# Inner workings of a multimeter

Discussion in 'General Electronics Chat' started by jaydnul, Oct 1, 2015.

1. ### jaydnul Thread Starter Member

Apr 2, 2015
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0
I have a few questions about multi meters in general:

1) How does the ohmmeter work? I can find general information online but what I want to know is does it just vary an internal resistance to send a constant current through the measured resistance? Or does the current through the measured resistance vary?

2) How does it measure the capacitance? Does it use an AC or DC signal?

3) How does it measure the voltage drop across a diode? Why doesn't this mode light up an LED and tell me the voltage drop?

Thanks

2. ### dannyf Well-Known Member

Sep 13, 2015
2,196
417
You are talking about a digital meter.

Ohm: measure voltage over a know resistor in serial with the dut.

Capacitance: depends.

Diode: depending on the meter s supply voltage. Some do and others don't.

3. ### jjw Member

Dec 24, 2013
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Ohmmeter usually puts a constant current through dut and measures voltage.

4. ### dannyf Well-Known Member

Sep 13, 2015
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The CCS approach can be very difficult to implement as it requires a precision current source.

5. ### saniyakhan69 New Member

Oct 2, 2015
1
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tell about the ohmmmeter and the resistive value of a capacitance
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Last edited by a moderator: Oct 2, 2015
6. ### OBW0549 Well-Known Member

Mar 2, 2015
1,389
932
What's difficult about it? Designing a precision constant current source is a simple matter for any experienced analog circuit designer. VERY simple.

7. ### dannyf Well-Known Member

Sep 13, 2015
2,196
417
Yeah, especially to someone who doesn't know how to do it.

8. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
The basic Ohmmeter measurement system:

Note that V(meas) is proportional to Runknown. Ohm's Law: E = I*R

In the second circuit, what value of the unknown capacitor would produce a slope of 0.667V/s in V(meas)? I am leaving this as an exercise to the student.

Last edited: Oct 2, 2015
9. ### OBW0549 Well-Known Member

Mar 2, 2015
1,389
932
I don't understand what you're trying to say here; are you saying that digital multimeters are designed by people who don't know how to design a precision constant current source circuit?

In any case, I just checked the owner's manual for my B&K 5491B DMM. Resistance is measured by passing a constant current through the D.U.T. and measuring the voltage drop across it. The current is auto-ranged in decades, from 0.5 mA on the lowest (500 Ω) scale down to 45 nA on the highest (50 MΩ) scale. Current source accuracy goes from ±0.1% on the lower ranges, up to ±0.3% on the highest range.