Introduction
This repair requires a thermal imaging camera (typically about £400)
I have a generic, 4 years old, 50 led light string powered by a small solar panel with one standard AAA NiMH battery to hold charge collected while the sun is shining. The coloured lights (green blue yellow red) come on automatically when dark, either steady or flashing depending on switch position.
A few of the lamps were not working, but the real problem was that the lamps got very noticeably dimmer as the distance from the power supply increased, such that the end ones were not lit at all. All the led's were in parallel running off what should be a near constant 1.3V, but clearly something was causing a voltage drop.
I examined the string of led's with a Topdon thermal imager (which is sensitive to about 0.1C) having left the lights on for about 15minutes to 'warm up'. One of the lamps (one of the many that wasn't working) showed it to be hot relative to the others, so I cut this led out and all the other working lights came to full brightness. (my fingers weren't sensitive enough to notice it was hotter than the rest.
Normally led's fail open circuit (high resistance) but the above problem led failed with a low resistance of 200 Ohms so was taking enough current to drop the surrounding voltages
Introduction image shows opened solar panel assembly with battery and circuit board and led string bagged in background. Images in 'step' section show thermal images and a stripped failed led.
Tools
Parts
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Thermal images of 'hot' and 'normal' leds. Green arrows show wires and red arrows shows leds. The 'normal' led and wire is hardly visible. Ignore temperatures they weren't positioned accurately. Colour scale auto-generated.
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Optical image shows a failed led with shrink-fit sleve removed (leg broken) and an intact led for comparison
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You don't have to reattach the section of leds and wire that has been cut off, but the string will of course be shorter. Just make sure the exposed ends cannot touch (ie cut one wire shorter than the other and insulate with tape or shrink-fit.)
Normally led's fail harmlessly open circuit (high resistance) but the above problem led failed with a low resistance of 200 Ohms so was taking enough current to drop the surrounding voltages (and thus heating itself up to be visible in the far infra red imaging camera).
Open circuit failure was due to corrosion and breakage of the led arm(s) due to rain ingress. Close circuit failure was probably due to corrosion product bridging the power wires. Which failure occurs depends on build quality ie the amount on water ingress/egress, how much corrosive soldering flux was not washed away, etc. etc
Garden solar lamps are often made of poor quality materials and poorly assembled. They seldom last more than 3-6 years. Perhaps they would last longer if sealed with silicone rubber or similar?
