# LEDs with resistors before and after... why?

The other day a friend asked me if I could mod one of those cheap chinese LED lamps they sell at the dollar store to use an AC adapter. So I did.

Taking it apart, as you'd expect the inside was horrible. Wires hanging onto each other with gobs of solder, switches in mid air with no support, stuff like that. Well, for \$2 what can you expect.

What I found interesting was that the circuit itself is 3 LEDs in parallel, with resistors before and after:

(Can't remember the resistor values, and I don't have any desire to take the screws off again and look :) )

Why the resistors before and after, and not simply in one place? Is there any special reason, or did they just have two types of resistors stocked up they needed to get rid of?

I think you're probably right: available stock/convenience/ignorance. It is definitely wrong! I expect you already know this but someone else reading this may not. No two leds have the exact same forward voltage due to manufacturing variations. Whichever led has the lowest Vf will draw more current than the others and burn out sooner. Once that happens, the series resistors are no longer the correct value, causing them to drop too little voltage which means too much voltage for the rest of the leds, causing one of them to burn out and so on. The moral of the tale is that when connecting leds in parrallel, each should have its own series resistor.

I only ever did a vaguely similar thing on one situation. Charlieplexing a bunch of leds. Only one led was lit at a time, so no need for transistor drivers. I just calculated the correct series resistor and halved its value. Then put a resistor on each Arduino pin. The current was sourced out of pin A, through one resistor, a led, a second resistor and sunk into pin B. (5 pins, 5 resistors, 20 leds)

Paul

The resistors might have been of a low enough wattage that the heat needed to be spread between two of them, but that doesn't explain why there's one on the front and one on the end. Like PaulRB (who kinda missed the question and overfocused on the parallel LED thing ;)) I'd guess it's either layout convenience or stock.

Good old chinese engineering... blue one is on the VCC terminal, grey one is on the ground terminal. They look a good 1/2W at least, so it doesn't look like they need to split up power.

As a side note, I swapped out the LEDs with some standard ones of a different color from my local shop, and left the circuit as it was. And not surprisingly, the new LEDs fried. They're probably relying on higher consumption LEDs to make up for the limiting resistor problem.

A resistor placed on both ends: No single point of failure.
This is “advised” [SOP] for the input of H11AA1 (and the like) when on the mains, too.

And a few drill holes and cut traces later...

No single point of failure.

That makes no difference, both resistors are now single points of failure.