Electronic Design

Use Throwaway Camera Parts For A High-Voltage Flash Unit

Parts of a throwaway camera's flash unit can be salvaged to build a 280-V capacitor charger for a flash unit that uses six or nine batteries (see the figure). Camera shops usually will give them away for free. The flash capacitor, output rectifier, and transformer are easily de-soldered for use in the new circuit.

Many circuits are available for such flash units. But the transformers are hard to get or can only be obtained in large quantities, and they work at just one voltage. Because the units in disposable cameras are designed for blocking oscillators with a 1.5-V supply, they have very low primary inductance, and peak primary current can easily exceed 2 A. But because the switching frequency will be very high, they can't be used in a blocking oscillator at high voltage without an expensive high-speed, low-saturation transistor or MOSFET. The switching transistor in the flash units can't be used with a 6-V battery because they only have a 25-V breakdown rating.

In the circuit shown, a low-cost Darlington transistor is driven by a 5-ms on-time astable oscillator formed with an NE555 timer, preventing transformer saturation. This reduces peak currents, and the 1.6-kHz frequency reduces transistor overheating. Low peak currents also will prolong rechargeable battery lifetimes.

Capacitor voltage regulation is achieved with a 280-V "zener diode" made with two reverse-biased 1N914 diodes operating in voltage breakdown. During zener operation, the NE555 is stopped by transistor Q2 being driven into saturation by the zener current. Charging will resume once some charge has leaked from output capacitor C1.

These 100-V diodes will typically have a 140-V breakdown, but some have been measured at 170 V. The total capacitor voltage should not exceed the 300-V rating of the flash capacitor used. A lower voltage also will prolong lamp and capacitor lifetimes.

Resistor R5 and capacitor C4 must be included to limit the current peak that's produced when the diodes break down. Otherwise, transistor Q2 will be destroyed. The astable oscillator directly drives the switching transistor, with turn-off voltage drive provided by the transformer secondary.

The maximum reverse voltage is limited by the forward voltage of D1. It also provides a charging path for the charging current. No heatsinking for the switching transistor is necessary because the peak and average currents are low. An LED indicates when the capacitor voltage is reached. With a 45-second charging time, average current is approximately 40 mA.

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