Voltage generated by an LED light??

bob280zx

Observer
I have 6 of these - Kawell K5-5118 18W. They are split into 3 different switched circuits left, rear, and right on the roof rack and work fine.
I noticed voltages on the light side of the relays - with the battery disconnected. To make a long story short, this is what I found:

Kawell.jpg

The individual lights vary from 1.35 to .39 volts. I know enough to attach lights and make them work. Anything past that and I'm liable to electrocute myself so I find this curious. Are LED's miniature generators? :)
 

Burb One

Adventurer
To get it straight, you are measuring a voltage on the positive wire going to the LED from the relay, even with the battery unconnected?

Could be a residual charge of some type in the light such as a capacitor or something that doesn't have enough juice to excite the diode, so it just stays there. Kind of like if turning off old or some electronics, it takes a second for the off light or sound to die out even after switching it off...

Maybe a good analogy is if you have a pipe with a "u" in it, like a sink catch. When the water is on, there is enough water flowing, to "overcome" the "u", but when it turns off, there is still some water stuck in the u.
 

bob280zx

Observer
To get it straight, you are measuring a voltage on the positive wire going to the LED from the relay, even with the battery unconnected?

That's where I first saw it. The test leads in the photo are connected only to the light itself which prompted the question.
 

dwh

Tail-End Charlie
Still seeing voltage with the lights covered? Then the voltage is coming from something else inside that housing.
 

Burb One

Adventurer
Good advice- Get a small incandescent light or something that draws a small current and connect it across the positive and negative of the light (the right tool would be a circuit tester with an light on it until the light went out), then retest and see if the current is still there.

If it were me doing it IE Bad advice- QUICKLY short the red and black of JUST THE LIGHT, as long as you are sure it's not connected to some other source, then retest and see if the current is still there.


*Blah, blah, I am not responsible for any actions and this is not sound advice, blah, blah
 
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bob280zx

Observer
Still seeing voltage with the lights covered? Then the voltage is coming from something else inside that housing.

See why I'm confused? :)

What in the world would be inside? I've got a bunch of different LED lights and none of them show this. All six of the Kawells do. I'm tempted to take one apart but then I wouldn't really know what I was looking at if it was more than LEDs and wire.
 

Burb One

Adventurer
See why I'm confused? :)

What in the world would be inside? I've got a bunch of different LED lights and none of them show this. All six of the Kawells do. I'm tempted to take one apart but then I wouldn't really know what I was looking at if it was more than LEDs and wire.

I'm 99% sure there some type of capacitor in the circuit holding a charge not enough to excite the LED , doing the good advice above should discharge it.
 

bob280zx

Observer
I'm 99% sure there some type of capacitor in the circuit holding a charge not enough to excite the LED , doing the good advice above should discharge it.

Lo and behold I believe you've got it. ( I think) Is the little brown thing a capacitor? Hooked a test light to it and after a minute or so, no voltage. I'm going to assume there's no issue with a volt or so hanging around?
Kawell.jpg
Thank you gents - learned something new so that makes it a good day.
 

Burb One

Adventurer
Cant see on my phone, what that "middle brown swirl thing" �� is, but that "cap looking thing" that is horizontal is definitely a capacitor. Yes, there's no harm with some voltage staying in that loop with just the light!

Also if it doesn't have a good rubber or other gasket/ seal, use some good black silicone putting it back together! It if doesn't have a good seal, I would take them all apart and seal them all!
 

dwh

Tail-End Charlie
Aye, that is certainly a cap. Like a very fast charge/discharge battery. You really wanna be careful if you ever horse around with big ones...they bite. Hard.

So it's there as part of the voltage regulation/power supply.

Lots of LED fixtures have s bit of circuitry inside. LEDs don't like over voltages.
 

4x4junkie

Explorer
It's 100% completely, totally, & entirely harmless.

That said, LEDs are semiconductor p-n junction devices and often will exhibit a slight photovoltaic effect when exposed to light (this is the same property on which solar cells work). Exposed to sunlight an LED appears to produce about 1.6 or 2.2V depending on type (red, blue, white, etc.), at a few microamps, so there is some possibility that could be what you are measuring.
 

dwh

Tail-End Charlie
Yes, that little one is harmless. Maybe that's why I specifcally said,"big ones". :)
 

rayra

Expedition Leader
I'm trying to fathom why that cap is attached in a manner to bridge that surface-mount resistor(?). It's like they're simultaneously throttling power on that pathway, while using the cap like a step-up transformer to spike the power at that point. It's like that overly complicated crappy 'common core' math, they want a different power level at at the other side of that gap but instead of getting there in one move with a resistor OR cap, they are making two moves in opposite value directions to arrive at a sum that gets what they want. Like wanting -1, but instead of subtracting 1 they are adding 5 then subtracting 6.

Like they built that circuit, did the production run, then discovered they'd throttled the power down too much, so the production fix was 'stick a cap on it at R3' to rectify the undervoltage and make the whole thing light up properly.


/typically you can get away with putting LEDs in a serial run, if their combined need is slightly under your input voltage. Their combined draw seems to exceed the available power. So they stick a cap on it to boost the juice enough to get by? When a proper series of lower-value resistors would have been the correct solution. Instead of re-working every resistor, they did it in one move by adding the cap. An efficient manufacturing fix to a poorly designed circuit. All depends how they laid out their circuit(s). Just seems like a hack(ed) layout.

(shrug) as long as it works. But that cap is a failure point. OP, if that light fails in the future, the first thing I'd do is take a look at that cap for typical signs of failure. Might even be wise to get some spares right now, while you have it open and can identify at part number on it.

eta
that's all supposition on my part, and only generally true if that's actually a resistor being bridged by that cap.
 

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