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Strange LED behavior: Why is there a voltage over the LED with only one wire connected to it?
Can LED be used for both light emission and sensingWould adding an LED to a multimeter's LCD screen affect readings?LED forward voltage - why does series resistor take excess voltage?Calculating forward voltage for a string of fairy lights: multimeter shows “1”How to replace a green LED with a red one in a simple transistor switch?I have an LED with no specs, but it does have a resistor. I want to double the voltageSame LED, different voltage drops with different supply voltagesLED driver putting out more voltage than rated?Why is voltage across LED filament bulb slowly decreasing?
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margin-bottom:0;
$begingroup$
I am encountering a strange problem when I measure the voltage across an LED. Please see below picture:
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?
led
edited Jul 23 at 7:43
jusaca
2,9782 gold badges11 silver badges35 bronze badges
asked Jul 22 at 8:57
Tom LinTom Lin
1721 silver badge6 bronze badges
$endgroup$
|
show 7 more comments
$begingroup$
I am encountering a strange problem when I measure the voltage across an LED. Please see below picture:
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?
led
edited Jul 23 at 7:43
jusaca
2,9782 gold badges11 silver badges35 bronze badges
asked Jul 22 at 8:57
Tom LinTom Lin
1721 silver badge6 bronze badges
$endgroup$
2
$begingroup$
Possible duplicate of Can LED be used for both light emission and sensing
$endgroup$
– Dmitry Grigoryev
Jul 23 at 6:01
3
$begingroup$
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.
$endgroup$
– 比尔盖子
Jul 23 at 8:25
3
$begingroup$
There are more than one wires connected to it. Otherwise how can you measure a voltage across it?
$endgroup$
– user207421
Jul 23 at 11:06
$begingroup$
@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
Jul 23 at 11:19
1
$begingroup$
@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
Jul 23 at 13:52
|
show 7 more comments
$begingroup$
I am encountering a strange problem when I measure the voltage across an LED. Please see below picture:
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?
led
edited Jul 23 at 7:43
jusaca
2,9782 gold badges11 silver badges35 bronze badges
asked Jul 22 at 8:57
Tom LinTom Lin
1721 silver badge6 bronze badges
$endgroup$
I am encountering a strange problem when I measure the voltage across an LED. Please see below picture:
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?
led
led
edited Jul 23 at 7:43
jusaca
2,9782 gold badges11 silver badges35 bronze badges
asked Jul 22 at 8:57
Tom LinTom Lin
1721 silver badge6 bronze badges
edited Jul 23 at 7:43
jusaca
2,9782 gold badges11 silver badges35 bronze badges
asked Jul 22 at 8:57
Tom LinTom Lin
1721 silver badge6 bronze badges
edited Jul 23 at 7:43
jusaca
2,9782 gold badges11 silver badges35 bronze badges
edited Jul 23 at 7:43
jusaca
2,9782 gold badges11 silver badges35 bronze badges
edited Jul 23 at 7:43
jusaca
2,9782 gold badges11 silver badges35 bronze badges
2,9782 gold badges11 silver badges35 bronze badges
asked Jul 22 at 8:57
Tom LinTom Lin
1721 silver badge6 bronze badges
asked Jul 22 at 8:57
Tom LinTom Lin
1721 silver badge6 bronze badges
asked Jul 22 at 8:57
Tom LinTom Lin
1721 silver badge6 bronze badges
1721 silver badge6 bronze badges
2
$begingroup$
Possible duplicate of Can LED be used for both light emission and sensing
$endgroup$
– Dmitry Grigoryev
Jul 23 at 6:01
3
$begingroup$
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.
$endgroup$
– 比尔盖子
Jul 23 at 8:25
3
$begingroup$
There are more than one wires connected to it. Otherwise how can you measure a voltage across it?
$endgroup$
– user207421
Jul 23 at 11:06
$begingroup$
@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
Jul 23 at 11:19
1
$begingroup$
@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
Jul 23 at 13:52
|
show 7 more comments
2
$begingroup$
Possible duplicate of Can LED be used for both light emission and sensing
$endgroup$
– Dmitry Grigoryev
Jul 23 at 6:01
3
$begingroup$
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.
$endgroup$
– 比尔盖子
Jul 23 at 8:25
3
$begingroup$
There are more than one wires connected to it. Otherwise how can you measure a voltage across it?
$endgroup$
– user207421
Jul 23 at 11:06
$begingroup$
@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
Jul 23 at 11:19
1
$begingroup$
@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
Jul 23 at 13:52
2
2
$begingroup$
Possible duplicate of Can LED be used for both light emission and sensing
$endgroup$
– Dmitry Grigoryev
Jul 23 at 6:01
$begingroup$
Possible duplicate of Can LED be used for both light emission and sensing
$endgroup$
– Dmitry Grigoryev
Jul 23 at 6:01
3
3
$begingroup$
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.
$endgroup$
– 比尔盖子
Jul 23 at 8:25
$begingroup$
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.
