Design an Ohmmeter: E & R1 Solutions

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SUMMARY

The discussion focuses on designing an ohmmeter using an ammeter with a resistance (Rm) of 50 ohms and a full-scale deflection current (Ifsd) of 0.5 mA. The objective is to determine the voltage source (E) and the series resistor (R1) needed to measure resistances below 110 kilo ohms, with a controlling resistance (R2) of 2 kilo ohms. The calculations involve using the equation R1 = E/Ifsd - R2/2 - Rm and considering various current sensitivity scenarios to derive the necessary parameters for the ohmmeter design.

PREREQUISITES
  • Understanding of Ohm's Law and basic circuit theory
  • Familiarity with ammeter and ohmmeter functionality
  • Knowledge of Kirchhoff's laws for circuit analysis
  • Ability to manipulate equations involving current and resistance
NEXT STEPS
  • Research the design principles of analog ohmmeters
  • Learn about voltage divider circuits and their applications
  • Study the effects of varying current sensitivity on measurement accuracy
  • Explore textbooks or online resources on electrical circuit design
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Electronics students, hobbyists designing measurement tools, and engineers involved in circuit design and analysis will benefit from this discussion.

JasonHathaway
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Homework Statement


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.
figure1-basic-series-type-ohmmeter1.jpg


Homework Equations


R1=E/Ifsd - R2/2 - Rm
CS=1/Ifsd

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 :)
 
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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).
 

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