# Homework Help: Designing an Ohmmeter

1. Jun 7, 2015

### JasonHathaway

1. The problem statement, all variables and given/known data
If you an Ammeter with Rm=50 ohms and Ifsd=0.5 mA, What is the voltage differece you should have if you want to design an Ohmmeter to measure the resistances below 110 kilo ohms? Also, determine the values of the minimum and maximum values of the voltage source that can be used, and sketch the scale with 5 values at least (The controlling resistance (R2) is 2 kilo ohms.

2. Relevant equations
R1=E/Ifsd - R2/2 - Rm
CS=1/Ifsd

3. The attempt at a solution
It's the first time I deal with an unknown voltage source, because in this case we have two unknowns: E and R1 (the series resistor).
If E is known, then we assume R short circuit. then we get R1 --> R1=E/Ifsd - R2/2 (the arrow in the middle) - Rm
Then we calculate the unknown resistor in different cases of the current (e.g. 1/3 Ifsd, 1/2 Ifsd, 3/4 Ifsd, 0.95 Ifsd (nearly maximum)).
But in this case - as I said before - there are two unknowns!

I tried to use the following:
I know that the current sensitivity is 1 over Ifsd ---> CS=1/Ifsd=1/0.5mA=10 kilo ohms/V, and Rscale=Vmax/CS.

Does that make any sense?

P.S. If anyone have any textbooks or educating sites with similar problem, I'll appreciate putting them here :)

2. Jun 11, 2015

### Svein

I agree with shorting A to B and that Im should then be equal to Ifsd. And it is sensible to start with R2/2. From there you need to follow Kirchhoff: Sum the currents in each junction point and solve (with E as a parameter).