PROTECTIVE CIRCUIT

• US 3,571,608 A
• Filed: 04/04/1969
• Issued: 03/23/1971
• Est. Priority Date: 04/04/1969
• Status: Expired due to Term

• Current Assignee: Hurd Edward T E
• Original Assignee: Hurd Edward T E
• Inventors: HURD EDWARD T E
• Application Number:
  US04/813,550
• Time in Patent Office: 718 Days
• US Class Current: 361/77
• CPC Class Codes: H02H 11/002 in case of inverted polarity or connection; with switching for obtaining correct connection
• CPC Class Codes: H02H 3/202 for dc systems
• CPC Class Codes: H02H 9/02 responsive to excess current current limitation for voltage regulators G05F1/573; disconnection after limiting H02H3/025

Abstract

The collector and emitter of a first transistor, which is normally biased on by a collector-base resistor, are connected, respectively, to positive circuit input and output terminals. The transistor base is connected to joined negative circuit input and output terminals through the collector-emitter path of a second transistor having its base connected through a zener diode to the positive input terminal. This diode breaks down and turns on the second transistor, which turns off the first transistor, whenever the value of the voltage between the input terminals exceeds a value determined by the zener rating of the diode. A pair of diodes connected across the series combination of an emitter resistor and the base-emitter junction of the first transistor limits the output current to a predetermined maximum value. A diode connected between the collector and emitter of the second transistor clamps the output voltage substantially to zero in the event that the polarity of the voltage between the input terminals becomes reversed.

Claims

1. A protective circuit comprising:

a pair of input terminals for connection across a source of voltage;

a pair of output terminals for connection across a load device;

a transistor having a collector, an emitter, and a base;

first means connecting one of said input terminals to one of said output terminals;

second means connecting the collector-emitter path of said transistor between the others of said input and output terminals so that current flowing between said output terminals flows into said collector and out of said emitter;

transistor turnon bias means connected between said collector and said base;

normally open switch means connected between said base and said one input terminal and operative to turn off said transistor when closed;

voltage sensitive means connected between said other input terminal and said switch means to close the latter when the voltage between said input terminals exceeds a predetermined value;

a resistor included in said second connecting means and having one end connected to said emitter so that current flowing between said output terminals flows through said resistor; and

a diode connected between the remaining end of said resistor and said base and oriented for current conduction in a direction away from said base.

2. Apparatus as specified in claim 1, wherein said switch means is a second transistor having a collector-emitter path connected between said base of the first-mentioned transistor and said one of said input terminals, and having a base connected to said voltage sensitive means.

3. Apparatus as specified in claim 1, wherein said voltage sensitive means is a Zener diode connected between said other input terminal and said switch means.

4. A protective circuit comprising:

a pair of input terminals for connection across a source of voltage;

a pair of output terminals for connection across a load device;

a transistor having a collector, an emitter, and a base;

first means connecting one of said input terminals to one of said output terminals;

second means connecting the collector-emitter path of said transistor between the others of said input and output terminals so that current flowing between said output terminals flows into said collector and out of said emitter;

transistor turnon bias means connected between said collector and said base;

normally open switch means connected between said base and said one input terminal and operative to turn off said transistor when closed;

voltage sensitive means connected between said other input terminal and said switch means to close the latter when the voltage between said input terminals exceeds a predetermined valve; and

a diode connected between said base and said one input terminal and oriented for current conduction in a direction toward said base.

5. Apparatus as specified in claim 4, wherein said second connecting means includes a resistor having one end connected to said emitter so that current flowing between said oUtput terminals flows through said resistor, and wherein a diode is connected between the remaining end of said resistor and said base and is oriented for current conduction in a direction away from said base.

6. Apparatus as specified in claim 4, wherein said switch means is a second transistor having a collector-emitter path connected between said base of the first-mentioned transistor and said one of said input terminals, and having a base connected to said voltage sensitive means.

7. Apparatus as specified in claim 4, wherein said voltage sensitive means is a Zener diode connected between said other input terminal and said switch means.