Zero crossing circuit for a relay

• US 5,804,991 A
• Filed: 03/19/1997
• Issued: 09/08/1998
• Est. Priority Date: 05/06/1994
• Status: Expired due to Term

• Current Assignee: The Watt Stopper Incorporated (Legrand SA)
• Original Assignee: The Watt Stopper Incorporated (Legrand SA)
• Inventors: Hu, Charles C.
• Primary Examiner(s): TRAN, TOAN V
• Application Number:
  US08/821,117
• Time in Patent Office: 538 Days
• Field of Search: 327/141, 327/142, 327/143, 327/162, 327/163, 327/186, 327/198, 327/79, 327/104, 327/263, 327/267, 327/451, 361/3, 361/5, 361/8
• US Class Current: 327/162
• CPC Class Codes: H01H 9/56 for ensuring operation of the switch at a predetermined point in the ac cycle
• CPC Class Codes: H03K 5/08 by limiting; by thresholding; by slicing, i.e. combined limiting and thresholding H03K5/07 takes precedence; comparing one pulse with another H03K5/22; providing a determined threshold for switching H03K17/30
• CPC Class Codes: H03K 5/1536 Zero-crossing detectors in measuring circuits G01R19/175

Abstract

A relay control circuit that includes a zero cross detector, a latch, and a delay circuit. The zero cross detector circuit detects when the voltage waveform or current waveform on an AC power line is at the zero crossing. The output of the zero cross detector clocks the latch (flip-flop), which receives a control signal at its data input. The flip-flop latches the control signal on the zero crossing point and outputs the latched signal to the delay circuit. The delay circuit delays the control signal for a predetermined time period depending on the make and break times of the relay so that the relay is switched ON and OFF substantially near the next zero crossing point of the AC voltage waveform.

Claims

1. A relay control circuit for controlling an electromechanical relay which switches a circuit, said relay control circuit comprising:

(a) a zero cross detector for detecting when an AC signal crosses a zero-crossing point; and

(b) a delay element, coupled to said zero cross detector and coupled to a coil of the electromechanical relay, configured to delay a relay ON/OFF signal a predetermined period of time, said delay element including;

i) a flip-flop for latching said relay ON/OFF signal to produce a latched relay ON/OFF signal; and

ii) a delay circuit for delaying the latched relay ON/OFF signal said predetermined period of time wherein said flip-flop is coupled at an input to said zero cross detector and coupled at an output to said delay circuit, and wherein said predetermined time period is selected so that the electromechanical relay switches substantially at a zero-crossing point.

2. A relay control circuit to prevent arcing that occurs in an electromechanical relay when relay contact is made to an AC power line and the energy level on the AC power line is at an energy level other than zero, said relay control circuit comprising:

(a) a zero cross detector, coupled to the AC power line, for outputting a signal which indicates each time the energy level on the AC power line becomes zero;

(b) a flip-flop, coupled to said zero cross detector, which receives a control signal at a data input and receives the signal output from said zero cross detector at a clock input, for latching the control signal when the energy on the AC power line is zero to produce a latched control signal; and

(c) a delay circuit, coupled to an output of said flip-flop and coupled to a coil of the electromechanical relay, for delaying said latched control signal for a predetermined time period so that relay contact is made to the AC power line substantially near the zero-crossing level.

3. A method of switching an electromechanical relay ON and OFF substantially near the zero-crossing of an AC power line, said method comprising the steps of:

(a) detecting a first zero-crossing point on the AC power line;

(b) latching a relay control signal, using a flip-flop, when said crossing point is detected to produce a latched relay control signal;

(c) delaying said latched relay signal a predetermined amount of time selected in dependence on a make time of the electromechanical relay, thereby creating a delayed latched signal; and

(d) supplying said delayed latched signal to a coil of the electromechanical relay, thereby causing the electromechanical relay to switch ON substantially near a second zero-crossing point.

4. A relay control circuit for controlling an electromechanical relay that switches a light coupled to an electronic ballast ON and OFF in response to a control signal from a motion detector, said relay control circuit comprising:

(a) a zero cross detector for detecting when an AC power signal powering said light crosses a zero-crossing point; and

(b) a delay element, coupled to said zero cross detector and coupled to a coil of the electromechanical relay, configured to delay said control signal a predetermined period of time so that relay contact is made and broken substantially near a zero-crossing point, said delay element including;

i) a flip-flop for latching said control signal, said latching of said control signal producing a latched control signal; and

ii) a delay circuit for delaying said latched control signal said predetermined period of time, wherein said flip-flop is coupled between said zero cross detector and said delay circuit.

5. A relay control circuit for controlling an electromechanical relay which switches a circuit comprising:

(a) a zero cross detector for detecting when an AC signal crosses a zero-crossing point; and

(b) a delay element, coupled to said zero cross detector, configured to delay a relay ON/OFF signal a predetermined period of time, said delay element including;

i) a flip-flop for latching said relay ON/OFF signal; and

ii) a delay circuit for delaying the coupled at an input to said zero cross detector and at an output to said flip-flop, for producing a pulse for clocking said flip-flop to change state after each zero-crossing.

6. A relay control circuit for controlling an electromechanical relay which switches a circuit, said relay control circuit comprising:

(a) a zero cross detector for detecting when an AC signal crosses a zero-crossing point; and

(b) a delay element, coupled to said zero cross detector and coupled to a coil of the electromechanical relay, configured to delay a relay ON/OFF signal a predetermined period of time, said delay element including;

i) a flip-flop for latching said relay ON/OFF signal to produce a latched relay ON/OFF signal; and

ii) a delay circuit, coupled to an output of said flip-flop and coupled to a coil of the electromechanical relay, for delaying said latched relay ON/OFF signal said predetermined period of time, wherein said flip-flop comprises a D flip-flop, and

wherein said delay circuit comprises a first resistor and a capacitor, and a second resistor and a diode coupled in parallel with said first resistor.

7. A relay control circuit to prevent arcing that occurs when relay contact is made to an AC power line and the energy level on the AC power line is at an energy level other than zero, said relay control circuit comprising:

(a) a zero cross detector, coupled to the AC power line, for outputting a signal which indicates each time the energy level on the AC power line becomes zero;

(b) a flip-flop, coupled to said zero cross detector, which receives a control signal at a data input and receives the signal output from said zero cross detector at a clock input, for latching the control signal when the energy on the AC power line is zero, said latching of said control signal producing a latched control signal; and

(c) a delay circuit, coupled to an output of said flip-flop and coupled to a coil of the electromechanical relay, for delaying said latched control signal for a predetermined time period so that relay contact is made to the AC power line substantially near the zero-crossing level, wherein said delay circuit comprises a first resistor and a capacitor, and

wherein said delay circuit further comprises an inverter coupled at an input to said first resistor and said capacitor.

8. A method of switching an electromechanical relay ON and OFF substantially near the zero-crossing of an AC power line, said method comprising the steps of:

(a) detecting a first zero-crossing point on the AC power line;

(b) delaying a relay control signal after detecting said first zero-crossing point by a predetermined amount of time selected in dependence on a make time of the electromechanical relay, thereby creating a first delayed latched signal;

(c) supplying said first delayed latched signal to a coil of the electromechanical relay, thereby causing the electromechanical relay to switch ON substantially near a second zero-crossing point;

(d) detecting a third zero-crossing point on the AC power line;

(e) delaying the control signal a second predetermined amount of time depending on a break time of the electromechanical relay, thereby creating a second delayed latched signal; and

(f) supplying said second delayed latched signal to a coil of the electromechanical relay, thereby causing the electromechanical relay to switch OFF substantially near a fourth zero-crossing point.