John Howell

I thought some may be interested in my experiences building a big LED torch.
I was surprised to see that large commercial torches still involved copious ‘D’ cells or 6 Volt lantern batteries powering Krypton-filled filament bulbs. Maglite has set an example, so surely the day has come for the LED?

Ever since my father bought a military surplus parabolic mirror in the 1950s it has been lying around looking for a project other than setting fire to things in sunlight. So I reckoned its time had come and researching powerful LEDs I was disappointed to find that they seemed to consist of  arrays of diodes packed together in parallel and series. These required, typically for a 100 Watt lamp, 32 Volts at 3 Amps via a constant current circuit.

Now having a parabolic reflector I needed a point source, a single LED as bright and small as possible. After much investigation I discovered the 10 Watt XM-L from Cree. This has a single 3mm square die, an intensity of 1,000 lumens, and cost £1.97 on eBay.

In July this year Cree announced an addition to this range, the XHP35 which has a 50% higher output.

So on to powering the torch. To get a useful battery life when consuming 3 Amps the only choice appeared to be a Lithium-ion battery. More reading and discovering how to manage lithium cells, but mostly how not to manage them! I learned enough to dismantle a laptop pack and assemble the cells to give me an 8 Volt 4 Amp-Hour battery, I finished off by installing protection circuitry and building a suitable charger.

How to turn all this into a torch? The tub from the base of a VAX vacuum cleaner presented itself. The 10 Inch reflector fitted perfectly with the battery behind. A sheet of perspex across the front to support the LED and its heatsink. I managed to find a focusing assembly from a projector and a radially finned heatsink to fit inside. The idea being that a computer fan would draw in air past the fins and exhaust it through the top of the case. The radial fins would offer the least obstruction to the light from the centre of the mirror.

I then discovered ‘buck’ and ‘boost’ inverters (I’ll leave you to Google those if you want more!) but suffice it to say one of these keeps the LED current at 3 Amps and consumes 2 Amps from the battery, now that’s clever!

Does it all work? Oh yes, it’s SO bright and the beam spreads only slightly such that I could be using a white laser. The fan was not needed, the heatsink runs quite cool.

It’s not really very useful, but it’s great fun!

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Alec Bray

Around the mid nineteen sixties, wasn’t there a fashion for bolting government surplus aircraft landing lights onto the front of your car?  Switch of the car, walk round to the front to witness the light slowly dimming out …

John Howell

I remember those, they were motorised so you could have them ‘appear’ like Lotus Élan headlights.

A school friend of mine once asked me, “Do car headlamps go out slowly because the electricity is still  running out of the wires?”