$endgroup$
– 比尔盖子
Jul 23 at 8:25
3
3
$begingroup$
There are more than one wires connected to it. Otherwise how can you measure a voltage across it?
$endgroup$
– user207421
Jul 23 at 11:06
$begingroup$
There are more than one wires connected to it. Otherwise how can you measure a voltage across it?
$endgroup$
– user207421
Jul 23 at 11:06
$begingroup$
@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
Jul 23 at 11:19
$begingroup$
@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
Jul 23 at 11:19
1
1
$begingroup$
@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
Jul 23 at 13:52
$begingroup$
@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
Jul 23 at 13:52
|
show 7 more comments
3 Answers
3
active
oldest
votes
$begingroup$
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.
answered Jul 22 at 9:04
jusacajusaca
2,9782 gold badges11 silver badges35 bronze badges
$endgroup$
4
$begingroup$
You'll probably need to add a large (e.g. 1M_ohm) resistor in parallel in order to see a response reliably.
$endgroup$
– MooseBoys
Jul 22 at 19:53
12
$begingroup$
A DMM is a large resistor in parallel.
$endgroup$
– Matt Young
Jul 23 at 15:43
6
$begingroup$
Darn reversibility of physics. Gets you every time. Or, is it actually you getting it?
$endgroup$
– Yakk
Jul 23 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
Jul 25 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
Jul 25 at 2:41
|
show 1 more comment
$begingroup$
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.
answered Jul 22 at 9:15
Ronald McFüglethornRonald McFüglethorn
5961 silver badge8 bronze badges
$endgroup$
4
$begingroup$
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
Jul 23 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
Jul 25 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
Jul 25 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
Jul 25 at 2:49
add a comment
|
$begingroup$
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.
answered Jul 23 at 8:25
uhohuhoh
1,6328 silver badges42 bronze badges
$endgroup$
1
$begingroup$
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
Jul 25 at 2:58
add a comment
|
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3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
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.
answered Jul 22 at 9:04
jusacajusaca
2,9782 gold badges11 silver badges35 bronze badges
$endgroup$
4
$begingroup$
You'll probably need to add a large (e.g. 1M_ohm) resistor in parallel in order to see a response reliably.
$endgroup$
– MooseBoys
Jul 22 at 19:53
12
$begingroup$
A DMM is a large resistor in parallel.
$endgroup$
– Matt Young
Jul 23 at 15:43
6
$begingroup$
Darn reversibility of physics. Gets you every time. Or, is it actually you getting it?
$endgroup$
– Yakk
Jul 23 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
Jul 25 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
Jul 25 at 2:41
|
show 1 more comment
$begingroup$
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.
answered Jul 22 at 9:04
jusacajusaca
2,9782 gold badges11 silver badges35 bronze badges
$endgroup$
4
$begingroup$
You'll probably need to add a large (e.g. 1M_ohm) resistor in parallel in order to see a response reliably.
$endgroup$
– MooseBoys
Jul 22 at 19:53
12
$begingroup$
A DMM is a large resistor in parallel.
$endgroup$
– Matt Young
Jul 23 at 15:43
6
$begingroup$
Darn reversibility of physics. Gets you every time. Or, is it actually you getting it?
$endgroup$
– Yakk
Jul 23 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
Jul 25 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
Jul 25 at 2:41
|
show 1 more comment
$begingroup$
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.
answered Jul 22 at 9:04
jusacajusaca
2,9782 gold badges11 silver badges35 bronze badges
$endgroup$
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.
answered Jul 22 at 9:04
jusacajusaca
2,9782 gold badges11 silver badges35 bronze badges
answered Jul 22 at 9:04
jusacajusaca
2,9782 gold badges11 silver badges35 bronze badges
answered Jul 22 at 9:04
jusacajusaca
2,9782 gold badges11 silver badges35 bronze badges
answered Jul 22 at 9:04
jusacajusaca
2,9782 gold badges11 silver badges35 bronze badges
2,9782 gold badges11 silver badges35 bronze badges
4
$begingroup$
You'll probably need to add a large (e.g. 1M_ohm) resistor in parallel in order to see a response reliably.
$endgroup$
– MooseBoys
Jul 22 at 19:53
12
$begingroup$
A DMM is a large resistor in parallel.
$endgroup$
– Matt Young
Jul 23 at 15:43
6
$begingroup$
Darn reversibility of physics. Gets you every time. Or, is it actually you getting it?
$endgroup$
– Yakk
Jul 23 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
Jul 25 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
Jul 25 at 2:41
|
show 1 more comment
4
$begingroup$
You'll probably need to add a large (e.g. 1M_ohm) resistor in parallel in order to see a response reliably.
$endgroup$
– MooseBoys
Jul 22 at 19:53
12
$begingroup$
A DMM is a large resistor in parallel.
$endgroup$
– Matt Young
Jul 23 at 15:43
6
$begingroup$
Darn reversibility of physics. Gets you every time. Or, is it actually you getting it?
$endgroup$
– Yakk
Jul 23 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
Jul 25 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
Jul 25 at 2:41
4
4
$begingroup$
You'll probably need to add a large (e.g. 1M_ohm) resistor in parallel in order to see a response reliably.
