I found a LED somewhere. How can I find out if it has a built-in resistor? As far as I can see, they look really equal to me. Measuring the resistance using the multimeter does show an infinite resistance - I don't know why. In case there is no built-in resistor, how can I find out the proper limiting resistor? How to find at what voltage the LED should be used?

Similar Q&A can be found here, but it does not oanswere the part about built-in resistors.

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    \$\begingroup\$ If it has a built-in resistor, it's probably spec'd for a much higher than Vf voltage, ie 5V or 12V. I'd use a coin-cell battery and connect it to the LED. If it lights up, it probably doesn't have an internal resistor. \$\endgroup\$
    – dext0rb
    Commented Jan 6, 2014 at 18:27
  • \$\begingroup\$ @dext0rb Oh, I had in mind that I always need a liminitng resistor when connecting a LED to battery? \$\endgroup\$
    – lumbric
    Commented Jan 6, 2014 at 18:31
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    \$\begingroup\$ A coin cell is a poor enough battery that it acts as its own limiting resistor. \$\endgroup\$
    – user16324
    Commented Jan 6, 2014 at 18:40
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    \$\begingroup\$ @lumbric coin cells have a high ESR, Equivalent Series Resistance, of about 25Ω or more. Along with the low voltage of 3v, which is less than the typical forward voltages of blue and white leds (3.2+). It's a happy convergence of coincidences. \$\endgroup\$
    – Passerby
    Commented Jan 6, 2014 at 21:40

3 Answers 3


I'd suggest connecting the unknown LED together with current limiting resistor to the regulated voltage source and measuring its voltage-current characteristics.


simulate this circuit – Schematic created using CircuitLab

The voltage at which the "reasonable" current starts to flow may be considered the "on voltage" of the diode. The slope of the I-V curve above this voltage will allow you to estimate the R2 value (if it is mounted in your diode).

$$R_2=\frac{\Delta U_{VM1}}{\Delta I_{AM1}}$$

Of course you should watch out to keep the current reasonably low to not overload your diode. (You can also control how bright light does it emit). Unfortunately there is no method to state whet is the maximum allowed forward current for the unknown diode.


To check the LED you should measure it forward and reversed. Ideally you will see it preform like a diode, which it is. The forward drop should be around 2VDC. My Fluke meter, on diode check, will cause a typical LED to flash.

With the leads connect in the forward direction the diode should show some resistance. In the reverse direction, it should look open.

The meter you use can have a significant impact on your results. A VOM which uses a single 1.5V cell will not work well for this.

I would do as wzab suggest. Start with a 5V supply and a resistor like 500 ohms. Put your LED in one way, check the drop and if it lights up, then reverse it. If you don't see 5V in one direction across the diode and something more like 2V in the other (LED will light), toss it.

  • \$\begingroup\$ I think your suggestion to measure resistance of an LED is not very helpful, and you grossly understated the variability of the results. You say that in the reverse direction it should "look open" but in the forward direction it should "show some resistance". An open circuit is some resistance! \$\endgroup\$
    – Joe Hass
    Commented Jan 6, 2014 at 22:42
  • \$\begingroup\$ splitting hairs I think Joe, "some resistance" as in a reading on the meter, any reading. \$\endgroup\$ Commented Jan 7, 2014 at 6:02
  • \$\begingroup\$ In the old days (30+ years ago) before DVMs were $10/each and found everywhere, we used VOM meter with analog meters. Checking diodes (not LEDs) was done by checking on the resistance scale, forward and revers on a diode for a 10X min difference. This is not very effective today because LEDs are not very good diodes. They drop 2 or more volts rather then 0.7 so 1.5V meters don't bias them. I dont think its a crime to use a DVM on a resistance scale to check an LED, it might show some results if it's using higher voltages, like 9V. \$\endgroup\$ Commented Jan 7, 2014 at 23:43

EDIT: Sorry about the advice to check the resistance. I posted it without checking, the comments are right, it really isn't the way to go. Other than that, I don't know of a good way to check if an LED has a limiting resistor. However, if you need to measure the forward voltage drop, as is required by the limiting-resistor calculators, I found a simple way of doing it:

Circiut to test forward voltage drop across a diode

The battery would be the same voltage that you would use in your finished circuit, and the number on the multimeter is the forward voltage drop.

Then, enter that on this site. Put your source voltage in the voltage field, the measured voltage drop in the forward voltage field, and unless it's a megasuperLED, put 20 in the last field.

Sorry about that piece of bad advice, hopefully I'll get some less down-votes now!

First of all, you said that when measuring it you got infinite resistance. That isn't a good thing. Either you were measuring it wrong or the LED is blown. Try it again, making sure one probe is contacting one leg, and the other is contacting the other leg, and press firmly. If you still get infinite resistance, the LED's dead. Get another.

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    \$\begingroup\$ -1 This is really bad advice. Trying to measure the characteristics of an LED with an ohmmeter is a crap shoot, and the results can vary dramatically depending on the exact meter used, the resistance scale selected, and the color of the LED. \$\endgroup\$
    – Joe Hass
    Commented Jan 6, 2014 at 22:38
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    \$\begingroup\$ Agreed, not sure measuring the resistance of a diode is the best procedure. I just tried it on my Fluke at work - infinite resistance both ways. \$\endgroup\$
    – dext0rb
    Commented Jan 6, 2014 at 22:43
  • \$\begingroup\$ @JoeHass Sorry, I've done a bit more research and changed the answer completely. Hopefully you could undo that downvote :-) \$\endgroup\$
    – felixphew
    Commented Jan 7, 2014 at 20:46

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