Multimeter problem-> values way off

[cornellian]

Hi everyone,

today I installed MultiSim 10 and to my surprise, the multimeter is giving me impossible values, even in the most basic circuits. My first thought was "Ok, I'm an undergrad; so I might miss something in the circuit", but even in the most basic one that contains a DC source of 3 volts, 2 serially connected resistors with 10 Ohms each, the multimeter readings are:

V:  3V, which is right

A:  300mA if multimeter is connected 'around' on a single resistor, and -r- if connected to both resistors

R: 150 M Ohms.. That's right.. I'm getting 150,000,000 in what is supposed to be a mere 20.

What might be the reason, any ideas? This has to have an easy solution, with something very obvious that I am overlooking. Thanks a lot...

[digitalguy]

Hi Cornellian,

I just created a circuit in Multisim 10, as you described, made the measurements you described, and all is well. Could you attach your circuit(s) and show how you made the measurements? For example, I'm not sure what you mean by "if multimeter is connected 'around' on a single resistor, and -r- if connected to both resistors".  I did, as an experiment, measure the resistance of my circuit with the power supply connected, and got a reading of 300 Megs. Of course, you should never do this in real life!!

Message Edited by digitalguy on 05-20-2007 01:48 AM

[Euler's_Identity]

Hello.  Assuming you aren't making an error, you aint seen nothin' yet!

Wait till you get output voltages from split-supplied op-amp circuits that, frankly, would burn down your house if they were true.   (There'd be arching to the lower potential lines inside the walls which would set the paint aflame and, therefore, burn down your house.  Welcome to the wonderful world of MultiSim math.)

[nestor-sg]

Cornellian,

Your V reading is ok... however, your A reading is incorrectly wired, you should connect the multimeter -working as an ammeter- in series... not 'around' the resistor since you are basically creating a shortcut.

That multimeter is a basic ohmmeter, so disconnect the power supply when measuring ohms.Use a SPST switch or something similar.

[bartgrefte]

Sorry for the kick, but I ran into the same weird problem with a variable resistor.

The value always (no matter the % setting) stays at 500MOhm, while without a powersupply hooked up, it's correct.

And to make things weirder(?), this is what happens after changing the power supply output voltage:

[Dennis_Knutson]

Not weird at all. Don't you know you can't do a resistance measurement on a hot circuit? An ohmmeter provides a known current and measures voltage. Ohms law does the rest.

[bartgrefte]

Even in a simple circuit with just a power supply and one resistor as component?

[Dennis_Knutson]

Yes, even if 🙂

Really fundamental electronics.

[bartgrefte]

But what if the circuit is not hot/active, then what? Multisim still displays the wrong value when the power supply is disconnected, while the resistor is still part of a(n inactive) circuit.

[digitalguy]

Not according to the pictures you show above.