How to cut mini light string?

[JBullard]

AndyK wrote:

I think I know the answer to this, but wanted to throw it out there to confirm. Can you cut a string of 25 (c9) into two strings of 12? My gut is saying no, since you are not dealing with a 50 100) string set which is really 2 sets 25 (50). Am I thinking correctly on this?

If they are Incadescent C9s then yes, you can cut them to any length. Each incadescent bulb is 120 volt rated, and they are wired in parael not series

[JBullard]

CKSedg wrote:

What I'd really like to know is:  Can an LED string be cut to EXACTLY the length wanted, using resistors to make up for the lights cut out?  Can anyone tell me how to calculate which resistors, and how to wire them in?  Thanks!

It will depend on how the LED string is made.

Some things will will need to know about your LED string.

Full wave or half wave.

How is it wired - all series or a series/parallel combination.

What is the forward voltage drop the LED is rated for. What is the current rating of the LED.

Then use this calculator:

http://www.theledlight.com/resistancecalculator.html

[CKSedg]

JBullard wrote:

CKSedg wrote:

What I'd really like to know is: Can an LED string be cut to EXACTLY the length wanted, using resistors to make up for the lights cut out? Can anyone tell me how to calculate which resistors, and how to wire them in? Thanks!

It will depend on how the LED string is made.

Some things will will need to know about your LED string.

Full wave or half wave.

How is it wired - all series or a series/parallel combination.

What is the forward voltage drop the LED is rated for. What is the current rating of the LED.

Then use this calculator:

http://www.theledlight.com/resistancecalculator.html

I don't think I'm smart enough to give you all the answers to your questions. I know that most of my lights are full wave, but I have a few half wave also. I would like to know for both of them how to cut them. I know the difference between basic series and parallel, but don't know how to tell if they are a series/parallel combination. And I don't know the voltage drop. Is this too complicated to worry about? I can just tape off the lights I don't want showing, if this is too much.

Thanks!

[JBullard]

Without the info on the LEDs then the calculations are impossible.

With incadescent minis, you are mainly dealing with voltage drop.

With LEDs, you first have to know the current in milliamps the LED is rated for, then the max forward voltage rating.

[Steven]

The forward voltage for an LED is constant. For red and orange, it is about 2.3 volts. Green, blue, and white are about 3.6 volts.

Maximum current varies, depending on the size of the junction and the ability to dissipate heat. It's usually about 30mA.

The LED resistance calculators you find online are designed for DC circuits, for example 12 volts DC. For AC (LED strings) it gets a little more complicated because unlike incandescent bulbs, LEDs are not linear, which means the RMS voltage is only an approximation.

I built a 50-bulb red LED string out of discrete LEDs I bought on eBay for my "Tune To" sign. I determined the resistor value by trial and error, by measuring the current while slowly raising the voltage using a variac.

Here's how I would determine what resistor to use for your string: Since you're planning to cut and splice the string anyway, first cut it near a bulb and measure the current using a multimeter. Be careful while doing this because you'll have line voltage! You'll use the DC current setting of the meter. This value won't be 100% accurate because of the non-linear effects mentioned above, but it will be close enough.

Next, add the voltages of the LEDs you plan to remove. Use the voltages based on the colors. Divide this total voltage by the current you measured. This is the resistor value to add in series. Don't remove the existing resistor.

To double-check, measure the current of your new string. If it's higher than what you measured originally, then you should increase the resistor value, or the lifetime of the bulbs will be reduced.

Don't forget to calculate the power used by the new resistor. If it's more than the power rating of the resistor, then it will get hot and fail.

Since the current will be the same as before, your new string will consume the same amount of power. Unless wire length and clutter is a problem, using black out caps is better.

[CKSedg]

Steven wrote:

The forward voltage for an LED is constant. For red and orange, it is about 2.3 volts. Green, blue, and white are about 3.6 volts.

Maximum current varies, depending on the size of the junction and the ability to dissipate heat. It's usually about 30mA.

The LED resistance calculators you find online are designed for DC circuits, for example 12 volts DC. For AC (LED strings) it gets a little more complicated because unlike incandescent bulbs, LEDs are not linear, which means the RMS voltage is only an approximation.

I built a 50-bulb red LED string out of discrete LEDs I bought on eBay for my "Tune To" sign. I determined the resistor value by trial and error, by measuring the current while slowly raising the voltage using a variac.

Here's how I would determine what resistor to use for your string: Since you're planning to cut and splice the string anyway, first cut it near a bulb and measure the current using a multimeter. Be careful while doing this because you'll have line voltage! You'll use the DC current setting of the meter. This value won't be 100% accurate because of the non-linear effects mentioned above, but it will be close enough.

Next, add the voltages of the LEDs you plan to remove. Use the voltages based on the colors. Divide this total voltage by the current you measured. This is the resistor value to add in series. Don't remove the existing resistor.

To double-check, measure the current of your new string. If it's higher than what you measured originally, then you should increase the resistor value, or the lifetime of the bulbs will be reduced.

Don't forget to calculate the power used by the new resistor. If it's more than the power rating of the resistor, then it will get hot and fail.

Since the current will be the same as before, your new string will consume the same amount of power. Unless wire length and clutter is a problem, using black out caps is better.

Steven, thanks for all the information. I wish I were that smart, but I think I understand how to do it now! Maybe your last statement about using black out caps is the better way to go. I really appreciate your help very much!

[AndyK]

perfect!!