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# Calculate current in shunt resistor

Junior Member
Join Date: Jun 2012
Posts: 13

 06-15-2012, 05:40 PM
Ok using the following:

Ish/Im = Rm/Rsh

Rsh = shunt resistance
Rm = meter resistance
Ish = shunt current
Im = meter current

A moving coil ammeter has a coil resistance of 28.5 ohms and a shunt resistor of 1.6 ohms. When the 0-740 mA display reads it maximum, how much current flows through the shunt resistor?

I need help on this. I have tried with calculating the voltage and power ... help

Senior Member
Join Date: Apr 2012
Location: Gurnee, Illinois - USA - North America - Planet Earth - Milky Way
Posts: 3,639

 06-15-2012, 05:44 PM
Homework?

If so show your work and answers and someone will hopefully guide you from there...

Senior Member
Join Date: Apr 2010
Location: Tombstone, Arizona
Posts: 693

 06-16-2012, 02:55 AM
You have one equation and two unkown variables. What is the second equation?

Senior Member
Join Date: Mar 2012
Location: Richmond, California
Posts: 694

 06-16-2012, 05:22 AM
This is not as much a mathematics problem as it is science problem. This is exactly why you should not be in trapped by memorizing formulas. You should be deriving all your formulas from definitions and scientific facts. If you cannot derive a formula you shouldn't be using it. Without looking at a formula tell me what happens when you apply a current to two resistances in parallel.

Junior Member
Join Date: Jun 2012
Posts: 13

 06-16-2012, 11:16 AM
Hello

This is a math problem and it needed to be rearranged. I did it!

I(sh) = (( Rm / Rs + Rm)). IT

IT= Total current

Last edited by inlineskater; 06-16-2012 at 11:22 AM..

Senior Member
Join Date: Mar 2012
Location: Richmond, California
Posts: 694

 06-16-2012, 04:33 PM
Sorry, I think you're missing the point. If all is you want is the answer you can do it using a formula. There are virtually an infinite number of combinations of parts and many types of parts involving inductors, capacitors, transistors, chemical batteries, and so forth, not just resistors. So are you going to memorize different formulas for all these different situations? Memorizing formulas is backwards. You must first understand the physics or the science and then wrap formulas around them. And if you do not understand how a resistor works get a resistor, a battery, and a multimeter and experiment with them. I design circuits for a living. I only rely on what I know about parts and data sheets. I see formulas on data sheets and I do not use them. I wrap my own numbers around the circuits based on what i know. If i relied only on formulas i never would be able to do my job. When I looked at your problem I realized instantly that you lacked understanding of the parts. So I say again that if you don't understand formula and you cannot derive if based on scientific fasts you should not be using it.

Junior Member
Join Date: Jun 2012
Posts: 13

 06-16-2012, 04:45 PM
No point was missed.

Thanks

Junior Member
Join Date: Jun 2012
Posts: 13

 06-16-2012, 04:51 PM
Quote:
 Originally Posted by CocaCola Homework? If so show your work and answers and someone will hopefully guide you from there...
Good Point and point taken, next time I will do a complete update to the problem. I was working from a mobile location while being on set.

Thanks

Junior Member
Join Date: Jun 2012
Posts: 13

 06-16-2012, 04:54 PM
Quote:
 Originally Posted by Laplace You have one equation and two unkown variables. What is the second equation?
One equation and 2 variable. The expression needed to be arranged as posted. Next time around I shall post all the steps I have taken to solve for the unknown.

Thank you

Senior Member
Join Date: Apr 2010
Location: Tombstone, Arizona
Posts: 693

 06-17-2012, 01:45 AM
From the wording of the question I can't tell whether the second equation should be (Im=740 mA) or (Im+Ish=740 mA).

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