led bicycle lights

Posted on Feb 7, 2014

Before getting started an acknowledgement is due, the circuit presented here uses an ingenious method of controlling a flyback converter by the voltage developed on a cur-rent sensing resistor, this was published by Andrew Armstrong in the July 1992 issue of ETI magazine. The reworked circuit is quite simple. At the instant that power is applied o

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nly a small current flows to charge C4 so insufficient voltage is developed on R3 to switch T2 on. Also, D1 allows C2 to charge from the 6 V battery, so R1 feeds enough voltage to switch on T1 this shunts the voltage across L1 and the current in it starts to rise. At a certain point the current which returns via R3 will develop sufficient voltage to switch on T2 which shunts the gate voltage to T1 causing it to switch off, initiating the flyback voltage from L1. The fly-back pulse forces a current around the circuit, charging C4 and feeding the LEDs. As the return current is via the current sensing resistor R3, this keeps T2 turned on and T1 turned off, so the flyback phase is not clamped until it has given up all its energy. Capacitor C3 provides positive feedback to ensure reliable oscillation and sharpen up the switching edges. Components D1, D2 & C2 form a bootstrap boost circuit for the MOSFET gate, although it is logic level it only guarantees the stated RD-S(on) at a Vg level of about 8 V ” by happy coincidence the combined Vf of four ultrabright red LEDs is about 8. 8 V and this is the value that the output is normally clamped to. There are some notes on the components specified. For position T1 an n-channel MOS- FET with a very low RD-S(on) of 15 m (at 10 V) Is suggested, although its high ID rating (35 A) is not strictly necessary. Purists may wish to use Schottky barrier diodes for D2 and D4, but a quick look at...

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