# Designing an Ohmmeter

#### SilverKing

Joined Feb 2, 2014
72
Hi everyone,

My problem says:
'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'

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Since the value of the measured resistance is mostly large, we use Series Type Ohmmeter.

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

#### WBahn

Joined Mar 31, 2012
25,590
One thing that the problem doesn't state, but which was probably covered in earlier material, is what is means "to design an Ohmmeter to measure the resistances below 110 kilo ohms".

Typically, for an ohmmeter such as this, that means that 100kΩ will result in a half-scale deflection. Does that sound familiar? If so, then you have two conditions that you want to meet, right? Namely, full-scale current when R=0 and half-scale current when R=100kΩ.

#### SilverKing

Joined Feb 2, 2014
72
To measure resistances below 110 kilo ohms, means that at 0 current the resistance will be infinity, and infinity is not a value to reach, so we take 95% of the resistance so it would the maximum resistance (it would match 5% of te current).

#### WBahn

Joined Mar 31, 2012
25,590
To measure resistances below 110 kilo ohms, means that at 0 current the resistance will be infinity, and infinity is not a value to reach, so we take 95% of the resistance so it would the maximum resistance (it would match 5% of te current).
Okay, so you still have two conditions that have to be satisfied. I'm assuming that the nominal position for the 2kΩ pot is in the center.

So what are the equations that govern the satisfaction of those two conditions.