I have a string of 10 LEDs in series. Occasionally some LEDs area breaking and the entire string stops working. Can a TDR (Time Domain Reflectometry ) or other measurement be used to determine which LED specifically is broken? I have never used TDR for anything other than impedance measurements. The LEDs are encased in plastic so cannot be measured or accessed directly.
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\$\begingroup\$ Please edit to explain what a TDR is. \$\endgroup\$– TransistorCommented Nov 11, 2020 at 23:45
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2\$\begingroup\$ Time Domain Reflectometry and frankly, I wouldn't even try. I'd consider a 4 or 5V Zener across each LED (assuming 20mA LEDs not 330mA ones) : the Zener will conduct and keep the string going; allowing you to find the faulty LED visually. \$\endgroup\$– user16324Commented Nov 11, 2020 at 23:49
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2 Answers
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No, a Time Domain Reflectometer won't help in this situation. A TDR sends a pulse down a cable and analyzes the reflection. The pulse will never get past the series LEDs. Well, maybe some might get past, but not nearly enough.
I have used a non-contact voltage tester to find breaks in line-powered LED strings. You may be able to inject AC into your string and do the same. You would need to find a safe voltage that was still high enough for the tester to detect.
Another option is to pierce the insulation with a very thin pin/needle. Use a binary search to find the issue. Recommended for low voltage circuits only.
answered Nov 12, 2020 at 0:06
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\$\begingroup\$ I used in the same case non-contact voltage tester. But it was LED string powered from 120 VAC and had diode and resistor in series. Works. \$\endgroup\$ Commented Nov 12, 2020 at 2:21
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I use an inexpensive Amprobe pocket multimeter PM55A. It has a position called "Electric Field" that beeps like a Geiger counter. The test leads are not used. The top left corner of the meter is moved toward the conductor, the beep rate indicates the strength. I have used it on cat6 POE cables, zone valve wires, and low voltage lighting. A tone generator/tone probe combo for telephone wiring will also work.
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1\$\begingroup\$ Geiger counters click. It is metal detectors that make beepy noises. \$\endgroup\$– JRECommented Nov 12, 2020 at 6:56