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?
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4\$\begingroup\$ You've tried to measure a resistor in situ? Disconnect at least one leg of the resistor before measuring. \$\endgroup\$– glen_geekSep 24 at 12:56
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4\$\begingroup\$ It's not the problem here (the 87V has a clear low-battery indicator and it's not on in the photo), but another thing to keep in mind when you see strange readings is to check if the multimeter needs a new battery. I've seen meters that give all sorts of wild readings when the battery is low. \$\endgroup\$– HearthSep 24 at 14:51
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2 Answers
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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.
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\$\begingroup\$ 1b, a long lead is picking something up. I've mostly seen when the meter leads complete a high-impedance earth loop (e.g. when testing for a common ground between measurement connections) \$\endgroup\$– Chris HSep 25 at 20:42
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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.
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\$\begingroup\$ In the last sentence "best" is imprecise word. There may be valid and important reasons to avoid desoldering, so deciding what solution is actually best may get complicated. Consider switching it to "most accurate", or something like that. \$\endgroup\$– MołotSep 26 at 10:48