Measure resistor with a Multimeter

kalemaxon89

Joined Oct 12, 2022
190
I have a 10k NTC at 25 C° and a 10k resistor in series, connected between the arduino's 5V and ground.

With the multimeter I would like to measure the 10k resistor (even though I know it is 10k) out of curiosity... but when I connect it to the circuit and connect the two probes of the multimeter to the resistor ends, the display shows "1." (i.e. out of scale).
This is impossible because I selected 20k in the multimeter .. so 10k should measure it.
Additionally, if I measure the same resistance disconnected from the circuit, the correct value of 10k is read from the multimeter.
What is happening?

EDIT: I've just tried to measure it again and now the display shows 14k .. how come?

ericgibbs

Joined Jan 29, 2010
17,981
Hi k88,
If you have +5v on the Resistor and NTC it will not give a resistance reading on a meter
The 5V will affect the reading.
E

BobTPH

Joined Jun 5, 2013
7,519
First of all, you should not expect correct readings of any component in circuit since there are other components involved.

But getting a higher reading in circuit for a resistor should not happen unless you are measuring with the circuit powered, which you should never do!

kalemaxon89

Joined Oct 12, 2022
190
First of all, you should not expect correct readings of any component in circuit since there are other components involved.

But getting a higher reading in circuit for a resistor should not happen unless you are measuring with the circuit powered, which you should never do!
Since this is my first time using the breadboard and making a circuit, I am measuring with the circuit powered. Thanks to you and @ericgibbs for the advice!

But I wonder, how can such a circuit (or whatever circuit it is) affect the resistance value ... shouldn't it remain constant at 10k ?

ericgibbs

Joined Jan 29, 2010
17,981
hi,
The fixed 10k series will retain its value, the NTC resistance will reduce as the temperature increases.
E

ericgibbs

Joined Jan 29, 2010
17,981
hi k89.
This is an LTS example for your circuit. Note the Plots. -5C to +40C, 36K down to 6k for the NTC.
E

BobTPH

Joined Jun 5, 2013
7,519
But I wonder, how can such a circuit (or whatever circuit it is) affect the resistance value ... shouldn't it remain constant at 10k ?
The resistance remains the same, but when you put the multimeter probes across the resistor it cannot measure the resistance of that resistor only, it measures the resistance of the entire circuit between those two points.

Think of a very simple circuit. Put two 10K resistors in parallel. Now try to measure the resistance of one of the resistors while in that circuit. What do you think you will get, and why?

kalemaxon89

Joined Oct 12, 2022
190
hi,
The fixed 10k series will retain its value, the NTC resistance will reduce as the temperature increases.
E
But why if I try to measure the 10k resistance with the multimeter while the power is on ... I see a value quite different from 10k ohms?

sarahMCML

Joined May 11, 2019
269
I have a 10k NTC at 25 C° and a 10k resistor in series, connected between the arduino's 5V and ground.

With the multimeter I would like to measure the 10k resistor (even though I know it is 10k) out of curiosity... but when I connect it to the circuit and connect the two probes of the multimeter to the resistor ends, the display shows "1." (i.e. out of scale).
This is impossible because I selected 20k in the multimeter .. so 10k should measure it.
Additionally, if I measure the same resistance disconnected from the circuit, the correct value of 10k is read from the multimeter.
What is happening?

EDIT: I've just tried to measure it again and now the display shows 14k .. how come?

View attachment 292206
Your multimeter works by passing a known, accurate current through the resistor being measured, in this case R1, and measuring the voltage across it. But in your circuit, when it's powered up, you also have current flowing through it from the power supply, so this confuses the multimeter, so it measures a different voltage. Hence the wrong readings!
Even un-powered you would have a current path around the circuit, through the PSU (unless disconnected) via the thermistor, which would also give an incorrect, lower, reading.

