Cutting LED Strings

[Ralph A]

I am planning my LED Pre Order sales for the year and had a question about cutting led strings. I have read that you can cut a string in half as long as it has a rectifier at the half way point. (i.e. one prior to the first light and one after light 50 on a 100 ct string) Is this true? Have any of you done this and were there any disadvantages to doing so, such as them burning out faster or not as bright etc. etc.?

Thanks Ralph

[Max-Paul]

Yes this is possible as you describe it. Although there needs to be some work done to get the other half to work again. Although it is possible. So you are spending lots of money to only get half of the string to work. Why not just buy a 50 ct string and be done with it? And no reason they should burn out faster. Now if you cut one or two lamps out of the string, then you increase the voltage across the remaining LEDs in the string. This will also increase the current through each lamp. This will decrease the life of the individual lamps.

[Ralph A]

Thanks Max! I only used 100 cout as an example. What I am doing specificly is this.

I am building a 5 person singing choir for 2012 all LED. The bottom of the mouth is a separate channel. It will only consist of 10 or 11 led lights. The most cost effective is a 32 count string. I wanted to keep as few wasted lights on the back of the prop as possible. Therefore I wanted to take a 32 count string and divide into a set of two or three for the bottom lip. Any Ideas?

Thanks Ralph

[Max-Paul]

Here is the problem Ralph. Lets say each of these LED in the string needs 3.5 volt each. And when you put a full bridge rectifier, you get 150VDC reading the RMS value. Ok for the math, 3.5 times 32 equals 103 volts drop across the LED. So we have 47 volts that needs to be dropped across something other than the LEDs. So we add a resistor based on the voltage that we need to drop across it and the current that needs to flow in the circuit.

Now I have been familiar with OHMs law for more than 3 decades. But I forgot how things work in a dirty DC circuit. And forgot about Peak voltage and Peak current and how a DVM reads RMS values. Thus I wanted 20mA but actually my meter did not take into account that I would have Peak values at the LED. So I do the math then tweek the circuit for 15mA.

BTW the above is an example of the 32 lamp circuit. That resistor will have to hold back more like 115VDC. But also the wattage of the resistor would have to be like 115V X .02 Amps, or 2.3 watts. So, your resistor needs to be 2.5 watt rating or better or it will burn up. BTW this will generate a lot of heat.

I do not know your situation, but I would recommend a DC controller and then a power supply with a voltage high enough to do the max LEDs with voltage dropping resistors for the circuits with less LEDs.

I can give more accurate info if I knew more about your project in details. Do you own a DVM or VOM? You need one for this kind of work. Nothing fancy, even a little pocket unit that can measure up to say 200 Volts AC and DC. And current up to 1 Amp.

[Ralph A]

Max I do own an old but very good meter. I have a basic understanding of electrical stuff, nothing fancy.

I am not worried about the few dollars that I'll save as much as keeping the lights on the back side of the prop to a minimum. The bottom lip will only use about 8 to 10 lights that leaves about 22 on the back side. However I do not like the idea of the resistor generating a lot of heat. If I split the lights in half 16 on each side would that make it any easier? If not I'll just order the 32ct strings and be done with it. What do you think?