5 volt LED

The little red LED on the dishwasher that shines on the floor to let you know it is running (fairly new, and pretty quiet) has stopped working again. First time it was fixed under warranty, but now 4 years after purchase (warranty was 3 years but supplier didn’t honour the free 2 year extension because I registered after 90 days :mad:) it's stopped again - well, it flashes briefly at the start, then stays off, but there's still (exactly) 4V going to it.

Most of the pre-wired (I'm lazy) 5mm LEDs for sale on eBay look to be 5-12V with "brightness varying with voltage" so with just 4V they might not be very bright. I want to buy a simple 5V (surprised that dishwasher only supplies 4V?) LED, maybe I'm better off trying Farnell? but don't want to pay the earth for postage...

Basically Dave, no such thing as a 5v red LED.

Different colours drop different voltages across them when operating due to the semiconductor materials used; e.g. red are usually 1.8v to 2v; green around 2.2v, blue somewhat higher.

The normal way of driving an LED is from a constant voltage via a resistor; the voltage drop in the LED will give the voltage across the resistor and therefore the current through the pair (e.g. a 5v supply to a red LED will drop 3v across the resistor, so a current of 1mA on a 3k resistor, 10ma on 300 ohm etc (V = IR; I = V/R))

So if there is nothing hanging on the back of the existing LED it is most likely a standard (i.e. cheap) led with a resistor on the circuit board feeding it. I'd be looking at something like this: https://www.ebay.co.uk/itm/152508668100 though they're expensive to a tight git like me - I just bought a couple of hundred for a project (surface mount, though) at 0.7c each

The LEDs you see advertised as '12v' include either an inline resistor or a constant current chip, but unless your existing LED has something hidden in it (or in heat shrink close to the LED) then it's most likely just a standard beastie.

Beware: they are polarity conscious; wired the wrong way around probably won't hurt them, particularly if there's a series resistor, but they won't light up. That style normally has one lead longer than the other and *may* have a flat on the case; observe the original.

Neil

Just to (possibly?) add to the above post, if you're measuring 4 volts at the LED, then you might have found the problem. Red LEDs run at the lowest voltage of any, and your machine might be supplying twice the required volts. Are you sure you're measuring at the LED; there's no other circuitry? It's interesting that the LED does still work to a degree.

If it is just a plain LED then I'd be tempted to add a an inline resistor (say around 4/500 ohm) to check that voltage. You only need a few ma to make the LED glow nicely. Factoring in more current doesn't always give a visibly brighter result. Hopefully wherever you buy from will have a spec sheet for the voltage and current and you'll be able to plug the numbers into Neil's formula. If you don't have a calculator to hand then there are loads of online tools - LED resistor calculator