| 摘要 |
<p>A voltage regulator and switching circuit for a flash lamp control unit. A silicon transistor (92) is used to switch on an incandescent lamp (24) and to regulate the voltage across the lamp. A driver transistor (94) is connected to the switch transistor in a darlington connection with the collectors connected by a diode (93). During initial turn on of the lamp, its filament draws a heavy current furnished by the darlington circuit. As the filament heats, its current drops to a low value. Drive current then flows through the drive transistor and a resistor (95) to ground, reverse biasing the diode and changing the circuit to a single transistor circuit. The driver transistor is controlled by a differential amplifier (98, 99) having a first transistor biased by a first voltage divider network (103, 205) connected across the power supply and a second transistor (99) biased by a second voltage divider network (197, 201) connected across the regulated voltage. A zener diode (97) provides a fixed reference voltage to the first transistor when the supply voltage is greater than the regulated voltage, and the first voltage divider supplies a variable reference voltage when the supply voltage is less than the regulated voltage. The action of the differential amplifier in the latter instance prevents the switch transistor from saturating so as to maintain a preselected low voltage drop thereacross. </p> |
