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50 LED Light String Answered

Im trying to wire a string of 50 LED's that are 200mA each. They are all connected in one long string, end to end (in series, I think). I'm having trouble trying to figure out what kind of power source and resistors to use and how to wire the resistors in. In the past I've wired 1 or 2 LED's to a 12 volt power source using a couple of resistors in parallel, but this project is much bigger and I'm worried about current loss and the very high voltage needed to power a string of 50 2 or 4 volt LED's in series... is that like, 100000 volts, or what?? Any guidance would be appreciated.


I tried this and it doesn't work. The problems that occur are LEDs light up with different brightness, and secondly my string got pretty hot on 120v. You have to go with much smaller groups in parellel.

were you trying to connect them to 120 AC? cause that would most likely explain the heat. LED's only pass electricity one way. AC tries to ram current back through the other way as well. should use an inverter unless the LEDs were designed for AC..

So if I use a power inverter I can keep the string wired in series? problem is- newbie that I am, I already wired the string, and just wired it in series because thats how it came in the 1st place... I think- (LEDs are from a GKI/Bethlehem light string- was too short and needed to be extended to fit the app.) Id really like to avoid rewiring, but will do it if I absolutely have to.

Well, most normal LED lights are designed to use DC current. What comes out of a wall socket is AC. I'm not entirely sure how most LED Christmas lights are set up, it could be that half are wired for one polarity, and half are on another circuit with the opposite polarity, or they could be designed for AC. Aditionaly, the strings were set up to have a certain number of lights to account for the voltage. removing half the lights could cause problems. What I was saying is that taking normal Radio shack LED lights and wiring a pile of them to an AC circuit is asking for a fire. Half the AV current is going to try to flow through the light the wrong way. this could, and almost certainly will generate a lot of heat and possibly small explosions I would have wired them all in parallel, then found a 2-5v supply that could provide the right amount of amperage. Much safer than trying to run 100v. If you are absolutely stuck with them wired as they are, you will have to find a 100V supply somewhere and do your best. You could try cutting every third or fourth light, and then rewiring all those series "chunk" sections into groups of parallel lights. then all you would need is a DC brick delivering 6-8v at an amp or so. Else, if you are brave and want to cheat, just go to Radio shack or Home Depot and find a beefy variable resistor (pot) and wire that in. Make sure it can handle the full voltage and then some. think wall style dimmer switch...then plug it in and crank on the pot til the lights look right. duct tape it down and away you go.

Very Helpful. Thanks. So the consensus seems to be that the best thing to do is to wire the LEDs in a parallel/series arrangement with say 5 LEDs connected in series with each string connected in parallel for 10 parallel strings. The current would be 200mA times the number of parallel strings - 2 amps and the voltage would be the 5 series connected LEDs or 17.1 volts, which means I could power the LEDs with an input voltage of 20V and 2 amps of current- then use resistors at the beginning of EACH series chain (10 resistors in all) to bring the 20v down to the 17volts of forward voltage needed for the LEDs. Does this sound right or do I need to do more homework? And can I just pick a 20volt2/Amp power supply up at a local Radio Shack or Home Depot... or do I need to start studying THAT now too? ;-)

20v over each LED is WAY too much. LEDs tend to range from 2-4v. Putting that much foreword voltage will just kill the LEDs

even in a series-parallel chain with 5 LED's in series?

wait, I think I misread -- you want 20V over 5 LEDs... that should work for 4v rated components ;) Remember, you can always go a little less and have your components last longer ;)

If you want to go DC - I'd use a laptop power cable. Mine is rated for and output of 18.5v and 6.5A max which should do the trick.

Half the AV current is going to try to flow through the light the wrong way.

Just to reiterate -- this is not a problem. The "D" in LED is diode, a diode's job is to make sure current can only flow in one direction. Thus, they are designed to take current in either direction (to a design limit) but only pass in one direction.

Perfect example is Fridge Lights which uses AC to power their LEDs ;)

Good reference. I had not really read that one. Have to try it. The only thing I want to warn about is that they are using a 4.5 Volt, 500 milliamp AC power supply. This limits any possible danger by providing a very low fixed voltage and amperage. (It would be hard to shock yourself with that amount of voltage). Attempting to do the same thing with current straight from the wall without any regulation on the amperage is a tricky proposition.

amperage on those wall worts are not fixed ;) Just voltage. The rating is a max. The same for mains power. The only current you have is what you draw from it - LED components tend to be "greedy" - the reason for a current limiter ;) I guess that's like a lot of people -- don't limit intake, and they'll take too much :P

Dealing with mains power really isn't magical when all you're only dealing with diodes and resistors :P

LEDNewbie, if you're still having problems after a few weeks, I'll be taking apart the strands I acquired after the holidays ;) I have a bunch of different types and brands and I'll post a circuit diagram for ya ;)

You need some sort of current limiter -- just a resistor to prevent too much from passing through. I'd follow a similar schematic to what the lights were originally wired for. Probably a 25x2 array (that is two sets of 25 LEDs in series). If you have a string still intact from the box, plug it in a remove one component -- did all of the lights turn off? Or just a portion? If they all turned off, then they are all in series which means 2.4V over each LED and a total current draw of 10A (pretty high).

if it doesn't get hot, you still have the other problem of different brightness of the LEDs as they all don't have exactly the same power consumption.

You can use any LED on AC power without a problem -- you're just going to get a 50% duty cycle.

I bought a bunch of LED strings after the holidays. They run on 120 no problem, except they oscillate at 60Hz which bothers me a bit (POV action when they move). There appears to be some sort of current limiter (probably just a resistor) and they're wired in parallel for two sections of 30 LEDs in series - 3V over each component. No heat issues, they rise about 5oC after a few hours when I bunch up about 8 strands in the same pile.

Just FYI, an inverter outputs AC (input DC). A rectifier outputs DC (input AC) ;)

Right. I think his problem is that the string of lights may have been trimmed down, and he needs to compensate for the other 20+ volts. Generally, I don't like messing with straight AC if I don't have to. kinda dangerous. (and I have kids..so .. you know... )


11 years ago

50 2-volt LED's in series gives you 100 volts, which should be your clue that they're probably not in a series ;) (actually it's possible they may be if they were originally designed to plug directly into 120V mains AC). Anyways, if they're wired in series then you'll only have two wires. If you have more than that then you likely have a couple of parallel chains of a smaller number.

Yeah, they were meant to be plugged in to 110Vac directly. So if I do that, what value of a resistor should I use? and wont I need something to bring down the Amperage that comes out of the wall to 200mA?

Each LED consumes about 2V . if you match the power source exactly say for example 5 LEDS with a 10volt source I found a current limiter(resistor) wasn't needed. If you open dollar store LED keychain/device you will find no resistors in the circuit.