This is a multi-part message in MIME format. ------=_NextPart_000_0050_01C542ED.51E26100 Content-Type: text/plain; format=flowed; charset="iso-8859-1"; reply-type=original Content-Transfer-Encoding: 7bit > I'm trying to wind a coil for the "White LED headlamp" at > . So far, I've made two > coils > (T1 in the schematic) that don't work and I can't see what I'm doing > wrong. > Has anyone built anything similar to the transformer used in the > LED > driver? Anyone got any tips? I consider the circuit suspect, although something SIMILAR will definitely work. It MAY work as shown. Are you matching the "dotting" shown. The dots mean that if the two coils are wound together the start of each winding is as shown by the dots. In this case winding start goes to collector for one coil and supply for the other. Reverse either one of these and the circuit is not as intended. Core needs to be large enough to support 500 mA odd pulses without saturating (much). many small cores wont. Lower operating current (and less LED drive) achieved by increasing R1. Maybe it will work. Operation: As shown the transistor Q1 is turned on by the DC bias via R1 and the coil AND it is turned on by the transformer pulse (with the dotting shown). So the harder it conducts the harder it is driven on. It will start to turn off only when collector current is no longer supported by Beta x Ibase. At this stage IC starts to collapse and the base is driven negative thereby enhancing the turn off. Say Beta = 100 then it turns off at about 300 mA collector current. Quite high. C3 is meant to be large enough that the voltage on the LEDs doesn't rise to operating voltage in one cycle. Too small and the LEDs will die before they can regulate. Q2 turns on when Iled is large enough and clamps Q1 off until LED current drops. 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