I have a Fluke 87V multimeter and sometimes when I measure some resistors (in power supply PCBs I saw it) it shows me negative value of resistance. Why this? It is some flaw of the multimeter or it happens something else?
There are two likely reasons:
- The circuit is powered and the voltage across the resistor under test is opposing the test voltage output by the meter in resistance mode.
- The circuit is off but there's a capacitor holding some charge and you're measuring that.
To measure resistance the meter must output a current through the resistor under test. It then measures the resultant voltage across the component and scales that to give the resistance value. Any voltage across the component will corrupt the measurement.
Other components (diodes, parallel resistors, transistors, inductors, chips, etc.) will appear in parallel with the component under test so many of us will desolder one leg to isolate the resistor under test.
Before you measure the resistance of an in-circuit resistor, measure the DC voltage across it first. If the meter reads 0 volts then you won't get a negative value for the resistance. However, measuring in-circuit resistors can be very hit and miss due to the effect of other components connecting to the same node. If the resistor value is critical to know, it's always best to remove the resistor.