Circuit Project Electronic

The 34063 is configured as a voltage inverter with current limiting resistors on pin 7. Pin 5 is the voltage detect pin and together with its 1.25V internal reference, a 240:1 voltage step-up ratio is configured giving an output voltage of around 300V. The emitter on pin 4 of the 34063 drives an IRF540 MOSFET through a diode. A 12V Zener diode is at pin 4 to keep the gate drive voltage at or below 12V in the case of higher input voltages. A PNP pull-down transistor is placed at the gate of the MOSFET to shorten the shutoff time. The MOSFET drives the primary of a step-up transformer, the secondary output of which is rectified by a diode and charges the main 68u capacitor. Across the source and drain of the MOSFET is another Zener diode to protect the transistor from high flyback voltages. The voltage of the capacitor is monitored by pin 5 of the 34063 through divider resistors. This capacitor is connected across the xenon flash lamp.

The trigger circuit consists of a 4070 quad XOR gate which has a Zener regulated supply. The first two XOR gates are configured as inverters and, with several external components, comprise an oscillator. The final two gates are configured to generate a pulse when the lo-hi state of the oscillator changes producing a double pulse. These pulses are directed to the gate of an SCR which dumps a small capacitor into the primary of the trigger transformer, thereby producing a high voltage in the secondary and thus an ionization of the xenon gas in the flash lamp. This creates an ion channel for the main 68u capacitor to discharge through. This double-pulse triggering cycle is self resetting and continues the whole while power is connected.

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