Multimeter problem-> values way off

[Dennis_Knutson]

You said that 'while without a powersupply hooked up, it's correct' and the image proves it.

I don't know your area of study but it's clear you need fundamental knowledge of dc, ohms law, and ohmmeters. Google can certainly help but perhaps you can find an EE student on your campus that can provide some tutorials.

[bartgrefte]

Did you both stop reading before "while the resistor is still part of a(n inactive) circuit." ? 😉

Just tried to measure a resistor in a real (but inactive) circuit, could measure it without problems. While in Multisim, I would have to separate the resistor from the inactive circuit to get an accurate reading.

As for study, it's been a while since I got stuff like this at school (decade a go) and I don't remember anything being said (at school) about not being able to measure resistors in active circuits. Measuring in an inactive circuit however, seems to be possible in a real life circuit, but not in Multisim.

Can anyone explain that?

[Dennis_Knutson]

Both of your images show an active power source - one with 5 volts and one with 2.5 volts.

[bartgrefte]

Forget the images for a sec 😉

The circuit I was talking about in my prevous post, which is not on the pics, is a LM393N circuit to hook up a sensor with analogue output to a digital input of an Arduino. Got it soldered together on a prototype-circuitboard.

If I disconnect the powersupply, I can measure the right value of the resistor while it is in a circuit, if I do the same in Multisim, that fails. Here it is in Multisim:

Wrong value, even though the circuit is not active.

[nestor-sg]

Hi, I can try to explain a bit...the ohmmeter needs to supply current into the resistor to take its measurement, one reason why in the real world you shouldn't measure resistance with a typical ohmmeter on a live circuit and you should turn it off, or isolate the resistor and take your measurement.

Now, in Multisim...in order for the ohmmeter (in the Multimeter instrument) to work, you need to run the simulation, remember, SPICE is running in the background, so, to invoke the simulation solver (i.e. create a circuit netlist and send it to the solver) we need to run the circuit. This is when the multimeter will establish the current source that will feed the resistor to take the reading.

With that said, if you run the circuit in Multisim with the resistor wired in a "live" circuit, the current flow from that live circuit will interfere with the reading of the Multimeter. Very similar to how it will interfere in a real circuit. That is why your connected resistor to the circuit is causing the Multimeter to give a bad reading.

The part where you may be getting confused (is my guess) is that once you stop the simulation, the reading is still there. That is as designed, we keep the measurements in the displays until you take another readind, that is, you run another simulation. When Multisim's simulation is not running, the Multimeter is NOT taking any new measurements, it is simply keeping the old measurement in the display. It will take a new measurement while the simulation is running.

I hope this helps, let me know if you have further questions with this.

Cheers!

[johnsold]

Consider that the measurement of a resistance is determined by Ohm's Law. You need to be able to measure simultaneously the voltage across the resistor and the current through the resistor.

A typical ohmmeter injects a current through the leads and measures the voltage across the leads.  (I have no idea how the Multisim multimeter works). In your image the meter is connected parallel to both R1 and V1. Regardless of the current injected by the meter, the voltage across R1 will be 5 V and this will have nothing whatsoever to do with the value of R1. You second image with V1 = 2.5 V is exactly what would be expected if the injected current is 100 nA. The algebra is left to the student as an exercise.

You would have the same problem ini a circuit with multiple resistors. Take your R2 circuit. Connect R3 = 250 ohms and R4 = 250 ohms in series. The connect the series combination parallel to R2. The meter will not read the value of R2 but the equivalent resistance of the three resistors combined or about 455 ohms.

What do you consider an "inactive circuit"?

I see there have been other posts while I was writing mine but I have not read through them yet.

Lynn

[nestor-sg]

I noticed I did not answered your last question in my previous post... in the picture you showed... you don't have any other source connected to the circuit... but you do have the whole circuit connected to the resistor... the Multisim Multimeter is taking a resistance measure between those two nodes where it is connected to the circuit wires... that includes of course the resistance or impedance of the rest of the circuit. Remember, the current injected from the Multimeter will flow to the whole circuit and affect the reading.

Isolate the resistor if you want to take the resistance measurement.

For instance... see this image, I can take the resistance measurement on R3, even on a live circuit running, because there is no other current flowing through the resistor, check it out:

I don't get a exact 1K measurement, but that's because of the internal SPICE structure of the current source of the ohmmeter and the other components it is made up, it does create a path to ground and a small current of the node between R1 and R2 is being leaked through the Multimeter.

Now, I have to ask you, what are you trying to do? And I ask this, because I maybe able to come up with a different solution for you if this is getting in your way. For example, if you are trying to have a "live" value reading of the variable resistor while your simulation is running, that would explain your concern with having the ohmmeter connected to the resistor, and we may need to tackle that issue in a different way.

So, we have explained why you are not getting the readings you are expecting, it is perhaps best if you instead explain what is your objective with taking that measurement because the way it is being done right now is not the right setup.

Cheers!

[Dennis_Knutson]

As a cautionary note, I have seen engineers and technicians banned from checking out instruments, fined for repairs, and even fired when they misuse the equipment.

[bartgrefte]

First off, thanks for all the info 🙂 , that's something that wasn't explained at school.

@nestor:

"The part where you may be getting confused (is my guess) is that once you stop the simulation, the reading is still there."

Nope, that didn't confuse me, I already expected that to be by design. What is confusing me, is that even with an inactive circuit (as in, no power source hooked up) the value is still wrong in Multisim. While it is correct on a real (not simulated) circuit that is not hooked up to a power source.

"I noticed I did not answered your last question in my previous post... in the picture you showed... you don't have any other source connected to the circuit... but you do have the whole circuit connected to the resistor... the Multisim Multimeter is taking a resistance measure between those two nodes where it is connected to the circuit wires... that includes of course the resistance or impedance of the rest of the circuit. Remember, the current injected from the Multimeter will flow to the whole circuit and affect the reading."

I always thought current takes the path of least resistance, which is in this case the variable 100kOhm resistor set at 5%, so the rest of the circuit shouldn't influence the measured value, or so I thought.

"Now, I have to ask you, what are you trying to do? And I ask this, because I maybe able to come up with a different solution for you if this is getting in your way. For example, if you are trying to have a "live" value reading of the variable resistor while your simulation is running, that would explain your concern with having the ohmmeter connected to the resistor, and we may need to tackle that issue in a different way."

Well, basically measuring the value of the variable resistor while it is in a (powered off) circuit. It seems that this is possible in a real life (but powered off) circuit, but for some reason I can't do the same thing in Multisim.

@johnsold:

"What do you consider an "inactive circuit"?"

No power source attached.

[nestor-sg]

Thanks for your confirmation... if then, all you want to do is measure the resistance, then, in Multisim, you do need to isolate the resistor, that's because for the Multimeter to work, the circuit needs to be simulated. This would be one of those cases in which a simulator will offer a different experience than in the real-world.

As far as your comment on the path of least resistance for the current flow, keep in mind that is not black or white, or all or none, current will be distributed along the circuit, with more current flowing through the path of least resistance, but other paths with more resistance will still get current flow, just not as much.

I hope this thread has solved the question and you can continue your project... cheers!