LED's Continuity Test

[Vasco]

I have some led lights and want to test them for continuity. Is it the same test as in any wiring? I think I read someplace that it's different or you can't test them. The light has two little wires coming out of it. I'm electrically challenged. Hope I'm mistaken.

[Lake-Effect]

Originally Posted by Vasco

I have some led lights and want to test them for continuity. Is it the same test as in any wiring? I think I read someplace that it's different or you can't test them. The light has two little wires coming out of it. I'm electrically challenged. Hope I'm mistaken.

You can't test LEDs for "continuity". If it's a packaged LED bulb replacement ready for 12v, then the easiest way to test is to put 12 v to it. make sure you get the polarity correct - the LED will light in one direction but not the other.

If it's an individual bare LED...

Here is a pretty thorough page on LED testing. Overkill, but you'll completely understand LEDs if you follow this way.

The easiest way to quickly test loose LEDs is to get a 9v battery and a small 1k (1000 ohm) resistor.

Wedge or tape one end of the resistor onto one terminal of the 9v battery, then take your LED and touch one lead to the battery terminal and the other to the free end of the resistor... the LED should light. Again it will light in one direction but not the other.

[belizesailor]

Caveat: I'm not an EE and I don't play one on TV, but here's my understanding (I do have a lot of practical electrical troubleshooting experience).

I don't think a simple continuity test on an LED will tell you anything (except maybe that it is totally fried and internally fused together)

Unlike an incandescent bulb, where there is a conductive filament bridging the positive and negative leads, there is no such simple physical connection in an LED. Light is emitted when applied voltage causes charged particles to travel between the N&P surfaces of the LED. With higher quality, current protected, LED's things get even more complicated because there is a bit of circuitry embedded in the bulb.

Because of these differences, it never crossed my mind to try a continuity test on an LED. So, to verify, I just did a continuity test, using a current protected known good LED....no continuity.

[Dsanduril]

If your multimeter has a diode test setting (the symbol looks like a little triangle with a vertical line crossing the pointy end) you can just use that. In the diode test setting it should read the forward voltage (somewhere around 1.5V for most LEDs) if the LED is working properly. It may even light the diode a little. If you're not sure about the proper polarity check both directions, one will show open, the other should show the forward voltage if the LED is working. If it shows open both directions then the LED is bad, if it shows a very low voltage both directions then there is a short.

[Paul Elliott]

Originally Posted by Dsanduril

If your multimeter has a diode test setting (the symbol looks like a little triangle with a vertical line crossing the pointy end) you can just use that. In the diode test setting it should read the forward voltage (somewhere around 1.5V for most LEDs) if the LED is working properly. It may even light the diode a little. If you're not sure about the proper polarity check both directions, one will show open, the other should show the forward voltage if the LED is working. If it shows open both directions then the LED is bad, if it shows a very low voltage both directions then there is a short.

This is exactly correct for a bare LED, but if we're talking about a led lamp assembly, that's going to have at least a series resistor, and probably an active current-regulator circuit. A simple diode test probably won't work.

[hellosailor]

Why would you do a continuity test on an LED fixture?

If you want to test it, apply twelve volts and see if it lights up. A 9v "transistor" battery is perfectly good enough for this job. The LED will ight if the polarity is correct, it will not be harmed if you get it backwards.

If you are trying to troubleshoot beyond that, you can use a multimeter probably set on the 2000 or 20,000 ohm scale. Reading "this way" it will show mainly the resistance of any dropping resistor that is in the fixture. Reversed, it show should infinite resistance (over range) since the LED is a diode, and it blocks all current flowing the wrong way. If the fixtures are fancy and have an active current regulator in them--blob, block, heat sink etc--this test is meaningless and you're better off using the 9v battery again, or a real 12v higher power source. The 9v battery may not have enough power to make a high power regulated LED fixture happy.

[rtbates]

sure you can ,with any ohm meter. with the negative lead(black) on the cathode and the positive (red) on the anode you'll read some resistance depending on the current limiting diode. Switch the leads and the resistance reading will be MUCH greater, like open or none. The key is whether the resistance changes when you swap the red and black meter leads.

Good luck....

[Paul Elliott]

Originally Posted by hellosailor

Why would you do a continuity test on an LED fixture? [...]

You wouldn't, and I wouldn't, but the O.P. said that he was unfamiliar with LEDs, and many people don't know the difference between an LED and an LED fixture, or an LED lamp with a built-in series resistor.

For the sake of useless completeness, while many LEDs show a 1.6V forward drop (more or less), for many of the high-efficiency ones the drop is about 2.1V. Gallium Arsenide Phosphide vs Indium Gallium Nitride, I think. There are other formulations as well, that have up to around 4V drop.

[hellosailor]

Randy, there are continuity testers sold and while you can use an ohm meter to test continuity, you still wouldn't do continuity tests on an LED as a means of accomplishing anything. You would do a forward/reverse resistance test, to see if it conducts in one direction versus the other, but that's not what a continuity test it. A continuity tester is an unpolarized device, typically a small battery and light bulb, which simply lights if the circuit is unbroken.

When you've got to start screwing around with meters and reversing and ranges, that's was beyond what 'continuity test' ordinarily means.

You can use a vicegrip as a hammer, that doesn't mean you should, or normally would.

Paul-
The traditional WHITE LEDs all had a 4.5v voltage drop or something like that. There are new ones that seem quite happy to work on a single 1.5v battery or a 3-volt battery though, so someone must have found a way to produce them with lower voltage drops now. And there's no sign of a DC-to-DC converter chip, the way they once did that. Just an LED, no outboard bits.

[Lake-Effect]

Originally Posted by Paul Elliott

For the sake of useless completeness, while many LEDs show a 1.6V forward drop (more or less), for many of the high-efficiency ones the drop is about 2.1V. Gallium Arsenide Phosphide vs Indium Gallium Nitride, I think. There are other formulations as well, that have up to around 4V drop.

Not useless...I too wondered whether to bring it up here or not.

I have LEDs in my personal inventory of all different colours, and they each have different forward voltage drops, depending on their colour. I have white-ish ones that drop 3.1v, some greens at 2.4 (I think) and red ones at around 1.9.

Another (possibly useless) point - many test meters with a diode test are fine for testing normal silicon diodes, which have forward drops of 0.6 to 0.7 v, but the meter might not have enough range to cope with the higher forward drops of bare LEDs. Check the manual, etc.

[Andina Marie]

Many, or perhaps most LEDs have a reverse breakdown voltage of less than 12 volts so if tested in reverse from a 12 volt source the results may be misleading or even destructive.

Caution relying on a meter measuring resistance. On the low resistance range, many meters use a 1.5 volt battery source which is too low to overcome the forward drop in many LEDs. However if you use one of the high ranges good meters will switch to an internal 9 volt battery.

Simple check, using the ohm meter, try it both directions on the LED in a dark location. Ignore the reading on the meter but if the LED is good the test current will cause a slight glow of light in the LED when you have the right polarity. However keep in mind that on cheaper meters using the resistance function you can't always rely on the polarity of the test leads corersponding to the actual voltage polarity, especially with analog meters.