Method and apparatus for protecting alternating current circuits

• US 4,271,449 A
• Filed: 04/04/1979
• Issued: 06/02/1981
• Est. Priority Date: 04/04/1979
• Status: Expired due to Term

• Current Assignee: Rockwell International Corporation
• Original Assignee: Rockwell International Corporation
• Inventors: Grogan, Martin L.
• Primary Examiner(s): Salce, Patrick R.
• Application Number:
  US06/027,129
• Time in Patent Office: 790 Days
• Field of Search: 361/94, 361/96, 361/97, 361/93, 361/110, 361/111, 361/87, 361/95, 361/98, 361/195, 361/196, 307/234, 307/293, 328/111
• US Class Current: 361/94
• CPC Class Codes: G01R 29/027 Indicating that a pulse characteristic is either above or below a predetermined value or within or beyond a predetermined range of values
• CPC Class Codes: H02H 3/093 with timing means in general H02H3/027; thermal delay H02H3/085; timing means for undervoltage protection H02H3/247

Abstract

A method and apparatus is disclosed for protecting a circuit from excessive current flow by pulse width modulation of a current sense signal and interrupting the circuit when the width of the pulse is less than a reference period.

Claims

1. An apparatus for protecting a circuit from excessive current flow, comprising:

sense transformer means for sensing the current flow through the circuit;

pulse shaping means for generating a sense pulse whose width is related to the rate of current flow into the sense transforming means;

1st single shot multivibrator means for generating a reference period at the occurrence of each sense pulse;

2nd single shot multivibrator means for providing a second pulse each time a sense pulse occurs;

comparator means for providing an enabling command as long as the sense pulse'"'"'s width is greater than the reference period and to provide a reset at a fault condition when the sense pulse'"'"'s width is less than the reference period;

selectable time delay means for providing a command signal after a fault condition and the occurrence of a selectable number of the second pulses; and

relay means activated by the command signal.

2. A method for protecting a circuit from excessive current flow, comprising:

sensing the current flow through a sensing circuit;

generating a pulse whose width is related to the rate of current flow through the sensing circuit;

generating a reference period;

comparing the width of the pulse to the reference period;

indicating a fault condition when the width of pulse is less than the reference condition; and

inhibiting the current flow through the circuit for a selectable period of time after the occurrence of the fault condition.

3. The method according to claim 2 wherein the step of generating a pulse whose width is proportional to the rate of current flow through the sensing circuit comprises:

passing the current flow in the sensing circuit through a first winding of a square loop transformer means, and rectifying the output of the transformer means.

4. The method according to claims 2 or 3 wherein the step of comparing the width of the pulse to the reference period comprises:

triggering a one shot multivibrator means whose time out period is equal to the reference period with the pulse.