I am encountering a strange problem when I measure the voltage across an LED. Please see below picture:

photo of test setup

As you can see, I only connected 1 wire ("-") of adapter and used a multimeter to measure the voltage drop across the LED and I found there is ~-2V on the LED! There is no loop in this circuit, so it should have no volts drop across the LED. I have used other multimeters, but I still measure that negative voltage so it's not a multimeter problem.

I'm really sure it's an LED problem, but I have never seen this behavior before. I'm also not familiar with the manufacturing of LEDs, so I don't know what's happening on this LED.

This LED correctly lights with a forward voltage and does not light with reverse voltage. However, the important issue is when I use this LED as a test fixture, it causes the reference voltage (GND) to shift so the output voltage is different.

My question is: Have you seen this behavior on an LED? What is the possible problem on this simple LED?

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    \$\begingroup\$ Possible duplicate of Can LED be used for both light emission and sensing \$\endgroup\$ Commented Jul 23, 2019 at 6:01
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    \$\begingroup\$ A fun experiment: if you have an oscilloscope, connect a LED to the probe, and (1) move the LED directly under a fluorescent lamp, you can see a nice and clear ~3 kHz sawtooth wave (or 100 Hz if it has an old ballster): this is the switching frequency of the ballast. (2) Turn up the brightness of your LED-backlighted monitor to 50%, move the LED next to it, you can clearly see a square wave due to PWM, and (3) You can even use this trick to check the quality of your LED lightblub/fixature, if you see a 50 Hz waveform, throw this light away, it flicks badly. \$\endgroup\$ Commented Jul 23, 2019 at 8:25
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    \$\begingroup\$ There are more than one wires connected to it. Otherwise how can you measure a voltage across it? \$\endgroup\$
    – user207421
    Commented Jul 23, 2019 at 11:06
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    \$\begingroup\$ @user207421 Read the comments and answers - the LED is generating voltage due to acting as a photovoltaic diodes - converting incident light to electrical energy :-). You can thus measure a voltage across it with NO wires (other than the meter probes) connected. \$\endgroup\$
    – Russell McMahon
    Commented Jul 23, 2019 at 11:19
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    \$\begingroup\$ @Tom Lin - Interest only. In addition to the interesting modes mentioned by others, I have seen a circuit for a light powered LED flasher :-). The received light produces voltage which charges a capacitor and then triggers a circuit to discharge the capacitor into the LED ! \$\endgroup\$
    – Russell McMahon
    Commented Jul 23, 2019 at 13:52

3 Answers 3


A LED is basically a photodiode. If you shine light with the corresponding wavelength onto the LED, it will generate a voltage over the pn junction. A red LED has a bandgap of ~2V, this is propably what you are seeing here.

Try to cover the LED with one hand and check if the voltage output stays the same.

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    \$\begingroup\$ You'll probably need to add a large (e.g. 1M_ohm) resistor in parallel in order to see a response reliably. \$\endgroup\$
    – MooseBoys
    Commented Jul 22, 2019 at 19:53
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    \$\begingroup\$ A DMM is a large resistor in parallel. \$\endgroup\$
    – Matt Young
    Commented Jul 23, 2019 at 15:43
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    \$\begingroup\$ Darn reversibility of physics. Gets you every time. Or, is it actually you getting it? \$\endgroup\$
    – Yakk
    Commented Jul 23, 2019 at 17:48
  • \$\begingroup\$ I don't think the root cause is photodiode. Because I remove the 1 wire of adapter and the volt on the LED is only ~0.2mv and I also moved the LED to underneath a spotlight, but the volt is ~0.3mv. The most important is I only connected 1 wire ("-") of adapter and the volt I measure is ~1-2V(The volt is shift and bigger than mV). \$\endgroup\$
    – Tom Lin
    Commented Jul 25, 2019 at 2:29
  • \$\begingroup\$ I appreciate you guys provide any experience and inputs to me although I still can't figure out this issue now. :-) \$\endgroup\$
    – Tom Lin
    Commented Jul 25, 2019 at 2:41

There is no problem with the LED, this is normal behavior.

LEDs produce a voltage when struck by incident light, much like a photodiode. The stronger the light, the higher the voltage.

For reference I just pulled out a 638nm (red) 3mm LED and measured it with my Fluke 189. It showed 0.3V. Moving the LED to underneath a spotlight and it showed 1.7V.

Different LEDs may produce different voltages with the same amount of incident light. Also, a multimeter with higher impedance will allow the LED to build up a higher voltage.

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    \$\begingroup\$ Indeed! This was the source of one of the hardest bugs I ever found!... wp.josh.com/2014/03/03/the-mystery-of-the-zombie-ram \$\endgroup\$
    – bigjosh
    Commented Jul 23, 2019 at 19:26
  • \$\begingroup\$ Did you connect any wires of adapters? I put the LED(without wires of adapter) under the flashlight(from my smartphone) to measure the volt and only got ~0.3mV(Maximum). \$\endgroup\$
    – Tom Lin
    Commented Jul 25, 2019 at 2:35
  • \$\begingroup\$ I do another test. I connect 1 wire of adapter to LED: (1) scale: 200mV, I got ~-3.18mV. (2) scal: 2V, I got ~1.6V. When I use flashlight on LED, the volt did not change. \$\endgroup\$
    – Tom Lin
    Commented Jul 25, 2019 at 2:36
  • \$\begingroup\$ In the beginning, I measure the LED because I used this LED to be a test fixture to indicate if there have correct output(-12V) and I found the output volt is incorrect by this bad LED. Once I change this bad LED to others the output volt(-12V) is correct. I think my MOSFET may be affected by bad LED. \$\endgroup\$
    – Tom Lin
    Commented Jul 25, 2019 at 2:49

It works both ways

As a supplemental answer to this and this excellent answer, the reverse process is also possible. Direct bandgap photodoiodes used in photovoltaic mode (it's the photovoltaic effect you are seeing here) can also luminesce or glow with recombination light when excess e-h pairs are produced.

This can be done with an applied electrical current or even an ion beam, but as explained in the excellent answer to Do III-V based photovoltaics “glow” (photo-luminesce) when illuminated but not loaded? the recombination light can be induced by a photocurrent within the junction, which itself is produced by incident sunlight.

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    \$\begingroup\$ I think something is wrong with my bad LED. I'm reading your link and try to figure out the root cause on my bad LED. Thank you for your feedback! \$\endgroup\$
    – Tom Lin
    Commented Jul 25, 2019 at 2:58

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