# LabVIEW

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## Reading a diode resistance by NI USB vs by Multimeter

Hello,

I am reading a resistance of diode at certain voltage input from power supply

and I found that

at 0.2 V

the resistance value by NI USB-6212 : 200 ohm

the resistance value by multimeter: 2 kohm

at 0.5 V

the resistance value by NI USB-6212: 500 ohm

the resistance value by multimeter: 5 kohm.

Would you please let me know why the values are different by 10 times?

Thanks.

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## Re: Reading a diode resistance by NI USB vs by Multimeter

If you don't want to endure the wrath of a certain Bob, you best post more information and code.

Me....I'm going to run and hide.

aputman
LabVIEW 2017
LabVIEW Programming
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## Re: Reading a diode resistance by NI USB vs by Multimeter

You set something up wrong?  You used the mutlimeter wrong?

None of those numbers sound right to me.  But we really can't help you since you didn't provide us any information or code such as the VI you used to measure with the USB-6212.

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## Re: Reading a diode resistance by NI USB vs by Multimeter

Hi nohmoses,

how do you measure resistance using a simple USB6212? This box can only measure voltages…

How does your multimeter measure resistance? Which type of DMM do you use? What's the specified accuracy of this DMM?

Clarify all those questions and we are on a way to solve this problem!

Best regards,
GerdW
CLAD expired, using LV2011SP1 + LV2017 (+LV2019 sometimes) on Win7+cRIO
Kudos are welcome

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## Re: Reading a diode resistance by NI USB vs by Multimeter

Resistance is typically not measured while the circuit is live.

-AK2DM

~~~~~~~~~~~~~~~~~~~~~~~~~~
"It’s the questions that drive us.”
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## Re: Reading a diode resistance by NI USB vs by Multimeter

Hello,

Simplely, I connected the NI board or multimeter as attached the figure "Picture1" to a LED

and monitored its resistance change.

In the block diagram, I chose the AI Input -> Resistance.

The DMM is Fluke 287.

Please let me know if you have any questions.

Thanks.

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## Re: Reading a diode resistance by NI USB vs by Multimeter

No - you cannot measure resistance that way.

To measure resistance, you normally pass a known current and then measure the voltage. A DMM will do this by generating a small known current and measuring the voltage internally. If you are supplying an external voltage as well, the DMMs internal resistance measurement will not work.

For the LabVIEW - the USB-6212 can only measure voltages. If you want to measure the resistance while the device is powered, you will need know/measure both the current (e.g. through a shunt resistor) and the voltage and then do R = V/I to get the resistance. I'm not sure what the 6212 does if you try to take a 'resistance' measurement as it doesn't have an internal current source.

Oh, I thought this all sounded familiar - here's a similar thread: https://forums.ni.com/t5/LabVIEW/daqmx-resistance-measurement-6251/td-p/3267084

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