How to wire LED christmas lights?

big_john · January 15, 2010, 11:52pm

I just bought a few strings of LED christmas lights for $4 each on clearance at the local Target store. I would like to take them apart and use the individual bulbs for lamp posts. The lights are white globes about 3/8" in diameter. They should make excellent old fashion street lamps. Each string has 60 bulbs. I read somewhere it takes 3 volts to light an LED. I plan to put several along a street in my down town area. I was going to just wire several of these lights in parallel and use my transformer to slowly apply voltage until they light? Then I planned to just measure the voltage output after and determing how much I need to supply them when they are installed? Before I start can someone tell me if this will work?

Big John

Torby · January 16, 2010, 12:52am

You need to figure out what these LED’s are. Do you have a meter and know how to measure current and volts?

Get a fairly big resistor, like maybe 1K. Set your transformer to give you about 12v. Put your resistor an 1 LED in series and see if it lights. If not, turn it around, 'cause an LED works one way but not the other. Now you know which wire is + and which is -.

Now, with the LED glowing, take your meter and measure the volts across the LED. You’ll find something between 1.5 and 3.5 v. Now you know the LED’s prefered voltage.

Next, put your meter in series with the resistor and the LED, and set it to measure milliamps. With the LED glowing, slowly increase the throttle, and you’ll see 2 things, the LED gets brighter, and the current gets higher. Turn the throttle up just a little at a time, and feel the LED for warmth. You’ll find a current where it starts to warm up. This is probably the max current you want to run your LED, and it is probably somewhere between 20 and 30 milliamps, though I have seen 50 ma leds.

Now that you know the volts and milliamps, you can figure out about anything you want.

Look around and find what kind of power supply you can scrounge. I’m thinking of one of those transformer things that plugs into the wall. You want one that says DC or has a symbol like a bar and dots under it. If you only find an AC one, we’ll need a diode. Likely you have one in a junk box. It will say somehwere how many volts it produces, but don’t believe that. Chop the plug off the end of the wire and strip the wires back a little so you can measure it with your meter.

Now, suppose your junk transformer gives you 9volts, and your LED’s run at 3v. 9 divided by 3 is 3, so you could put 3 LEDs in series and they’ll light, assuming you have the +'s and the -'s right. If your transformer isn’t an even multiple of the LED voltage, and it likely won’t be, you’ll need to put a resistor. Figure how many LED’s you can put and need less than the voltage from the transformer. Multiply this by the LED voltage.

So maybe you have 2v LED’s and a 9V transformer. 4 LED’s makes 8v, so you need to account for 1v. Multiply this 1v by the current you measured, maybe 30mA, to get 33. In this case, you need a 33 ohm resistor. Use your own figures to get the number you need.

Somewhere on your transformer, it should specify the maximum current it’s good for, maybe 300mA. You can divide this number by the current each LED draws, and get the number of strings of LED’s and resistor you can use.

So next time you’re shopping, drop by the local Radio Shack, and tell the guy you’re looking for a resistor, and he’ll show you a cabinet with lots of drawers. You’ll find little resistors, each has 2 wires and some colored stripes, in packages of 5 for a couple bucks. Pick some that are the value you figured, or just above that. The package will say how many ohms they are. You’ll find they come in sevaral sizes, itty bitty, small, medium, big, and great big. You won’t need big ones, so get a pack of the smallest you can find of the value you want. If the package says K, Kilo, M or Mega, that’s thousands or millions of ohms, too many.

Run home with your treasure, get out your transformer, and hook up a string of leds with your resistor. Be careful to get the leds hooked + to -, + to -… because LED’s only work one way.

You can always use a battery instead of a transformer. Figure it the same way. AA size batteries will run a string of however many LED’s for a long time.

big_john · January 16, 2010, 8:21pm

Tom thanks for the reply. Yes I have a decent digital meter. I built a few Heath kits when I was younger and know what resistors are and what they do. I am probably average when it comes to soldering and basic wiring. I also had a rather large O gauge and HO gauge layout at one time that I did all the wiring on. I am converting all my engines to RC/Battery power. I purchased three RCS systems at the last ECLST show with batteries and a charger.

What is the reason to wire the lights in series rather than parallel? Do I have to have a resister in series with the lights for them to work? I figure I will end up with around 12 street lights. I thought putting them in parallel would make the wiring simpler as I could get by with one feed wire down the entire street and just tap in for each light. If I have to wire the LED’s in series of 3 or 4 lights per set that would require three or four wire feed lines. I have several yard light systems and they are all wired in parallel but they do not have LED lights. In fact I was planning on using one of the transformers from an old set that I removed. I thought I could put in a dimmer switch to operate the lights at a certain brightness.

Big John

Torby · January 16, 2010, 10:31pm

You’ll be in great shape.

An LED is quite different from a light bulb. First, they are low voltage things, usually between 1 1/2 and 3 v. Second, an LED has a specific operating voltage. With the resistor in series with the led, you can adjust the throttle, but the voltage on the LED will stay the same. Odd, eh?

You can put them in parallel, and run them with something their voltage, like 1 or 2 batteries. It’s also common to run 3v LED’s from those little flat lithium cells. In parallel, they add their current, so if each draws 20 mA, 12 in parallel will draw 240 mA, which will run a battery down pretty fast. If you have a low voltage power pack, this will work great. If you have a higher voltage, you can string several in series to adapt to the higher voltage.

Your Christmas lights are wired in series so their current limiting resistor doesn’t have to drop a hundred and some volts, and dissipate lots of watts. Since each string has 60 bulbs, I’d guess they’re 1.7 or 2v leds.