Wednesday, 16 April 2008

LED lights

In a comment on an earlier post Brian from NB Harnser asked for some more details on some LED lights that I'd discovered.

new light unit and modified unit

The lights I'd bought were from Poundland and can be seen here. The packaging reads: "Outdoor Solutions LED Camping Light". The first one I bought was last year just before a camping trip to the Netherlands. It lit up our large tent a treat. The units take four AA batteries. Inside there are four curved printed circuit boards, each with six white LEDs connected in parallel; each circuit board connected in parallel. There's also a switch, and a hook for hanging it in your tent, or you can slide it over a patio umbrella pole.

I suggested that they would be easy to convert to 12V use, just by arranging two pairs of circuit boards in series, so there would still be 6V across each LED. When I did this, and connected it across my car battery, several of the LEDs immediately burned out.


I think I know why this happened. When I first investigated these lights I was surprised to see all the LEDs connected in parallel, as the usual voltage across an LED is just 2V. There is 6V of battery power, from four 1.5V AA cells, in the unadulterated unit. But the actual voltage across the LEDs while battery powered I measured to be only just above 3V. At the time I assumed my batteries were a bit low, but perhaps it's because the LEDs draw so much current (say 20mA each, times 24 gives almost half an amp) that the internal resistance of the batteries limits the current. And anyway, you're not supposed to put more than 2V across an LED - that's why they usually have current limiting resistors in series. Not in this cheapo unit, though!

modified unit - each circuit board wired in series with the next (switch taken out of circuit here)

What I did next, to another unit, was to connect each of the four groups of six LEDs in series, so the voltage across each LED should now be only about 3V. This is a very simple mod, and it worked perfectly. I don't yet know how it will cope with alternator charging voltage of 14.7V. When I connected my modified unit across 12.8V car battery it took 73mA (i.e. consuming just under 1W); when the engine was running the voltage was 13.6V and the unit drew 134mA (i.e. nearly 2W). This suggests that the light unit is quite sensitive to fluctuations in input voltage. A simple transistor regulator would probably be a good idea, or, even simpler, a series resistor. I'm going to have to try this after dark: comparing unmodified battery unit with modified one across car battery.

showing how easy it is to modify: just snip off one lead and reposition the other so as to connect outside edge of one board to inside edge of next

A word of warning though - I've found several of the Poundland units with very dodgy soldering. The last one I opened up had the no solder at all joining the links between each AA battery position. And the previous one had some dry joints at the LEDs.

There's a discussion of LED lighting for boats on the OwnerShips discussion forum here.

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