Testing LED w/ Digital Multimeter

Thread Starter

Lightningbolt

Joined Feb 4, 2013
9
I was working on a project that involved LED’s. I tried to check them with my digital multimeter (DMM) set to the Diode function. No mater which way I connected the test probes, the DMM read "OL". Yet, when I tested a normal diode, the DMM read "OL" when the probes were connected the wrong way, and the correct Voltage drop when connected the correct way. The LED’s were typical with 20mA, 2-4V ratings.

I know the LED’s I was trying to test were good, because they worked just fine when installed in my project.

Why did my DMM read "OL" testing the LED and the voltage drop when testing the normal diode?

Thanks in advance for your help!
 

Wendy

Joined Mar 24, 2008
23,415
LEDs especially are non-linear devices. With a diode, you can get 0.2VDC to 0.7VDC forward voltage drop, which is what a meter is looking for. With a LED you can get 2.0VDC (really old, obsolete LEDs) up to 3.8VDC drop, which is a lot different.

One of the devices in my toolbox is a really simple LED tester, which is a battery and a resistor (more or less). With modern LEDs this can still go wrong, due to the fact some LEDs have a PIV of 5V! In other words, if you connect more than 5V backwards to a LED it can blow.

Then add the ESD sensitivity to some LEDs (part of the PIV I mentioned) and you get the idea.

Here is a commercial tester I bought a long time ago, but it is easy enough to roll your own.



http://www.bgmicro.com/LED1063.aspx

A really simple LED tester
 

Thread Starter

Lightningbolt

Joined Feb 4, 2013
9
I am unfamiliar with the acronyms "ESD" & "PIV"; would you please provide an explanation?

If I understand you correctly, the reason my procedure failed for the LED test is because the DMM did not supply a great enough voltage (2-4 VDC) to overcome the forward voltage of the LED (to open the gates of the LED using the hydraulic analogy); therefore, it determined there was no continuity between the test leads which resulted in the "OL" display.

While for the normal diode test procedure, the DMM was able to supply enough voltage to overcome its forward voltage (~ < 1 VDC); therefore it determined the forward voltage drop and displayed that result.

Is this a correct interpretation?
 

KJ6EAD

Joined Apr 30, 2011
1,581
I am unfamiliar with the acronyms "ESD" & "PIV"; would you please provide an explanation?
Electrostatic discharge and peak inverse voltage.

If I understand you correctly, the reason my procedure failed for the LED test is because the DMM did not supply a great enough voltage (2-4 VDC) to overcome the forward voltage of the LED (to open the gates of the LED using the hydraulic analogy); therefore, it determined there was no continuity between the test leads which resulted in the "OL" display.

While for the normal diode test procedure, the DMM was able to supply enough voltage to overcome its forward voltage (~ < 1 VDC); therefore it determined the forward voltage drop and displayed that result.

Is this a correct interpretation?
That's correct. Some meters will supply enough voltage to dimly light some LEDs and some won't. I like to use a constant current supply made from an LM317 voltage regulator and a 9V battery to test and identify LEDs since it works with all colors equally well.
 
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