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I have had this cheap set of fairy lights for a few years. Powered by 3xAA batteries.

Recently when they are turned on they start bright as they always were then proceed to get dimmer and dimmer until they reach a low light after a couple of minutes.

The lights stay dim but leaving them off for a few hours causes them to ‘reset’ and then the bright to dim behaviour happens again.

I want to fix them and I can see a resistor inside which I suspect could be broken? Does anybody know why this could be happening and what I might need to fix them?

enter image description here

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  • \$\begingroup\$ This would be a very strange failure mode for a resistor. \$\endgroup\$
    – rdtsc
    Jan 9, 2020 at 21:15

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There is only 6 failure points in no particular order.

  1. The batteries. Especially rechargeable ones at end of life. Try new ones or good alkaline.

  2. The leds. As the hours rack up, especially with high current flows, they can start to dim. As these are in parallel and we don't know the resistor value or the current involved total or individually, we can't say if they are near their end of useful life. Additionally overcurrent may cause failures in the diode doping mechanism which also causes shorts.

  3. The resistor. The behavior you describe is unlikely but again without knowing the current involved we can't say. Thats a 1/4 Watt resistor I think so too high a current can lead to failure. Measure its resistance without load. Measure the voltage across it under load. Compare using ohms law and its color code.

  4. The switch. With the likely current involved its unlikely, but these switches can start to corrode or oxidize due to the dc current and their resistance can go up. If its over lets say 5 ohms in resistance between terminals then it is bad.

  5. The wire. Same as the switch, its resistance may have increased due to physical damage or corrosion. In that case the far end of the led string would be dimmer than the near end so unlikely to be the issue.

  6. Battery terminals. Same as switch and wire. If dirty or corroded, say from battery leaks, resistance will build up. An eraser is a quick cleaning method for minor oxidation.

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Replace the batteries with fresh alkaline ones.

What you describe is normal behavior for batteries that are either capable of supplying the current draw or are close to the end of their life. The "recovery" is due to polarization.

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  • \$\begingroup\$ Shouldn't we give the OP the benefit of the doubt that he didn't overlook the batteries given that he has had this thing for a few years? \$\endgroup\$
    – DKNguyen
    Jan 9, 2020 at 20:59
  • \$\begingroup\$ Thanks for the answer I am using Energizer rechargeable NiMH batteries. I am positive these used to work ok! I will try with non rechargeable but that seems a shame. \$\endgroup\$
    – greenimpala
    Jan 9, 2020 at 21:29
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    \$\begingroup\$ @DKNguyen I'm more of an Occam's razor kind of troubleshooter. NiMH batteries die over time especially if overcharged (irreversible damage). Eneloop ones seem to be a cut above. \$\endgroup\$
    – Spehro Pefhany
    Jan 9, 2020 at 21:52

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