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I had one doubt regarding resistance measurement from multimeters. I cannot show the schematic because it is confidential but I have added a circuit diagram. It refers to a gate driver circuit. I am measuring the resistance between gate and source of a MOSFET as a part of the impedance measurement tests on the PCB using two multimeters (Fluke 287 and Fluke 115). Both of them are giving different readings. What could be the reasons for this? I wanted to what in the multimeter makes the difference? The current paths in the schematic definitely include the gate driver, some diodes etc.

enter image description here

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    \$\begingroup\$ Vgs is a voltage reading. Why would you use a resistance measurement device for this? \$\endgroup\$
    – Transistor
    Feb 13 at 10:51
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    \$\begingroup\$ Are you saying you're connecting a DMM in resistance mode across a powered circuit? \$\endgroup\$
    – TonyM
    Feb 13 at 11:04
  • \$\begingroup\$ Please draw a schematic of your setup. What is your end goal? \$\endgroup\$
    – winny
    Feb 13 at 11:09
  • \$\begingroup\$ The current paths in the schematic definitely include the gate driver, some diodes etc. You have to allow for this. If the meters drive the diodes differently, you will get different readings. Neither may be correct. Draw the schematic of all components connected to your Rgs that will take current when to try to measure it, then we can advise on how to measure it correctly. \$\endgroup\$
    – Neil_UK
    Feb 13 at 11:26
  • \$\begingroup\$ I have updated the question, please check \$\endgroup\$
    – Andr7
    Feb 13 at 11:35

1 Answer 1

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Multimeters measure resistance by injecting a known current into the circuit under test and measuring the resultant voltage drop. From that the resistance is calculated by \$ R = \frac V I \$.

Your circuit has non-linear elements in it - the diodes - and these will have a non-linear relationship between voltage and current so the voltage drop across them will depend on the meter's test current.

Learning exercise:

  • Switch meter 1 to resistance mode and meter 2 to mA mode. Connect red to red and black to black and record the current reading (that's meter 1's test current) and the resistance reading (that's meter 2's shunt resistance).
  • Repeat the exercise with meter 1 in mA mode and meter 2 in resistance measurement mode.
  • If you have a diode to spare try measuring the "resistance" of the diode using each meter.

Report back on your findings and your learning.

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  • \$\begingroup\$ Can we know that known current from the multimeter specification? \$\endgroup\$
    – Andr7
    Feb 13 at 11:57
  • \$\begingroup\$ Read the datasheets and user manuals to see if they're listed. \$\endgroup\$
    – Transistor
    Feb 13 at 12:32
  • \$\begingroup\$ Meter 1 (Fluke 287) Test current : 1mA , Meter 2 shunt resistance : 0.3 ohms , Meter 2 (Fluke 115) Test current : 0.4 ohms , Meter 2 shunt resistance : 0.3 ohms \$\endgroup\$
    – Andr7
    Feb 14 at 5:00

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