High reliability AC load switching circuit
A high reliability AC load switching circuit is disclosed. In some embodiments, the AC load switching circuit includes a high-speed switch connected between the load and the voltage source, a cutoff switch connected between the load and the voltage source in parallel with the high-speed switch, and a level detector connected to the voltage source and to a control input of the high-speed switch. The high-speed switch may be a solid-state switch, for example, a TRIAC or a bidirectional switch, and the cutoff switch may be an electromechanical switch, for example, a relay. In some embodiments a snubber is connected in parallel with a solid-state switch. In some embodiments a microcontroller is connected to an electromechanical switch and the level detector. In some embodiments, both a first cutoff switch and a second cutoff switch are used.
1. A switching circuit to selectively connect a load to a voltage source, the switching circuit comprising:
a high-speed switch connected between the load and the voltage source;
a snubber connected in parallel with the high-speed switch;
a cutoff switch connected between the load and the voltage source in parallel with the high-speed switch;
a level detector and control circuit connected to the voltage source and to a control input of the high-speed switch, wherein the control input is further connected to a portion of the switching circuit that is between the load and an output of the high-speed switch and between the load and an output of the cutoff switch; and
a controller comprising a first enabling input connected to the level detector and control circuit and a second enabling input connected to the cutoff switch, the controller further comprising a processor and memory storing at least one of firmware and software that is executable such that: the first enabling input is activated in response to a powering on signal received from the controller and the level detector and control circuit turns on the high-speed switch at a zero crossing of the voltage source, and the second enabling input is activated independent of the operation of the high-speed switch after the high-speed switch is turned on to turn on the cutoff switch, the high-speed switch remaining turned on after the cutoff switch is turned on.
2. The switching circuit of claim 1 wherein the high-speed switch comprises a solid-state switch.
3. The switching circuit of claim 2 wherein the solid-state switch comprises a triode for alternating current (TRIAC).
4. The switching circuit of claim 2 wherein the solid-state switch comprises bidirectional switch.
5. The switching circuit of claim 1 wherein the cutoff switch comprises an electromechanical switch and further comprising:
a transistor connected to the controller; and
a resistor connected between the transistor and the electromechanical switch.
6. The switching circuit of claim 1 further comprising a second cutoff switch connected in series with the high-speed switch.
7. The switching circuit of claim 6 wherein the cutoff switch, the second cutoff switch, or both the cutoff switch and the second cutoff switch each comprise an electromechanical switch.
8. The switching circuit of claim 7 wherein the electromechanical switch comprises a relay.
9. The switching circuit of claim 6 wherein the second cutoff switch is connected between the high-speed switch and the voltage source.
10. The switching circuit of claim 6 wherein the second cutoff switch is connected between the high-speed switch and the load.
11. A method of selectively connecting a load to an AC voltage through a switching circuit comprising a controller comprising a first enabling input connected to a level detector and control circuit and a second enabling input connected to a cutoff switch, the controller further comprising a processor and memory storing at least one of firmware and software, the method comprising:
activating the first enabling input from the controller in response to a powering on signal received from the controller to thereby enable the level detector and control circuit to turn on a solid-state switch between the AC voltage and the load at zero-crossing of the AC voltage;
suppressing voltage spikes and/or dampening ringing to prevent misfiring of the solid-state switch using a snubber connected in parallel with the solid-state switch; and
activating a second enabling input from the controller to close a first electromechanical switch connected in parallel with the solid-state switch independent of the operation of the solid-state switch after the turning on of the solid-state switch to turn on the cutoff switch, maintaining the solid-state switch turned on after the cutoff switch is turned on using a control input of the solid-state switch that is connected between the load and an output of the solid-state switch and between the load and an output of the cutoff switch.
12. The method of claim 11 further comprising:
in response to a powering off signal received by the controller, deactivating the first electromechanical switch; and
turning off the solid-state switch after deactivating the first electromechanical switch.
13. The method of claim 11 further comprising activating a second electromechanical switch connected in series with the solid-state switch prior to turning on the solid-state switch and in response to activation of the switching circuit.
14. The method of claim 13 further comprising:
deactivating the first electromechanical switch;
turning off the solid-state switch; and
deactivating the second electromechanical switch.
15. The method of claim 11 wherein the solid-state switch comprises bidirectional switch.
16. The switching circuit of claim 1 wherein the high-speed switch is configured to remain turned on for an entire duration that the cutoff switch is turned on.