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Triac light bulb flasher 2010-07-05

This 800 W light bulb flasher operates directly off the line and needs no transformer. Power for the timer circuit is derived by limiting the current using a 330 nF capacitor (acts like a 9.6 k resistor at 50 Hz), rectifying with a full-wave rectifier composed of four diodes (you may also use a pre-made bridge rectifier instead of the diodes, of course, but make sure the voltage rating is 400 V, or 250 V RMS). Then the voltage is limited with a 9 V zener diode (almost any of this voltage will work), a 1 W type. The 100 µF capacitor filters the power, a 16 V rating may be a bit safer. Remember: if the zener diode fails, the capacitor will blow because it gets peaks of up to 330 V, although current-limited). In this configuration, the timer gives long pulses at 1.3 Hz.

Now there's one problem: we can't drive the triac directly, because the controlling voltage is not isolated from the line since there is no transformer. The easiest way to drive it is thus by using a triac optocoupler. The K3021 or MOC3021 is well suited for this purpose, as it works like a small triac and thus allows it to directly drive the gate of the larger triac. The coupler is connected to turn on when the timer outputs a low, so we get short pulses.

Please note that this only works with resistive loads like incandescent light bulbs or heaters. It does not work with fluorescent lamps (need a snubber network to do that).

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