$endgroup$
– MooseBoys
Jul 22 at 19:53
$begingroup$
You'll probably need to add a large (e.g. 1M_ohm) resistor in parallel in order to see a response reliably.
$endgroup$
– MooseBoys
Jul 22 at 19:53
12
12
$begingroup$
A DMM is a large resistor in parallel.
$endgroup$
– Matt Young
Jul 23 at 15:43
$begingroup$
A DMM is a large resistor in parallel.
$endgroup$
– Matt Young
Jul 23 at 15:43
6
6
$begingroup$
Darn reversibility of physics. Gets you every time. Or, is it actually you getting it?
$endgroup$
– Yakk
Jul 23 at 17:48
$begingroup$
Darn reversibility of physics. Gets you every time. Or, is it actually you getting it?
$endgroup$
– Yakk
Jul 23 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
Jul 25 at 2:29
$begingroup$
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
Jul 25 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
Jul 25 at 2:41
$begingroup$
I appreciate you guys provide any experience and inputs to me although I still can't figure out this issue now. :-)
$endgroup$
– Tom Lin
Jul 25 at 2:41
|
show 1 more comment
$begingroup$
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.
answered Jul 22 at 9:15
Ronald McFüglethornRonald McFüglethorn
5961 silver badge8 bronze badges
$endgroup$
4
$begingroup$
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
Jul 23 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
Jul 25 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
Jul 25 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
Jul 25 at 2:49
add a comment
|
$begingroup$
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.
answered Jul 22 at 9:15
Ronald McFüglethornRonald McFüglethorn
5961 silver badge8 bronze badges
$endgroup$
4
$begingroup$
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
Jul 23 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
Jul 25 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
Jul 25 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
Jul 25 at 2:49
add a comment
|
$begingroup$
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.
answered Jul 22 at 9:15
Ronald McFüglethornRonald McFüglethorn
5961 silver badge8 bronze badges
$endgroup$
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.
answered Jul 22 at 9:15
Ronald McFüglethornRonald McFüglethorn
5961 silver badge8 bronze badges
answered Jul 22 at 9:15
Ronald McFüglethornRonald McFüglethorn
5961 silver badge8 bronze badges
answered Jul 22 at 9:15
Ronald McFüglethornRonald McFüglethorn
5961 silver badge8 bronze badges
answered Jul 22 at 9:15
Ronald McFüglethornRonald McFüglethorn
5961 silver badge8 bronze badges
5961 silver badge8 bronze badges
4
$begingroup$
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
Jul 23 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
Jul 25 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
Jul 25 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
Jul 25 at 2:49
add a comment
|
4
$begingroup$
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
Jul 23 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
Jul 25 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
Jul 25 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
Jul 25 at 2:49
4
4
$begingroup$
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
Jul 23 at 19:26
$begingroup$
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
Jul 23 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
Jul 25 at 2:35
$begingroup$
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
Jul 25 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
Jul 25 at 2:36
$begingroup$
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
Jul 25 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
Jul 25 at 2:49
$begingroup$
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
Jul 25 at 2:49
add a comment
|
$begingroup$
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.
answered Jul 23 at 8:25
uhohuhoh
1,6328 silver badges42 bronze badges
$endgroup$
1
$begingroup$
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
Jul 25 at 2:58
add a comment
|
$begingroup$
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.
answered Jul 23 at 8:25
uhohuhoh
1,6328 silver badges42 bronze badges
$endgroup$
1
$begingroup$
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
Jul 25 at 2:58
add a comment
|
$begingroup$
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.
answered Jul 23 at 8:25
uhohuhoh
1,6328 silver badges42 bronze badges
$endgroup$
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.
answered Jul 23 at 8:25
uhohuhoh
1,6328 silver badges42 bronze badges
edited Jul 23 at 8:31
answered Jul 23 at 8:25
uhohuhoh
1,6328 silver badges42 bronze badges
answered Jul 23 at 8:25
uhohuhoh
1,6328 silver badges42 bronze badges
answered Jul 23 at 8:25
uhohuhoh
1,6328 silver badges42 bronze badges
1,6328 silver badges42 bronze badges
1
$begingroup$
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
Jul 25 at 2:58
add a comment
|
1
$begingroup$
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
Jul 25 at 2:58
1
1
$begingroup$
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
Jul 25 at 2:58
$begingroup$
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
Jul 25 at 2:58
add a comment
|
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$begingroup$
Possible duplicate of Can LED be used for both light emission and sensing
$endgroup$
– Dmitry Grigoryev
Jul 23 at 6:01
3
$begingroup$
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.
$endgroup$
– 比尔盖子
Jul 23 at 8:25
3
$begingroup$
There are more than one wires connected to it. Otherwise how can you measure a voltage across it?
$endgroup$
– user207421
Jul 23 at 11:06
$begingroup$
@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
Jul 23 at 11:19
1
$begingroup$
@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
Jul 23 at 13:52