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There are 9 LEDs (3 V each) connected in series in the LED strip. I tested individual LED with 3 V applied across, and it works fine, but when 9 × 3 V is applied across the whole strip, the LEDs are barely lit; each one shows very dim light and the power supply shows only 1 mA or less being drawn (the power supply only shows 0.00).

Another strip that works fine draws 100 mA. There are no resistors or caps on the strip.

What could be causing such high resistance in the path when individual LEDs work fine? If it was an open connection, the LEDs should not be lit at all, but they do light up faintly.

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  • \$\begingroup\$ Please clarify your specific problem or provide additional details to highlight exactly what you need. As it's currently written, it's hard to tell exactly what you're asking. \$\endgroup\$
    – Community Bot
    Commented Oct 27, 2022 at 5:22
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    \$\begingroup\$ How do you know you're applying 3*9 = 27 V? Might you be applying 26 V? LEDs have a very steep I/V curve, which is why we current drive rather than voltage drive them. Is your DVM accurate on the higher range? Are you simply putting 3 x 9 V batteries in series and hoping the voltage is 27 V? \$\endgroup\$
    – Neil_UK
    Commented Oct 27, 2022 at 6:11
  • \$\begingroup\$ I have a lab DC power supply that is very accurate. I tried varying the voltage above 27. The brightness stays the same (i.e., very dim, hardly visible). Tested all 9 LEDs with 3V and they all works fine and show bright light. Other strips works ok (there are 5 strips). There must be some very high resistance in the path. \$\endgroup\$
    – Allen King
    Commented Oct 27, 2022 at 15:58
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    \$\begingroup\$ There must be either broken PCB trace or broken solder join on that LED strip. I would suggest testing 2 LED's at a time (using 6V) - finding 2 LED's next to each outher in series chain and applying 6V - you should do 8 of those tests (first-second, second-third,..., eghts-nineth LED). Also do testing strip positive to first LED negative and strip negative to last LED positive (3V here, of course). \$\endgroup\$
    – Volodymyr Kalinyak
    Commented Oct 29, 2022 at 7:32

1 Answer 1

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These strips are long, so they come in two parts connected together with a small connector. Strips are secured to the chassis using screws and also double-sided tape so they don't move at all. However, it turned out that the connector that joins two parts of a strip has a design flaw. It doesn't provide a solid connection. I soldered and shorted the connections (+ to + and - to - across the connectors) and everything works now. The last suspect turned out to be the issue. 4 out of 5 failed. Kudos to the designer of the connector.

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