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I'm trying to check the resistance of an LED with DMM. I know the LED works but when I set the DMM to resistance, it says O.F no matter which way I put the probes. I've tried manually setting the range but that didn't help. Is something wrong with my meter?
Problem testing resistance of LED with DMM
- Thread starter Circuits123
- Start date
An LED is a diode with no resistance.
A digital voltmeter uses a low voltage when measuring resistance so it does not cause a silicon diode or silicon transistor junction to conduct. My fluke DMM uses 0.2V when measuring ohms but uses about 2V on its "diode test" which causes a 1.8V red LED to conduct but not a 3.2V blue or white LED.
A digital voltmeter uses a low voltage when measuring resistance so it does not cause a silicon diode or silicon transistor junction to conduct. My fluke DMM uses 0.2V when measuring ohms but uses about 2V on its "diode test" which causes a 1.8V red LED to conduct but not a 3.2V blue or white LED.
A diode isn't a resistor, so don't be too surprised when you don't get meaningful results.
The O.F probably means "overflow" meaning that, if it WERE a resistor, it's resistance is higher than the range your meter is on. This make sense as you are probably not getting any current through the diode because you meter isn't putting out enough voltage to forward bias it.
What is 0.F???? Sounds like you have it on the wrong function for one. You need a diode test setting on your meter to test the LED. Even then it may not work if the meter does not put out enough volrage to over come the forward voltage of the LED.
Diodes, including LEDs, have a very non-linear resistance with current.
You can measure the apparent resistance at a single point by putting a known current from a power supply through the LED (Id) and measuring the voltage drop across the LED (Vd).
The equivalent diode resistance at that point will then then be Vd/Id.
But if you measure at a different current, you will get a different resistance value because of the logarithmic relation between voltage and current in a diode.
You can measure the apparent resistance at a single point by putting a known current from a power supply through the LED (Id) and measuring the voltage drop across the LED (Vd).
The equivalent diode resistance at that point will then then be Vd/Id.
But if you measure at a different current, you will get a different resistance value because of the logarithmic relation between voltage and current in a diode.