BobTPH

Joined Jun 5, 2013
7,519
But why if I try to measure the 10k resistance with the multimeter while the power is on ... I see a value quite different from 10k ohms?
Go back and read my post #7 and @sarahMCML’s post #9. If you don’t understand them, tell us what you are not getting.

MrChips

Joined Oct 2, 2009
29,200
Your resistance meter does not measure resistance. It measures current.
Think about that for a moment.

The meter applies a voltage and attempts to read back current.
If the circuit is still powered and the circuit voltage opposes the meter voltage, the measured current will be lower than expected. Hence the readout resistance will be higher than expected.

crutschow

Joined Mar 14, 2008
32,848
The meter applies a voltage and attempts to read back current.
Analog meters do that.
Digital multimeters apply a constant-current and read the voltage.

ElectricSpidey

Joined Dec 2, 2017
2,593
Measure the current thru the resistor, then measure the voltage across the resistor...then apply Ohm's law.

kalemaxon89

Joined Oct 12, 2022
190
The resistance remains the same, but when you put the multimeter probes across the resistor it cannot measure the resistance of that resistor only, it measures the resistance of the entire circuit between those two points.

Think of a very simple circuit. Put two 10K resistors in parallel. Now try to measure the resistance of one of the resistors while in that circuit. What do you think you will get, and why?
Done.
When the circuit (powered with 5V from Arduino) is off of course the multimeter reads 10k .. when it's on, it reads "1" out of scale on the multimeter for both.
Indeed, when I touch with the terminals of the multimeter the ends of one of the 2 10k resistors ... the display of the multimentro goes out. When I disconnect the terminals (probes) it turns back on.

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kalemaxon89

Joined Oct 12, 2022
190
Analog meters do that.
Digital multimeters apply a constant-current and read the voltage.
My multimeter is DT830B digital

kalemaxon89

Joined Oct 12, 2022
190
Il multimetro funziona facendo passare una corrente nota e accurata attraverso il resistore da misurare, in questo caso R1, e misurando la tensione attraverso di esso. Ma nel tuo circuito, quando è acceso, hai anche corrente che scorre attraverso di esso dall'alimentatore, quindi questo confonde il multimetro, quindi misura una tensione diversa. Da qui le letture sbagliate!
Anche non alimentato si avrebbe un percorso di corrente attorno al circuito, attraverso l'alimentatore (a meno che non sia scollegato) tramite il termistore, che darebbe anche una lettura errata, inferiore.
Your resistance meter does not measure resistance. It measures current.
Think about that for a moment.

The meter applies a voltage and attempts to read back current.
If the circuit is still powered and the circuit voltage opposes the meter voltage, the measured current will be lower than expected. Hence the readout resistance will be higher than expected.
Measure resistance in a built (like PCB) circuit is difficult because current choose the "way" with least resistance, in fact the measuring current doesn't flow only in the resistor.

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BobTPH

Joined Jun 5, 2013
7,519
but I didn't think it would happen even with two resistors in parallel on a breadboard
Are you saying you don’t think the current would split half and half between two equal resistors in parallel on a breadboard?

kalemaxon89

Joined Oct 12, 2022
190
Are you saying you don’t think the current would split half and half between two equal resistors in parallel on a breadboard?
Sorry, I misspoke. Please forgive my english.
I just edited the previous answer because it seems just as you said ... that is, the current does not split into two branches.
Of course I know that this is not the case.

kalemaxon89

Joined Oct 12, 2022
190
Are you saying you don’t think the current would split half and half between two equal resistors in parallel on a breadboard?
I have put the two resistors in parallel and cannot display the resistance (of either one) on the multimeter. I would like to know physically (I haven't figured it out yet) why this happens

MrChips

Joined Oct 2, 2009
29,200
Resistors in parallel

Any number of resistors in parallel will give you a single effective resistance between points A and B.
You cannot determine the value of each individual resistor by measurement with any type of meter.

If you know the values of all but one resistor, you can calculate the unknown value using the parallel resistor equation.