Remote Living > Lighting

Christmas Lighting

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DRAYCO:
Ok, first I'm not sure that this is the best place to ask a question about this,

But this is the only place I know where there are smart ppl in electronics, and well I've always get many good answers to my questions in the past, so thanks for that.
Well, alright, the one thing that is related to this site, is that I'm saving allot of power to go with LED Christmas lights only, this year and last, so all my displays are so cheap in running that its about 2 dollars (CDN) I figure I'm spending on running them.
Well, you may or may not know, but LED Christmas lights are fixed, they cannot be removed and switched around like the traditional ones, so that brings me to my problem, I've got some of those spinning things and light up little figures that used to use. Besides other then just putting one string of traditional to solve that, I'd rather not do that.
So, I figured that, each light, most introduce a voltage drop as the more you put on, If say I have a set of 50, and 25 of them are dead or just weird to bypass the light part, the rest of the set becomes allot brighter,

I took apart a set, and cut out a section, leaving only 5 lights, and as soon as I connected it, it all blew; they became real bright for a sec, and then died completely.

So I figure I need to create some sort of dummy load to give the impression that there are 45 lights that are not there, so that the remaining 5 will work normally, and allow me to connect up my little tree figures, with out blowing them up
now I know this is likely AC power, on a string of Christmas lights, so I'm not sure, if you can connect up those, um forget their names, to it to create that load or not
That is what I want to know, what I'll need to pull this trick off?
If anyone can suggest or help me in this question, I'd appreciate it.
p.s. I'd think that next year, someone will create a consumer product similar to what I'm trying to do, I can see it now, but I don't want to wait

Nando:
DRAYCO:
Short to the point : LEDs run from 1.8 ( Red, green, yellow) to 3 volts ( white)
The do not need voltage they need current so the voltage source needs to have a resistance in series to protect them.
Regular diodes use around 20 milliamps.

If you have a string of 5 LEDs: 1.8 * 5 = 9 volts and if you have 12 volts dc then

(12 v - 9 V)/20 milliamps = 150 OHms in series with the string,

Power = (V^2)/R = (3*3)/150 = 0.06 watts is the dissipation of the resistor, therefore you need to get one that is at least 3 times higher wattage,

Use 1/4 Watt 150 ohms resistor and you will be safe.
Regards
Nando

LEXX:
I think what nando is trying to say is this: If you have a string of lights that needs a certain amount of current then your resistance has to be of a certain value.  If you have cut out 45 of the 50 lights then then voltage across each one will be 10 times as high as it should be and the current will also be 10 times as high resulting in a POOF!  What you need to do is measure the resistance across the 5 LEDs that you have remaining, lets say for the sake of arguement that it is 10 ohms, that would mean that you need to have a 90ohm resistor for a total of 100 to get the voltage and current to the proper level.  like he said a resistor in the 150 ohm range would be big enough for sure but it might be too big, just measure the resistance out, multiply by 9 and that will be the value of the resistor that you need.  Remember that resistors create heat as their way of dissapating energy so don't cover them with anything very flammable, it will only be a very small amount but still, be on the safe side.

LEXX

nothing to lose:
The other guys gave more detailed info but this is simple and what I do many times.
Take a few 1.5v batteries like AA, C, D ect.. and try one led. If it lights nicely and does not burn out then 1.5V is what to use. So wire 8 leds in series for 12Vdc  or 4 leds for 6Vdc ect.. If they don't light then take one out.

 If the leds do not light at 1.5v then I connect 4 leds in series and 3 batteries, That's about 6Vs of leds on 4.5Vdc source, if works nicely I use that, if not I take out 1 led and see if that works, if not I take out another.
Nice to know that a color LED is 1.8V and white is 3V, you can add that up for close to the correct volts then go an extra led or two for safty. Then if not working take out 1 led or two if needed. 12vdc source with all color leds then should be 6.66 leds, 7 should work nicely and not fry them. Myself I would rather heve the extra light instead of wasting the power with a resister.

For all White at 3V that would be 4 leds even at 12v.
I would like to point out that as I recall the leds Christmas lights I used "SEEMED" to be a little different than normal ones. They may be the same 1.8V for colors but I think they seemed to be a little over maybe. Been a year, so not sure. This year I can't find them, I have 1/2 a string I never used yet  but where did they go?? Any way I think the ones I had worked fine at 1.5-2v each.
Also

"but LED Christmas lights are fixed, they cannot be removed and switched around like the traditional ones"
Either I read that wrong or you have something a bit different than I had. My string was exactly like normal little twinkle lights except they were leds instead. They had they little sockets, pull them out and the led leads were bent around the sides same as the twinkle lights. Just pull the leads straight and pull them out of the holder. I could put any light in any holder, mine were all color leds. No whites.

wooferhound:
 Here is a handy series resister calculater for LEDs
http://ourworld.compuserve.com/homepages/Bill_Bowden/led.htm

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