Mechanical relays, contactors, and switches control the flow of electricity by the mechanical action of two conducting elements. The physical contact of two conducting elements creates a path for electricity. Remove that physical contact, and the electrical pathway is broken. Mechanical relays, contactors, and switches operate in this manner.
What must be further understood is that mechanical relays, contactors, and switches operate in a random manner with respect to the electrical signal to be switched. Imagine a 60-Hz ac power circuit. A mechanical relay, contactor, or switch will interrupt this circuit in a random manner with respect to the ac power waveform passing through it.
If the circuit is interrupted at a moment when the ac current flow is low, then little reverse voltage will result. However, if the circuit is interrupted at an instant when the ac current flow is high, then a corresponding high reverse voltage will result.
What does this mean for the electronic product design engineer? Mechanical circuits are, in fact, circuits that interrupt the flow of electricity in a “random” manner—random, of course, with respect to the ac current flowing through that circuit at any particular instant.
In a circuit that exhibits high inductance, such as a motor circuit, a solenoid circuit, or an electromagnetic circuit, reverse voltage can get very high. High voltages can damage and pit mechanical contacts. They also can cause noise and disturb other nearby sensitive circuitry.
Energy is stored in the electromagnetic field that surrounds an inductive circuit. The stored energy is high when the corresponding current flow in that circuit is high. Upon circuit interruption, the flow of current through the circuit immediately stops.
As current flow stops, the circuit’s electromagnetic field collapses, sending its unit of stored energy back into the wires from which it came. If the current is high, and the field is large, then a high voltage will be created. Mechanical relays, contactors, and switches all will exhibit this type of behavior.
Solid-state triac devices may be used to interrupt the flow of electricity in a manner that is synchronous with the ac current waveform. Triacs are three-terminal, solid-state semiconductor devices that permit the flow of ac current through two terminals so long as the third, the trigger, is energized. They also have the additional, highly desirable property that electrical current flow stops when current flowing through the device is zero.
This may seem obvious, but imagine that ac current is flowing through your inductive circuit. You now wish to turn that circuit off. You do so by removing the trigger signal from your solid-state triac device. The device continues to conduct electricity until the ac current waveform reaches zero, at which time no further ac current flows until the device is triggered again. The action of the solid-state triac device ensures that the circuit is broken only when zero current is actually flowing.
Now return to our inductive circuit. The solid-state triac device ensures that the circuit’s energy flow is interrupted only when the ac current waveform reaches its instantaneous zero. The stored energy in the circuit’s electromagnetic field will be at or near zero when the current flowing through that circuit is at or near zero.
With no field energy, there will be nothing to cause the high voltages we previously observed in mechanical switching systems. The solid-state triac device guarantees that circuit interruption will occur only at the zero-current instants, eliminating the worry of high-voltage switching transients.