Shunt Resistance Calculation for a DC Circuit with 2 Parallel Resistors

  • Thread starter Thread starter damon669
  • Start date Start date
  • Tags Tags
    Resistance Shunt
AI Thread Summary
The discussion focuses on calculating the required shunt resistance for a DC circuit with two parallel resistors, where one is a meter with a resistance of 0.1 ohms and a maximum current of 1 mA. The total current in the circuit is 200 A, leading to a shunt current calculation of 199.9 A. Participants emphasize the importance of maintaining precision in calculations, particularly with decimal places and units, to avoid significant errors. The final calculation yields a shunt resistance of approximately 0.5 microohms, highlighting the need for scientific notation and appropriate significant figures. Accurate voltage and current values are crucial for determining the correct shunt resistance in this scenario.
damon669
Messages
33
Reaction score
0

Homework Statement


DC circuit
2 resistors in parallel Rm and Rs
one resitance will be a meter (Rm =0.1 ohm), the other will be the shunt
Max current is 200A
Im (meter current is 1mA)



Homework Equations


I= Is + Im
V=IRt



The Attempt at a Solution


I am thinking first to find shunt current which Is
Is = I - Im
=199.9A ?
 
Physics news on Phys.org
damon669 said:

Homework Statement


DC circuit
2 resistors in parallel Rm and Rs
one resitance will be a meter (Rm =0.1 ohm), the other will be the shunt
Max current is 200A
Im (meter current is 1mA)



Homework Equations


I= Is + Im
V=IRt



The Attempt at a Solution


I am thinking first to find shunt current which Is
Is = I - Im
=199.9A ?

Hi damon669, Welcome to Physics Forums.

Yes, the shunt current will likely come in handy. What is it you're trying to find? Your problem description doesn't explicitly pose a question...
 
The question is
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 0.1Ω
 
damon669 said:
The question is
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 0.1Ω

Okay. So you know the currents that are flowing through each of the components (you might want to hang on to a few more decimal places for the shunt current for intermediate steps, since you are dealing with a difference between a large number and a tiny one). What else can you determine from the given values?
 
V=IR

so 0.1 x 0.00001
= 0.0000001
 
damon669 said:
V=IR

so 0.1 x 0.00001
= 0.0000001

Yes, you can determine the potential across the meter resistance. However, be careful of your decimal places... check the value you've used for current. (it's often advantageous to use scientific notation, particularly when metric prefixes are given). Also be sure to include units on results.

So then, what's the potential across the shunt resistance?
 
Rs = V/Is

10^(-6)/199.9
 
I read you can use product over sum rule to get V
V=I x RsRm/Rs+Rm
can I use this? if so how to make Rs the subject?
 
5mV?
 
  • #10
damon669 said:
Rs = V/Is

10^(-6)/199.9

Your result will be out by a few orders of magnitude. Did you check the current value that you used for I am as I suggested?
 
  • #11
damon669 said:
I read you can use product over sum rule to get V
V=I x RsRm/Rs+Rm
can I use this? if so how to make Rs the subject?

You could do that. It's just algebra, rearranging the expression. Start by getting rid of the fraction (multiply both sides by the denominator expression).

There's a more straightforward option, too. Look up "current divider". It would go well with the starting values you've been given.
 
  • #12
damon669 said:
5mV?

What is that supposed to be?
 
  • #13
current divider gives

I = I am + Is
so Is = I - I am (already done)
I - V/Rm
 
  • #14
gneill said:
What is that supposed to be?

thought i worked out the voltage to be 5mA, which would be the same in all parts of this parallel circuit
 
  • #15
damon669 said:
current divider gives

I = I am + Is
so Is = I - I am (already done)
I - V/Rm

That doesn't look like the current divider rule, and you're not given V ahead of time...

But before going on with a new method you need to fix the one you've used first. Check the numerical value you've used for Im!
 
  • #16
damon669 said:
thought i worked out the voltage to be 5mA, which would be the same in all parts of this parallel circuit

While it is true that the voltage will be the same across both resistances, that voltage is not 5mV !
(and mA is a current: milli-Amps).

Redo your voltage calculation and pay particular attention to the magnitudes of the values!
 
  • #17
V = IR
= 0.001 x 0.1
=0.0001
= 10^(-4)
 
  • #18
damon669 said:
V = IR
= 0.001 x 0.1
=0.0001
= 10^(-4)

That looks better :smile: Be sure to add units to your results! Always! You will lose marks if you fail to include units!

So, with an appropriate metric prefix that would be 0.1 mV.
 
  • #19
So Rs = V/Is
=0.0001/199.9
=0.0000005003 ohms
 
  • #20
damon669 said:
So Rs = V/Is
=0.0001/199.9
=0.0000005003 ohms

Yup. Now's the time to use scientific notation and round results to the appropriate number of significant figures.
 
  • #21
0.5003 microhms
 
  • #22
and that's it

must admit did not expect an microohm answer
 
  • #23
damon669 said:
0.5003 microhms
Too many significant figures... take a look at the original given values to determine how many significant figures are appropriate. Otherwise it looks good.
 
  • #24
0.5 microohm
 
  • #25
damon669 said:
0.5 microohm

Looks good! :approve:
 
  • Like
Likes 1 person

Similar threads

Replies
4
Views
2K
Replies
12
Views
13K
Replies
41
Views
7K
Replies
1
Views
4K
Replies
3
Views
2K
Replies
6
Views
2K
Replies
16
Views
4K
Back
Top