Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific current responsive fault sensor
Patent
1987-09-25
1989-09-26
Jennings, Derek S.
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific current responsive fault sensor
361 98, 323516, 323907, 330207P, 330298, H02H 3087
Patent
active
048705330
ABSTRACT:
An output transistor (T.sub.1) is to be protected from damage due to excessive currents. This transistor supplies a current (I.sub.o) to a load (3) and is driven by a driver transistor (T.sub.2). The base-emitter junction of a current-sensing trnsistor (T.sub.3) is connected in parallel with that of the output transistor (T.sub.1). The collector current of transistor (T.sub.3) is a measure of the current (I.sub.o) of the output transistor (T.sub.1). A current generator (5) supplying a reference current (I.sub.ref) is coupled to the collector of the current-sensing transistor (T.sub.3). If the collector current of the current-sensing transistor exceeds the reference current (I.sub.ref) the protection circuit is activated so that the driver transistor is bottomed via an emitter-follower (T.sub.4). The base current of transistor T.sub.1 is thereby limited. Thus, the output current of the output transistor is limited to a maximum value which is substantially equal to the product of the reference current (I.sub.ref) and the ratio between the emitter areas of the output transistor (T.sub.1) and the current-sensing transistor (T.sub.3).
REFERENCES:
patent: 3792316 (1974-02-01), Bondini et al.
patent: 3845405 (1974-10-01), Leidich
patent: 3924158 (1975-12-01), Fransworth
patent: 4021701 (1977-05-01), Davies
patent: 4321648 (1982-03-01), Kaplan
patent: 4330757 (1982-05-01), Fukaya et al.
Franzblau Bernard
Jennings Derek S.
U.S. Philips Corp.
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