Time delay circuit employing field effect transistor and differential operational amplifier

Claim 2

In an electrical control system for controlling the state of energization of a load responsive to an input control signal, a timing circuit for providing a predetermined time delay between initiation of the control signal and a change in the state of energization of the load, said timing circuit comprising a time delay network including an interconnected resistor and capacitor, selectively energizable input switch means connected to said time delay network for effecting charging and discharging of said capacitor in predetermined time intervals responsive to the initiation of the control signal, a voltage sensing switch control means connected to said capacitor for sensing the voltage thereacross and producing an energizing signal systematically related to said voltage, comparison means including first and second input terminals and an output terminal, means for providing said first input terminal with a bias signal having a preselected voltage level relative to the voltage level at said second input terminal for maintaining said comparison means in a de-energized state and said second input terminal being connected to said voltage sensing switch control means for receiving said energizing signal, said comparison means being energized only when said energizing signal is of a predetermined magnitude relative to said bias signal so as to provide an output signal responsive to energization thereof, and selectively operable output switch means connected to said output terminal for controlling the state of energization of the load.

Claim 3

In a system in accordance with claim 1 wherein said comparison means comprises a bi-stable device which provides an output signal to effect operation of said output switch means in only one of its states.

Claim 4

In a system in accordance with claim 2 wherein said comparison means is maintained in said one state in response to the establishment of a preselected voltage level across said capacitor.

Claim 5

In a system in accordance with claim 3 wherein said comparison means comprises a differential operational amplifier.

Claim 6

In a system in accordance with claim 4 wherein said voltage sensing switch control means comprises a buffer amplifier having an input terminal coupled to said capacitor and having an output terminal coupled to said second input terminal of said differential operational amplifier for effecting an impedance match therebetween.

Claim 7

In a system in accordance with claim 5 wherein said buffer ammplifier has a relatively high input impedance and a relatively low output impedance so as to generate said energizing signal at its output terminal in relatively close correlation with the voltage level across said capacitor.

Claim 8

In a system in accordance with claim 6 wherein said buffer amplifier comprises a field effect transistor.

Claim 9

In a system in accordance with claim 7 wherein said field effect transistor is connected in a source follower configuration having its gate connected to said capacitor and having its source coupled to said second input terminal of said differential operational amplifier.

Claim 10

In an electrical control system for controlling the state of energization of a load responsive to a control signal, a timing circuit for providing a predetermined time delay between initiation of the control signal and the production of an output signal, said timing circuit comprising, a time delay network including an interconneted resistor and capacitor, selectively energizable input switch means connected to the time delay network for effecting the charging of said capacitor to a first preselected voltage in a predetermined time interval, a voltage responsive control means connected to said capacitor for generAting a varying energizing signal which varies in close correlation with the voltage level established across said capacitor during charging thereof, bi-stable voltage threshold detector means having first and second input terminals and an output terminal, means to maintain said first input terminal at a substantially constant reference voltage level relative to the voltage level at said second input terminal for maintaining said voltage threshold detector means in its first state, said second input terminal being connected to said voltage responsive control means so as to receive said varying energizing signal thereat for effecting operation of said voltage threshold detector means in its second state when said varying energizing signal reaches a preselected magnitude relative to said reference voltage level at said first input terminal upon expiration of said predetermined time interval during charging of said capacitor, said detector means generating an output signal only during operation in its second state, and selectively energizable output switch means connected to said output terminal of said threshold detector for controlling the state of energization of the load, said output switch means being energized by said output signal.

Claim 11

In an electrical control system for controlling the state of energization of a load responsive to a control signal, a timing circuit for providing a predetermined time delay between initiation of the control signal and the production of an output signal, said timing circuit comprising, a time delay network including an interconneted resistor and capacitor, selectively energizable input switch means connected to the time delay network for effecting the charging of said capacitor to a first preselected voltage in a predetermined time interval, a voltage responsive control means connected to said capacitor for generAting a varying energizing signal which varies in close correlation with the voltage level established across said capacitor during charging thereof, bi-stable voltage threshold detector means having first and second input terminals and an output terminal, means to maintain said first input terminal at a substantially constant reference voltage level relative to the voltage level at said second input terminal for maintaining said voltage threshold detector means in its first state, said second input terminal being connected to said voltage responsive control means so as to receive said varying energizing signal thereat for effecting operation of said voltage threshold detector means in its second state when said varying energizing signal reaches a preselected magnitude relative to said reference voltage level at said first input terminal upon expiration of said predetermined time interval during charging of said capacitor, said detector means generating an output signal only during operation in its second state, and selectively energizable output switch means connected to said output terminal of said threshold detector for controlling the state of energization of the load, said output switch means being energized by said output signal.

