Author Topic: Current sense resistor and dmm reading  (Read 1810 times)

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Offline StuartambientTopic starter

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Current sense resistor and dmm reading
« on: August 31, 2016, 02:19:32 pm »
I've done a few tests with these resistors and they were all spot on, but the latest is driving me a bit battery. 

Here's my problem.  I have the voltage range on the dmm set to 200mV.  The reading I get off this simple circuit is .2 which I'm interpreting as 2 micro Volts. 

With the circuit I have 4 resistors (including the shunt) in serial with a battery that has 6.11V on load.
R1- 984 Ohm
R2- 2.160k
R3- 21,500k
CSR - 1.018 Ohm

Aside from the CSR, the voltage drops added up correctly and using resistance I calculated to see about 247mA in current.
I'm confused because if I use 2 micro Volts and divide by the resistance (1.018) I don't seem to get the right answer. I do understand that if I am in fact reading micro volts, accuracy beyond even one number is not going to happen on my DMM. 


 

Offline StuartambientTopic starter

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Re: Current sense resistor and dmm reading
« Reply #1 on: August 31, 2016, 02:31:41 pm »
Wait, I take it back.  Using the 197mA's i get with E=IR, 197ma*1.018 = .2
So it's correct outside of lack of precision, which I'm looking into.  Maybe not another DMM but something I can build.
« Last Edit: August 31, 2016, 02:33:49 pm by Stuartambient »
 

Offline Dave

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Re: Current sense resistor and dmm reading
« Reply #2 on: August 31, 2016, 02:59:34 pm »
You're getting 247uA, as in microamperes, not milliamperes. Multiply that by 1R and you get 247uV, which your meter will correctly read as 0.2mV.
<fellbuendel> it's arduino, you're not supposed to know anything about what you're doing
<fellbuendel> if you knew, you wouldn't be using it
 
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Offline StuartambientTopic starter

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Re: Current sense resistor and dmm reading
« Reply #3 on: August 31, 2016, 03:31:55 pm »
Ok, I guess my math is still a bit fuzzy.
So how do calculations get performed, meaning, the voltage drop on R1 (984 ohms) was 242mV.  I was dividing 242/984. Which is why I was thinking mA.   If I do a .242/984 then it looks like it is micros, except I need to be better at scientific notation.  Looks like this number would be the micros,  2.459349593495934959349593495935e-4
 

Offline Dave

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Re: Current sense resistor and dmm reading
« Reply #4 on: August 31, 2016, 03:37:18 pm »
2.459E-4 = 245.9E-6 = 245.9 micro

I'd suggest you practice until it becomes second nature. ;)
<fellbuendel> it's arduino, you're not supposed to know anything about what you're doing
<fellbuendel> if you knew, you wouldn't be using it
 

Offline StuartambientTopic starter

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Re: Current sense resistor and dmm reading
« Reply #5 on: August 31, 2016, 03:39:08 pm »
2.459E-4 = 245.9E-6 = 245.9 micro

I'd suggest you practice until it becomes second nature. ;)

Yep, that is what I was thinking.  I flew through that chapter a bit to fast.  :-//
 

Offline StuartambientTopic starter

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Re: Current sense resistor and dmm reading
« Reply #6 on: August 31, 2016, 09:17:22 pm »
Sorry, I had  confused exp button for exponent button.

« Last Edit: August 31, 2016, 09:40:08 pm by Stuartambient »
 


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