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Short Circuit Protection with Optical Isolator

IP.com Disclosure Number: IPCOM000089457D
Original Publication Date: 1977-Nov-01
Included in the Prior Art Database: 2005-Mar-05
Document File: 2 page(s) / 34K

Publishing Venue

IBM

Related People

Cappa, M: AUTHOR [+2]

Abstract

An optical isolator is used to sense low voltage conditions in a load. The optical isolator responds quickly to remove the base drive to a series-pass transistor and shut-off the transistor before it is damaged.

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Short Circuit Protection with Optical Isolator

An optical isolator is used to sense low voltage conditions in a load. The optical isolator responds quickly to remove the base drive to a series-pass transistor and shut-off the transistor before it is damaged.

Zener diode 1 is referenced to ground. This makes it difficult to limit the base current in series-pass transistor 2 because diode 1 appears as a very low impedance. Transistor 3 controls the base current to overcome this problem.

Under normal load the voltage between nodes 4 and 5 is sufficient to keep optical isolator 6 on. This turns transistor 3 on, and thus supplies current to the base of transistor 2. Zener diode 7 stabilizes the voltage at node 5.

Under excessive load the current through the load and the resistor 8 rises, and the voltage at node 4 starts to fall. The current through variable resistor 9 will decrease. When resistor 9 is properly adjusted, the current through isolator 6 will fall below the minimum operating current for isolator 6 when the voltage at node 4 reaches the desired trip point. Isolator 6 shuts off, thereby turning transistor 3 off and removing the base current from transistor 2.

Diodes 10 and 11 protect isolator 6 from reverse current and voltage under full short-circuit conditions. Resistors 12 and 13 provide reverse bias currents through diodes 1 and 7.

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