How does digital multimeter work?

  1. 1. The problem statement, all variables and given/known data
    If I had two DMM's, how would I use one of them to measure the current that another DMM uses to measure the resistance of a circuit element.

    How would I use a DMM to measure the resistance of a shunt resistor that another DMM uses to measure current in a circuit?


    2. Relevant equations

    V=IR


    3. The attempt at a solution
    Well for the first part, I know that when a DMM measures resistance, it sends a constant current through resistor and the voltmeter part of the DMM measures the voltages across that element which is caused by the current, and then simply divides that voltages by current to get resistance. So in order to find the constant current that the DMM outputs, would I just put another DMM in series with the first DMM and resistor ?

    For the second part, I'm still trying to figure out how a DMM measures current. All I know is that it is a voltmeter attached to a shunt resistor. Once I can figure this out, I can figure out how to measure the resistance of the shunt resistor.

    Any help is appreciated and I will use the rep system to those who help.

    Thank you.
     
  2. jcsd
  3. berkeman

    Staff: Mentor

    For the 2nd question, model the current measurement input circuit as the shunt resistor in parallel with a very high input impedance voltmeter circuit. So if you want to measure the shunt resistance, how would you use your 2nd DVM? BTW, the shunt resistances used on the low-current scales are moderate resistances. You should measure them sometime -- you may be surprised how large they are, and how they can affect the measurement in some circuits.

    For the 1st question, you can use your 2nd DVM still in voltage mode instead of current mode. How would you use it to measure the current flowing through the external resistor...?

    (and I don't think the reputation feature is turned on at the PF... :smile: )
     
  4. By putting it in series ( in current measuring mode) with the first DMM and the external resistor...?

    Or put it in parallel ( in voltmeter mode) with the ext resistor and divide the measured voltage by resistance.
     
    Last edited: Jul 6, 2011
  5. berkeman

    Staff: Mentor

    Not if it is in Voltage mode... :smile:

    You can certainly put the DVM in current mode and put it in series with the resistor to measure the test current being output by the first DVM. But as I alluded to when I cautioned about the moderate size of the test shunt resistances in current mode, you may get a more accurate current reading by putting the 2nd DVM in voltage mode, and (doing what?)...
     
  6. Or put it in parallel ( in voltmeter mode) with the ext resistor and divide the measured voltage by resistance.

    I'm still having trouble with the second question....use voltage division ?
     
  7. berkeman

    Staff: Mentor

    Good. You can also separately measure the resistance of the external resistor all by itself, to improve the accuracy of your numbers (instead of relying on the 1% value printed on the resistor).

    How do you measure the resistance of a resistor that is in parallel with another very high resistance (assume infinite)? The answer is simpler than you may be thinking....
     
  8. You would have to either know the current going through that resistor or voltage across the two resistors in parallel ( assuming that you know the value of the higher resistor) and use voltage division.

    I'm not quite sure...
     
  9. berkeman

    Staff: Mentor

    You are over-thinking it. How do you measure a resistor with a DVM?
     
  10. By just connecting the DVM in parallel w/ resistance measuring mode.

    So I should just measure the shunt resistor by connecting the two prongs of both DVMs together (one of them turned off and the other to resistance measuring mode)? Isn't there any other conducting material in the DVM that will affect the reading other than the shunt resistor?
     
  11. berkeman

    Staff: Mentor

    Keep the 2nd DVM turned on, so that its voltage measuring circuit in parallel with its shunt resistor is high impedane. If it is turned off, there will be some diodes across the shunt resistor that may affect your resistance measurement, depending on how high the measuring voltage is.

    EDIT -- BTW, try measuring the shunt resistor(s) for several different current scales, if you can. You'll start to see why I said that the shunt resistance is non-trivial for some current measurement situations...
     
  12. So I asked the instructor if the two circuits were correct and he said that for the first one, we don't need to hook any DMM to a resistor. So I guess just hook both the DMMs together with one measuring resistance and the other measuring current?

    He said the second part wasn't quite right either but I was close. He didn't say anything else except said that both circuits are the same for each case....

    I would try to just experiment with two DMMs but I only have one :(
     
  13. berkeman

    Staff: Mentor

    Okay, that works too.
    Ah, I see where he is going with this. He wants you to see that the same hookup can be used to get both the value of the test current of one DMM and the shunt resistance value of the other. Clever. Think through the 2nd case, and I think you'll see what to do to get the shunt resistance value....
     
  14. I still don't understand why it wouldn't work to just use the same circuit as before with the same settings (two DMMs connected, one on measuring current, the other measuring resistance). The reading on the resistance measuring DMM wouldn't be the value of the shunt resistor of the other?
     
  15. berkeman

    Staff: Mentor

    Yes, I think you are right. Sorry if I got confused by the permutations we talked about.
     
  16. It's ok. I guess we were over thinking it a little... But thank you for your help. It is much appreciated.
     
Know someone interested in this topic? Share this thead via email, Google+, Twitter, or Facebook

Have something to add?