 2.20

    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
    Rload = 4.4Ohm
    Vload = 0.054V
    Panel meter reads 4.80

    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
    Rload = 6.8Ohm
    Vload = 0.082V
    Panel meter reads 7.53

    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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  13. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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  14. Ron H

    AAC Fanatic!

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

    What does Walt stand for?
     
  15. Ron H

    AAC Fanatic!

    Apr 14, 2005
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    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.
    2. Apply Isrc to Rload.
    3. Measure Vload with the Fluke.
    4. Calculate Rload=Vload/Iload.
    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
    4,853
    767
    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.
     
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