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I have an LED flashlight with 13 LEDs. Some of the LEDs do not light, but some flash regularly at different rates, and some flash irregularly. The circuit is simply a resistor in series with the lamps, which are in parallel, and a six volt lantern battery. I have disassembled the flashlight, bypassed all contacts, and checked the resistors. The voltage across the LEDs is a steady 6.04 volts. Why do they flash? Posts suggest flashing is not a recognised failure mode of LEDs.

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  • \$\begingroup\$ Flashing is a normal mode of flashing LEDs. \$\endgroup\$ – Ignacio Vazquez-Abrams Nov 12 '17 at 14:16
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    \$\begingroup\$ It's a flashlight. \$\endgroup\$ – Olin Lathrop Nov 12 '17 at 14:45
  • \$\begingroup\$ I've noticed that the solder connections and/or components (e.g. switch) in these cheap LED flashlights are sub-par, and so just holding them at a different angle can change the number of LEDs that light. Have you tried replacing the switch? That's always my first step. \$\endgroup\$ – calcium3000 Nov 12 '17 at 15:13
  • \$\begingroup\$ Thanks, I've bypassed the switch. I think Passerby's answer says it all. \$\endgroup\$ – fbanks Nov 13 '17 at 15:42
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Posts suggest flashing is not a recognised failure mode of LEDs.

Wrong. Flashing, or intermittent connections causing the led to turn on and off, is a typical failure mode of leds which are cheaply constructed, physically damaged, or forced with a higher current.

When some die, the rest of your parallel leds will get a higher current which will lead them to die as well. As this happens, the leds can go through thermal expansion which breaks the internal connection then cool which causes the connection to reconnect, then expand, etc. Bond wires can do the same. Or internal shorts of the led substrate material. This can also lead to leds flashing between different brightness.

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  • \$\begingroup\$ Great answer. Thanks. I'm going to replace the LEDs with high power ones - 35cd each and see how it works. I should wind up with a "tactical flashlight". LOL. Perhaps I should upgrade the circuitry to avoid the failure mode you mention? \$\endgroup\$ – fbanks Nov 13 '17 at 15:42
  • \$\begingroup\$ Make sure you meet the recommended current & voltage and but from a legit source, that's all you need to do. \$\endgroup\$ – Passerby Nov 13 '17 at 19:10

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