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Capacitor Discharge Circuit

IP.com Disclosure Number: IPCOM000042160D
Original Publication Date: 1984-Mar-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 2 page(s) / 35K

Publishing Venue

IBM

Related People

Hitchcock, LJ: AUTHOR

Abstract

The stored energy from a rectifier input filter capacitor is rapidly removed by this discharge circuit. The circuit is particularly applicable for three-phase rectification, but is also applicable to single-phase as well as other multiple-phase configurations. When used in a multiple-phase configuration, it discharges the capacitor only when two or more of the phases are turned off. With AC power on, bridge rectifier 1 rectifies the 3-phase line input AC power, e.g., 220 volts, and charges the schematically shown input filter capacitor C2, which may be a bank of capacitors, for example, to an unregulated DC voltage, e.g., 300 volts.

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Capacitor Discharge Circuit

The stored energy from a rectifier input filter capacitor is rapidly removed by this discharge circuit. The circuit is particularly applicable for three-phase rectification, but is also applicable to single-phase as well as other multiple-phase configurations. When used in a multiple-phase configuration, it discharges the capacitor only when two or more of the phases are turned off. With AC power on, bridge rectifier 1 rectifies the 3-phase line input AC power, e.g., 220 volts, and charges the schematically shown input filter capacitor C2, which may be a bank of capacitors, for example, to an unregulated DC voltage, e.g., 300 volts. The discharge circuit 2, that is, diode rectifier CR1-CR3, voltage divider network R1- R2, R3 and reference Zener diode CR4, co-acts to provide base drive current for transistor Q1, causing it to conduct through resistor R6 and its collector voltage to drop. As a result, the Darlington transistor pair Q2 and Q3 are not conducting. Capacitor C1 is charged to approximately 200 volts through the series connected elements R7, R4, CR4 and R3 and maintains the ON bias at the base of Q1, and hence Q2 and Q3 remain off. Now, if the unregulated 300 VDC load is removed across the output terminals 3-4 and the AC input power is then turned off, the 300-volt output will remain high. Capacitor C1 continues to supply bias for a short time period to keep Q1 on and Q2, Q3 off. This time period, e.g., 30 milliseconds, is jud...