Ohm meter onto a LED...

Hi.
What to expect checking an LED with an ohmeter; forward and reverse; or on 20 Mohm scale ? What about a string of series LEDs ?

It will depend on what the excitation voltage is of the meter.

My 34401A bench meter uses 2.6 Volts. Enough to forward bias some LEDs, but not all. So a low voltage LED would show something forward biased, but open if reverse biased. It would be useless for a string of LEDs as the individual forward voltages add.

The excitation voltage from my Tek DMM916 hand held meter is only 0.8 Volts. So it wouldn't forward bias any LED. A yellow LED I just checked showed open in both directions.

My fluke 87V applies 0.6mA to a diode or LED up to 3.9V so a white and blue LEDs are easily seen. The general iisers manual for 80- series fluke meter says 0.8v bjt the 87 is an oddball.

I have used meters that supply 1.5V on the "diode" test. It looks like meters vary, voltage from ohm-meter mode and diode-mode. And LEDs vary as much. Some meters have a low voltage mode (does not turn on silicon) and a high voltage mode (for testing silicon).

So it is hard to say what happens with your meter and your LEDs.

the best way to check LEDs is with a constant current source set for 10mA fed from a 5V power supply. if you want to read the forward voltage, connect a voltmeter to the output of the current source.

IIRC, one of my DMMs applies a constant current of 1 mA. Have to measure that to confirm. For diode test, I mean.

My multimeter does not light all LED colors so I use a 9V battery in series with a 390 ohms resistor.