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How does digital multimeter work? 
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#1
Jul611, 02:39 PM

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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
Jul611, 06:17 PM

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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... ) 


#3
Jul611, 06:47 PM

P: 22

Or put it in parallel ( in voltmeter mode) with the ext resistor and divide the measured voltage by resistance. 


#4
Jul611, 06:51 PM

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How does digital multimeter work?
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?)... 


#5
Jul611, 07:01 PM

P: 22

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 ? 


#6
Jul611, 07:17 PM

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#7
Jul611, 07:37 PM

P: 22

I'm not quite sure... 


#8
Jul611, 11:24 PM

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#9
Jul711, 12:06 AM

P: 22

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? 


#10
Jul711, 01:19 PM

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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 nontrivial for some current measurement situations... 


#11
Jul711, 02:24 PM

P: 22

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 :( 


#12
Jul711, 02:36 PM

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#13
Jul711, 03:06 PM

P: 22

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?



#14
Jul711, 03:11 PM

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#15
Jul711, 03:15 PM

P: 22

It's ok. I guess we were over thinking it a little... But thank you for your help. It is much appreciated.



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