Zero crossing AC relay control circuit

Details Zero crossing AC relay control circuit Zero crossing AC relay control circuit

1976-03-15

1977-09-27

Miller, Jr., Stanley D.

Electrical transmission or interconnection systems

Personnel safety or limit control features

Interlock

307296A, 307318, 307354, 361173, H03K 1700

Patent

active

040513947

ABSTRACT:
An AC relay control circuit comprising a pair of back-to-back, series connected zener diodes control current flow through a control resistor. The control resistor is connected in parallel with parallel, reverse-connected light emitting diodes forming the light emitting portions of optical isolators. The photo transistor portions of the optical isolators are connected in parallel and control the generation of a trigger signal adapted to control the closure of relay contacts. Regardless of the instantaneous state of the AC voltage waveform (e.g., positive or negative), during the higher portions of the AC voltage waveform, one or the other of the LED's is forward biased and emits light, causing its related photo transistor to be turned on and the trigger signal to be at a low level. As the AC waveform approaches the next zero crossing point, it drops below the breakdown level of the zener diodes, causing current flow through the control transistor (and the forward bias voltage) to terminate, causing the previously lit LED to go out. The trigger signal then rises to a high level adapted to cause relay contact closure. The time between trigger signal rise and the zero crossing point is exactly related to the relay closure time.

REFERENCES:
patent: 3223850 (1965-12-01), Carrozza et al.
patent: 3342996 (1967-09-01), Crusinberry
patent: 3537757 (1970-11-01), Griffin
patent: 3697782 (1972-10-01), Matoaka
patent: 3816763 (1974-06-01), Korn et al.

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