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Jeremys LED Lighting Page.pdf

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A lower overhead version would be:

R3 100k

R2 100k

T1 IRFU8729 or similar

Battery, 11.7 to 14.5 volt
Pot1: 10k
T2 BC546B

T3 BC546B

R1 300m

R4 10k

Notice that R1 is now 0.3Ω, which would allow 2A to flow before turning on T2 but Pot1 now
cannot reach the actual sense voltage, R4 prevents T2base getting closer than half way to it, so it is
effectively dimmed to half, keeping it at 1A. The lowest full-power operating voltage is now just
11.4V. This could be taken further but it must be borne in mind that the junction voltage of a bipolar
transistor changes with temperature and that if, for example, a 0.1Ω R1 is used, dropping just
100mV at 1A, a 50mV change in the voltage drop of T2 would change the output by 50%.
The IRFU8729 N-Channel Enhancement HEXFET MOSFET is vastly over-specified for this job,
having an on-resistance of 0.006Ω a switching time of 25ns and a current handling capacity of 50A
but costs only 41p and does the job. Its in an IPAK package, a bit like a miniature TO220 but with
no fixing tab, and in this circuit needs heatsinking – up to 5W dissipation if the battery is being
overcharged at the time. I have been known to fit it between the dimmer pot and an aluminium wall
plate with a little CoolTape double sided sticky pad to stop it sliding out. 100cm² of cooling surface
should be enough in any case, but it may get hot.

Mounting Power LEDs
The Z-LED P4 similarly needs cooling, as it has to absorb 3.5 watts of power and only turns about
15% of this into light. Therefore a suitable fitting must be constructed. 200cm² each is about right as
these LED's need to stay cooler than the MOSFET at about 70°C maximum, and lifetime is cooling
dependent so it's best to stay cooler. CoolTape is not sticky enough to keep an LED stuck on its
own, so m3 screws or pop-rivets are needed to hold it on. Be creative with your design, as this is the
visible bit – but sometimes simplicity is easier to keep neat. I find a hand plane makes a nice edge
to an aluminium plate, and keeping the LED's shielded from sight is essential since this is a lot of
light to be coming from a 2mm square. Simply bending a 100cm² bit of aluminium into a square Ushape with the LED in the bottom, then screwing one side to the wall with a spacer makes a nice
invertible uplighter/downlighter for a small space.