Off delay solid-state time delay apparatus

Claim 1

Off delay time delay apparatus comprising, in combination, first and second teRminals adapted to be connected to a singlephase alternating current line, first and second lead means connected to the first and second terminals respectively, a full wave rectifier having one input directly connected to the second lead means and having the other input thereof adapted to be connected by way of a relay coil to the first lead means, a controlled rectifier having a control element and having the anode-cathode path thereof connected across the output of the full wave rectifier, circuit means connecting the controlled rectifier anode to the positive output lead of the full wave rectifier and connecting the controlled rectifier cathode to the negative output lead of the controlled rectifier, the controlled rectifier while gated on forming a closed alternating current switch and causing full energizing current to flow from the alternating current line through the relay coil, said controlled rectifier being normally in a nonconductive condition, other circuit means including in series a first diode, a first resistor and a first Zener diode connecting the control element of the controlled rectifier to the first lead means, a second resistor connecting the control element of the controlled rectifier to the negative output lead of the full wave rectifier, a pair of complementary NPN and PNP transistors, the emitter of the NPN transistor being connected to the negative terminal of the full wave rectifier, means forming a connection between the collector of the NPN transistor and the base of the PNP transistor, an R-C timing circuit including a timing capacitor and an adjustable timing resistor connected in parallel, one terminal of the parallel timing circuit being connected to the base of the PNP transistor and the other terminal of the parallel R-C timing circuit being connected to said aforementioned negative output lead of the full wave rectifier, a second diode connecting the emitter of the PNP transistor to the junction between the first resistor and the first Zener diode, current flowing through said second diode and through the emitter-base junction of the PNP transistor producing a voltage across the base thereof determined by the voltage drop in the timing resistor, said last-named voltage causing the pair of complementary transistors to conduct whereby the voltage at the junction between the first resistor and the first Zener diode is reduced to a value less than the Zener voltage of said first Zener diode and no gate current is supplied through said first Zener diode to the control element of the controlled rectifier, an initiating switch having one terminal thereof connected to the first lead and to the first terminal, circuit means including in series a third resistor, a third diode and a fourth diode connecting the other terminal of the initiating switch to the timing capacitor, a second Zener diode connected between the junction between the third and fourth diodes and said negative output lead of the full wave rectifier, closing the initiating switch causing the aforementioned timing capacitor to be quickly charged to a voltage which when applied to the base of the PNP transistor renders the pair of complementary transistors nonconductive thereby causing current to cease to flow through the first diode connected between the emitter of the PNP transistor and the junction between the first resistor and the first Zener diode, thereby causing the voltage at said junction to rise to a value which exceeds the Zener voltage of the first Zener diode thereby causing a gating current to pass through the first Zener diode and to be applied to the control element of the controlled rectifier causing said controlled rectifier to be gated on and causing full current to flow through said relay coil, said initiating switch being adapted to be thereafter opened, said timing resistor discharging said timing capacitor within a predetermined time interval after the switch is opened to a voltage at which current begins to flow through The emitter-base junction of the PNP transistor and through the second diode connected to the junction between the first resistor and the first Zener diode thereby causing the voltage at said junction to fall to a value less than the Zener voltage of the first Zener diode thereby causing the controlled rectifier to assume a nonconductive circuit open condition and thereby causing current through the relay coil to fall to zero.