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Browse Prior Art Database

Power Failure Warning Circuit

IP.com Disclosure Number: IPCOM000042413D
Original Publication Date: 1984-May-01
Included in the Prior Art Database: 2005-Feb-03
Document File: 2 page(s) / 25K

Publishing Venue

IBM

Related People

Kinnard, JR: AUTHOR

Abstract

This circuit provides a warning signal of an impending drop of power supply voltage to a critical level within the normal operating range. This condition is recognized in a power supply employing linear regulation by detecting an increase in base current in the pass transistor beyond the maximum normal operating base current. Transistor Q1 serves as the pass transistor in a linear regulation mode. Vin is an unregulated DC voltage suitably higher at its minimum level than the regulated output voltage VO. An integrated circuit (IC) regulator monitors VO and supplies an appropriate base current to Q1 to maintain VO at the desired level. When Vin is in the normal operating range, the base current to Q1 is known to be some maximum value by the gain specification for Q1.

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Power Failure Warning Circuit

This circuit provides a warning signal of an impending drop of power supply voltage to a critical level within the normal operating range. This condition is recognized in a power supply employing linear regulation by detecting an increase in base current in the pass transistor beyond the maximum normal operating base current. Transistor Q1 serves as the pass transistor in a linear regulation mode. Vin is an unregulated DC voltage suitably higher at its minimum level than the regulated output voltage VO. An integrated circuit (IC) regulator monitors VO and supplies an appropriate base current to Q1 to maintain VO at the desired level. When Vin is in the normal operating range, the base current to Q1 is known to be some maximum value by the gain specification for Q1. Resistor R1 is selected to keep transistor Q2 off for this normal base current. With Q2 off, transistor Q3 is also off and the PFN (Power Failure Not) line is at an up level. As Vin drops below the minimum value necessary to maintain VO within tolerance, Q1 begins to saturate and its gain drops. The integrated circuit regulator responds by increasing the base current of Q1 to compensate for the lower gain. At some point the regulator can no longer supply more base current and VO drops out of regulation. Since the regulator can normally supply much more than normal operating current for short durations, a value of R1 can be selected to meet the criteria above and also cause...