Another question on shortening a full wave rectified LED strip

T

TraxtarXKR

New member
I have a dimmable full-wave rectified series LED string that I need to shorten. It's the high-end type from one of the major online suppliers.

The set has 100 LEDs in total, separated into 3 sections of 33 or 34 LEDs.

I've found the following PDF, but I'm not sure how it applies to what I'm doing since I already have a full rectified string.

Here are the parameters for the string:
  • Rectifier: Full Wave Rectified
  • Voltage: 120 Volts
  • Watts Per Set: 6.9 Watts
  • Amps Per Set: .06 Amps
  • Dimmable: Yes, through voltage control
You can see that the cut section is not illuminated, and I haven't connected power in fear that they will immediately burn out.

I've measured 2.5v DC across the first LED in the series.

20260219_162733[1].jpg

I've cut the string almost exactly in half to fit my application, so I'm left with a full section of 33 LEDs, and a half section of 15 LED's. I simply need to know what components need to be added to the end of the 15 LEDs to have them light up the same as the rest of the string.

I've read through the different forum threads, but many of the links are dead and I'm at a loss with how to proceed.

My initial thought is to add a single (2.5vDC / 0.06A) * 15 LEDs = 625 ohm resistor @ 3W, but that feels wrong.

Can anyone assist?
 

LED String Shortening Resistor Calculator

I know there are always questions about shortening LED strings and what resistor values to use. I created an excel calculator to determine the value of resistor needed. I created it in excel 2007 and I don't know if it will work properly in 2003 or earlier. You put in the peak voltage, LED...
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This string calculator might be helpful, and it's giving a different result from the 625 ohm resistor:

Inputs
Voltage: 125 * 1.414 = 176.75V
LEDs: 15 remaining in the set
LED Current: 0.06A per string / 3 sets of 34 LEDs per string = 0.02A per set
LED Voltage: 2.5vDC
Total Voltage Drop: 37.5V
Resistor Voltage: 139.25V

Calculated Values
Resistor Current: 0.02 Amps
Resistor Value: 6962.5 Ohms
Resistor Wattage: 2.785 Watts

Standard Values
Resistor Current: 0.02 Amps
Resistor Value: 7500 Ohms
Resistor Wattage: 3 Watts
 
It's similar to cutting the old incandescent strings. Each bulb (LED) shares the voltage, so when you remove nodes, you would need to make up that much voltage drop.

In the incan days people sometimes would use a large resistor to do this. I've never seen it done with LEDs but I would assume it's the same concept.

Honestly I think your best bet is to just mask off the unused nodes, or better yet buy strings that match the length you need, if possible.
 
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