Destructive Physical Analysis (DPA) Testing for Relays

There are two common categories of relays that are subjected to DPA testing: electromechanical relays (EMRs) and solid-state relays (SSRs). EMRs have contacts that are open or closed using magnetic force. EMRs employ at least two circuits; the energizing (input) circuit and the contact (output) circuit(s). The energizing circuit consists of a coil, similar to an inductor, that produces a magnetic field when current flows through it that moves an armature. The armature pivots and usually has arms with glass beads that move a center common contact known as a moveable contact. The moveable contact functions to select the position of the contacts – open or close.

Pole is the number of completely isolated circuits that can pass through a switch. Current can be carried through only one circuit at a time for single-pole contact while double-pole contact can carry current through two isolated circuits simultaneously. Throw is the number of closed contact positions per pole that are available on a switch. A switch with a single throw contact can control one circuit, whereas a double-throw controls two.

SSRs differ from electromechanical relays as they do not employ mechanical metal contacts and the switching occurs electronically. They also have an input control circuit side and an output circuit. The input circuit performs the same function as the coil used in electromechanical relays, however, the circuit is activated or deactivated when voltage is applied. The voltage needs to be higher than the pickup voltage to activate the circuit. When the voltage applied is less than the required dropout voltage, the circuit is deactivated. The control circuit determines when the output circuit is energized or de-energized. The output circuit performs the same function as the mechanical contacts of electromechanical relays by switching on the load. Typically, SSRs have only one output contact. DPA of SSRs are performed as integrated circuits.