关键词：
Power resources
Multivibrators
Relaxation oscillators
Capacitors
Electric apparatus & appliances
Electrical engineering
摘要：
THE TRICK IS TO USE A RETRIGGERABLE MONOSTABLE MULTIVIBRATOR TO CONTROL THE MOSFET. Universal power supplies must work from mains power lines ranging from 90 to 264V ac at 50 or 60 Hz. Directly rectifying the input voltage charges the filter capacitor to 120 to 370V dc. Such voltages present a serious threat to personnel who are prototyping or repairing the power supply. It is desirable to discharge the filter capacitor when mains power is off so workers can safely deal with the power supply. An intuitive solution is to use an ac relay. However, relays cannot operate in a wide range of input voltages, they consume significant power and space, and they have a limited number of cycles. Figure 1 shows an alternative circuit, which you can apply to a filter capacitor of almost any value. It uses a MOSFET, Q{sub}1, and a current-limiting resistor, R{sub}D, to discharge the high-voltage filter capacitor, C{sub}P within one second after you switch off the mains power. The trick is to use a retriggerable monostable multivibrator to control the MOSFET.
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