Claim 12

In a system in accordance with claim 9 wherein said voltage responsive control means comprises a buffer amplifier coupled between said resistor and said second input terminal for effecting an impedance match between said resistor and said second output terminal.

Claim 13

In a system in accordance with claim 10 wherein said buffer amplifier includes a field effect transistor having a relatively high input impedance and a relatively low output impedance.

Claim 14

In a control system in accordance with claim 11 wherein said bi-stable voltage threshold detector comprises a differential operational amplifier.

Claim 15

In a control system for controlling the state of energization of a load responsive to a control signal, a timing circuit for providing a predetermined time delay between initiation of the control signal and the cessation of an output signal, said timing circuit comprising a time delay network including an interconnected resistor and capacitor, selectively energizable input switch means connected to the time delay network for effecting charging of said capacitor to a first preselected voltage level and for subsequently effecting discharging of said capacitor to a second preselected voltage level in a predetermined time interval, voltage responsive control means connected to said capacitor for generating an energizing voltage signal which varies between first and second levels in close correlation with the first and second preselected voltage levels respectively across said capacitor, a bi-stable voltage threshold detector means having a first and second input terminals and an output terminal, means to maintain said first input terminal at a first substantially constant reference voltage level effecting operation of said voltage threshold detector in its first state, said second input terminal being connected to said voltage responsive control means so as to receive said energizing voltage signal thereat for effecting operation of said voltage threshold detector means in its second state when said energizing signal is at said first level corresponding to charging of said capacitor to said first preselected voltage level and for effecting operation of said voltage threshold detector means in its first state when said energizing signal is at said second level corresponding to discharge of said capacitor to said second preselected voltage level upon expiration of said predetermined time interval, said voltage threshold detector means generating a signal at its output terminal only during operation in its second state, and selectively energizable output switch means connected to said output terminal of said voltage threshold detector means for controlling the state of energization of the load, said output switch means being de-energized responsive to operation of said voltage threshold detector means in its first state accompanying discharge of said capacitor to said second preselected voltage level upon expiration of said predetermined time interval.

Claim 16

In a control system for controlling the state of energization of a load responsive to a control signal, a timing circuit for providing a predetermined time delay between initiation of the control signal and the cessation of an output signal, said timing circuit comprising a time delay network including an interconnected resistor and capacitor, selectively energizable input switch means connected to the time delay network for effecting charging of said capacitor to a first preselected voltage level and for subsequently effecting discharging of said capacitor to a second preselected voltage level in a predetermined time interval, voltage responsive control means connected to said capacitor for generating an energizing voltage signal which varies between first and second levels in close correlation with the first and second preselected voltage levels respectively across said capacitor, a bi-stable voltage threshold detector means having a first and second input terminals and an output terminal, means to maintain said first input terminal at a first substantially constant reference voltage level effecting operation of said voltage threshold detector in its first state, said second input terminal being connected to said voltage responsive control means so as to receive said energizing voltage signal thereat for effecting operation of said voltage threshold detector means in its second state when said energizing signal is at said first level corresponding to charging of said capacitor to said first preselected voltage level and for effecting operation of said voltage threshold detector means in its first state when said energizing signal is at said second level corresponding to discharge of said capacitor to said second preselected voltage level upon expiration of said predetermined time interval, said voltage threshold detector means generating a signal at its output terminal only during operation in its second state, and selectively energizable output switch means connected to said output terminal of said voltage threshold detector means for controlling the state of energization of the load, said output switch means being de-energized responsive to operation of said voltage threshold detector means in its first state accompanying discharge of said capacitor to said second preselected voltage level upon expiration of said predetermined time interval.

Claim 17

In a control system in accordance with claim 13 wherein said voltage responsive control means comprises a buffer amplifier coupled between said resistor and said second input terminal for effecting an impedance match therebetween.

Claim 18

In a control system in accordance with claim 14 wherein said buffer amplifier includes a field effect transistor having a relatively high input impedance and a relatively low output impedance.

Claim 19

In a control system in accordance with claim 14 wherein said bi-stable voltage threshold detector comprises a differential operational amplifier.

Claim 20

In a control system in accordance with claim 1 including power input terminals, the selectively energizable input switch means includes an NPN transistor whose emitter-collector is connected across the power input terminals, a junction formed between the resistor and capacitor of the time delay network, the collector connected to the junction, a voltage divider having two legs and a junction therebetween connected across the input power terminals, the base of the transistor connected to the voltage divider junction and a switch in one leg of the voltage divider such that when the switch in the voltage divider leg is open the transistor is conductive shunting current from the capacitor and upon closing of the switch the transistor is rendered non-conductive and the capacitor is charged.

Claim 21

In a control system in accordance with claim 1 the time delay network includes a double pole double throw relay to obtain in one position an on delay and in the other position an off delay.