Series / Parallel LEDs and resistors


While not arduino related (yet) I have a question about LEDs in series and parallel.

I have a strip of leds on a specialty circuit board that are about 80% burned out. The manufacturer, if they still sell them, probably wants eleventy million dollars!!! So, I am going to get some practice soldering SMT leds.

My power source is 36v, and there are 6 banks of 5 LEDs wired in series for a total of 30 LEDs. Interestingly enough, each bank of 5 LEDs has 4 resistors (read 121), one in between each LED. So, there's six banks of the following:

power - LED - resistor - LED - resistor - LED - resistor - LED - resistor - LED - power

I'm not quite sure how to calculate the voltage each LED receives, my first inclination is to go with 36v received by each bank wired in parallel becomes 36 / 5 = 7.2v. But I'm not quite sure what is the point of wiring so many resistors in. With my limited electrical knowledge, I would have put in:

power - resistor - LED - LED - LED - LED - LED

So, anyone have an idea of the replacement values of these PLCC LEDs?


Resistors generate heat.* By using multiple resistors each resistor can be smaller, and spreading the resistors around on the board means you don't get all of the heat in one spot. Your one-resistor circuit design would work just as well as long as the resisor can handle the heat.

I have a strip of leds on a specialty circuit board that are about 80% burned out.

SOMEHTING'S WRONG! Is that board rated for 36V? LEDs do fail once in awhile, but like most solid state electronics, they normally "last forever". I've worked for a small electronics company for more than 10 years. We probably go through about 1000 LEDs a month, and I don't remember ever seeing a returned unit with a dead LED. LEDs are sometimes "dead on arrival", so we have to change one in a new unit before we ship once in awhile, and we've probaly had a few "early failures" during our 1-week elevated temperature burn-in. But after they are "proven good" and shipped to customers, they just don't fail.

I'm not quite sure how to calculate the voltage each LED receives

You can't calculate it. You can measure it, or check the specs. LEDs (like all diodes) are non-linear devices. Once the LED comes-on, the voltage remains (almost) constant and as you increase current, the brightness goes up. You can calculate the current by measuring the voltage across one of the resistors, and then use Ohms' Law.

  • Power can be calculated as Voltage squared/Resistance. (i.e. You measure the voltage across one resistor, and calculate the power for that one resistor.)

36V when the 5 LEDs only drop maybe 18V (blue or white LEDs, and even less for Red, like 11-13)? Seems high.
121 should be 120 ohm resistor.

LEDs are pretty inexpensive - 12-20cents each kind of price.
What physical size are they? 0603? 0805? 1206?

See "package/case" filter here:

They are a little shy of 1/8" square. It's a pretty standard size, but I don't have a metric ruler around to check right now. I'm sure at whatever size, LEDs are cheap compared with what they want for the board.

Heat dissipation for the resistor pattern makes sense. It's a part to an espresso machine, and thus is subject to heat around 100C pretty much 24/7. The LEDs aren't indicators either, it's part of a backlight that's on, 24/7. So it's possible they have reached end of life, though by the pattern of failure (dead near the hottest parts, still alive further away) , I'm going with them just getting cooked.

So, anyone have a trick for replacing the LEDs with something that can take the heat a bit more? I don't think I can just run a larger LED like you could run a higher wattage resistor...

What's the voltage across a working LED?
Just apply 5V across an LED and one resistor.