Solid State Relays or SSR Relays

Solid State Relays operates like electromechanical relays except that these relays are contactless. They work by employing electronic components such as triacs, thyristors, and power transistors as part of the switching element.​ An input signal to an SSR switches the output from a non-conducting state to a conducting state, switching the load circuit on and off. Instead of using a magnetic circuit for the intermediate signal to achieve galvanic isolation between the input and the output, as in the electromechanical relay, SSRs use optoelectronics, capacitive connection, and electrical field coupling as the intermediate signal. Therefore, SSRs respond quickly, are highly resistant to vibration and shock, quiet when switching, and unaffected by the presence of dust, gases, and other contaminants.

Because of material design, solid state relays have a restricted switching range and capacity, limited by the size and the thermic resistance of the switching components. While SSRs have only normally open (NO) output and need different switching elements for AC and DC, these products offer longer switching life due to the contactless system, have high switching reliability under certain conditions, and can switch AC loads at specific points in the cycle, such as switching off when the load current crosses zero and switching on when the load voltage crosses zero. And, in most cases, SSRs can be controlled directly from other electronic circuits.

Conversely, solid state relays have no galvanic separation in the load circuit when in the off state and because these are highly susceptible to external electrical influences – such as surges, spikes, and strong electrical fields, SSRs require protection circuits and ultra-fast fuses. Another usage consideration is the need for switching higher power loads. These types of applications require higher input power for higher output switching. As a result, these requirements will likely drive the device to utilize a heatsink when switching high currents which is due to the heat generated by the switching element. As with any application consideration of this type, the addition of a heatsink will increase the size/weight of the overall component.

Our solid state relays are engineered for ultra-reliable switching of AC, DC, or bi-directional loads, featuring inherently low switching acoustical noise and extremely long life. These SSR designs use the electrical and optical properties of semiconductors to perform switching functions and output isolation. With no moving parts, contact wear bounce issues are eliminated. Additionally, low voltage turn-on and zero current turn-off eliminates electrical noise (i.e., back EMF) transients.

TE SSRs can be used in a variety of applications where fast switching times and a high number of life cycles are required. Further design features include utilization of thick film hybrid circuit technology, panel mount configurations, screening to W or Y MIL levels (where applicable), and approval to MIL specs or applicable DSCC dwgs.

As an example, the KILOVAC brand MS14 series subminiature SSRs employ state of the art photo-voltaic optical isolation providing 1000Vrms input/output isolation and power Mosfet output chips for ultra-reliable high speed switching of DC or bidirectional loads up to 350mA and 400Vdc. The input is current regulated and buffered to minimize power dissipation and permit driving the relay direct from CMOS or TTL. The relay is packaged in a custom hermetically sealed low-profile .100 grid configuration which conserves space for high density PC board circuitry.