See photo. Is this a normal LED? I'm trying to troubleshoot it and can't tell if the bulb\diode is fine.

enter image description here enter image description here

  • I was going to write, “what is this, electricalengineering.SE?” Then I went on there and it scared me. But srsly, I don’t know, sorry
    – Swifty
    Aug 9, 2018 at 18:12
  • Is this a bicycle headlamp or something? If not, I'm not sure this question belongs on bicycles.SE
    – SSilk
    Aug 10, 2018 at 13:14

2 Answers 2


It's an LED.

Easiest way to see if it is damaged is to connect it to a battery and see if it lights up.

The difficulty is that you can't disassemble the the white plastic unit, so if the LED does not light, the connection wires, resistor or LED itself may be damaged.

If you have a voltmeter you could measure the resistance across the terminals. It should be infinite one way, tens of ohms the other. If it's infinite both ways or zero either way it's trash.

Good news is that decent quality rechargeable LED lights are now inexpensive and ubiquitous.

  • Around 1400 Ohms one way and 700 the other way
    – PIXP
    Aug 9, 2018 at 18:18
  • 2
    If it doesn't light up with a new battery or a fully charged one it's trash. LED lights can't be repaired by standard users. You'd need some skills in applied electronics.
    – Carel
    Aug 10, 2018 at 9:20
  • 3
    Please remove the stuff about the voltmeter: What a voltmeter displays for an LED is totally down to the wiring of the voltmeter, not the LED. The point is, that any LED has a certain threshold voltage, and the voltmeter must apply a certain voltage to the LED to measure its "resistance". If the applied voltage exceeds the threshold, you get a low reading, if its below, you get an infinite reading, and if its close, you get some value in between. As such, if resistance seems to be infinite in both direction, this result is totally inconclusive. Dec 18, 2020 at 18:52

There's a circuit board inside the white plastic bit, which is called an LED driver.

Here's an example http://www.dx.com/p/3-6v-16v-925ma-constant-current-led-driver-board-for-cree-and-ssc-leds-4pcs-506226#.W26FJ3b-hhE which looks like this:

http://img.dxcdn.com/productimages/sku_506226_4.jpg http://img.dxcdn.com/productimages/sku_506226_5.jpg

The red and black leads go to the LED and the two round metal tracks on the other side are for interfacing to supply battery. The specs relate to voltage out, and supply voltage available as well as maximum currents.

Given your light is not working now, you're not going to make it worse with some exploratory surgery.

Now some observations:

  • There is no thermal conductor around that unit. Its all plastic. Normally a high-power LED makes a lot of heat that must be removed by passive heatsinks.

  • That LED is... tiny. Normally bike lights come with MUCH higher power LEDs that look something like this: http://img.dxcdn.com/productimages/sku_186908_1.jpg Note the complete aluminium back to allow heat transfer to a heatsink?

So if your light isn't obviously damaged on inspection (loose wire or leaked battery or corrosion) then its probably best to junk the light and buy something newer. Its done its service, time to retire it.

  • 1
    I've done this recently, converting the light to run off 5V at the same time. The driver board needs to be matched reasably well to the current needed by the LED, which is an unknown. If you know the rated power of the emitter in W, divide that by 3.5V to get the current, and most white LEDs are 100-150 lumens per Watt.
    – Chris H
    Jun 4, 2023 at 18:18

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