Tuesday, 12 December 2017

LED Emergency Lights - Arranging for 12v supply

Apart from the need to test a number of individual LEDs on each of these Emergency Lamps, which are considerably dimmer than the others (so might need replacing from my LED stock), the big problem with repurposing these lamps is the need to arrange for the 3.9v constant currant LED array to be fed from a 12v source. The on-board 7135 current regulator cannot handle above 6v input, so I need an efficient way to drop 12v at about half an amp, down to between 4-6v for the current regulator.

I had planned on using 7805 linear positive 3-terminal devices, but using these to drop 8v at 500mA will consume 4W and they will get damn hot. Plus theres then the hassle of arranging smoothing and decoupling.

So, at the risk of having to wait for them to arrive from China, ive opted to go for pre-built Buck Converter modules based on the LM2596 device.

At under a sov each, I cant build them for this price! And being adjustable, I can set the output of the module to correspond with a sweet spot in the input range of the 7135 device. These devices, being switching converters, are considerably more efficient than the linear 7805, so should run much cooler.

I could probably arrange to trickle charge the 3.9v 1500mA NiCd packs in each lamp as well.

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