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Electrical dc shunt resistace question 
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#1
Mar1412, 12:51 PM

P: 144

1. The problem statement, all variables and given/known data
A meter is shunted by a parallel resistance. a) Determine the required value of the shunt resistance if the maximum value of the current I, is 200A. The meter can read a maximum of 1 mA and has a resistance of 0.1 Ω. b) If the shunt is made of copper and has a crosscectional area of 25cm^{2} calculate its required length. (for copper take ρ as 1.7x10^{8}Ωm 2. Relevant equations ohms law (V=I.R) R = ρ.(l/A) 3. The attempt at a solution a) full scale voltage = 0.001 * 0.1 = 0.0001 v shunt resistance R_{shunt} = 0.0001/200 = 0.5x10^{6}Ω R_{shunt} = 0.5 mΩ b) 0.5x10^{6} = (1.7x10^{8}) * ( l / 0.25^{2}) (0.5x10^{6} / 1.7x10^{8}) * 0.25^{2} = l l = 1.83824 cm does any of this look right? Thanks. 


#2
Mar1412, 01:52 PM

Mentor
P: 11,675

The cross sectional area of the shunt is already given in cm^{2}. You don't want to square it.
Everything else looks okay for the required accuracy. 


#3
Mar1412, 02:50 PM

P: 144

ok great so is this correct?
0.5x10^{6} = (1.7x10^{8}) * ( l / 0.25) (0.5x10^{6} / 1.7x10^{8}) * 0.25 = l l = 7.353 cm 


#4
Mar1412, 03:01 PM

Mentor
P: 11,675

Electrical dc shunt resistace question



#5
Mar1512, 01:56 AM

P: 1,506

I think you should double check your area units. An area of 25cm^2 is 0.0025m^2.
the resistivity is given in ohm.m Are you certain you have been given the cross sectional area as 25cm^2 ? This is a very large area, even for a shunt resistor Also, you have written R = 0.5m.ohms it should be micro.ohms......does not affect your calculation though. 


#6
Mar1612, 09:01 AM

P: 144

Does this look better? 0.5x10^{6} = (1.7x10^{8}) * ( l / 0.0025) (0.5x10^{6} / 1.7x10^{8}) * 0.0025 = l l = 0.7353 mm 


#7
Mar1612, 01:53 PM

P: 1,506

I would say this question does not make sense and is completely unrealistic.
A shunt resistor 0.7mm LONG!!!!!!! I would check the source of the question and double double check the numerical information. There is nothing wrong with your method ! 


#8
Mar1612, 02:15 PM

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P: 11,675




#9
Mar1612, 03:16 PM

P: 1,506

I still don't believe it!
A 0.7mm thick sheet fitted in clamps connected somehow to the terminals of a 1mA movement. Won't the clamps be part of the shunt? Has anyone ever met such a thing? I think there is something wrong with the wording of the original question 


#10
Mar1612, 04:11 PM

Mentor
P: 11,675




#11
Mar1712, 03:26 AM

P: 1,506

The calculation should be double checked.... The answer is not 0.7mm
There is nothing wrong with your method....check the calculation 


#12
Mar1912, 03:15 PM

P: 3

Is the answer 7.353 cm or 7.353 m?



#13
Mar1912, 03:34 PM

P: 1,506

I got 7.3cm
The answers previously given were a mixture of wrong units 


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