Low noise, reliable switching solid state relays
An SSR or solid state relay is a relay with no moving contacts. They work by employing semiconductor switching elements like triacs, thyristors, and diodes. Solid state relays are designed for applications that require high switching speed and feature low switching acoustical noise and extremely long life. These relays are used when direct control by electronic circuits is necessary and switching without arcing or contact chatter is required. SSRs are not a replacement but complement electromechanical power relays.
Specifications for Solid State Relays
- AC SSRs are qualified to MIL-PRF-28750/9 for 2A rated JDS9 series, and /10 for 25A rated JPS10 series, and to DSCC drawing 86031 for the 10 A PS12 series.
- DC SSRs are qualified to loads from low level to 2 Amps/60V.
- Some TE DC SSR relays are available with optional status monitoring and/or short circuit protection.
- 10 and 25A rated AC SST mounting configurations are designed primarily for panel or chassis mounting.
- The 2A rated AC JDS9 series and all DC models are designed for printed circuit board mounting, with some models configured for surface mount connection.
- Most models are approved to the applicable military specification, MIL-PRF 28770, or associated detail drawings.
- Special application specific models can be made available.
In general, SSRs function similar to electromechanical relays except that these relays are contactless, using 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.
Frequently Asked Questions
Solid State Relays (SSRs)
Q: How are SSRs different from electromechanical relays?
A: Both perform the same switching function, but their constructions are different. SSRs do not have any moving parts such as an armature or metal contacts, making them free of noise and arcs.
Q: Are SSRs expensive compared to electromechanical relays?
A: Initial cost of purchase is high for SSRs. But due to their long life capactiy and ability to handle heavy loads, SSR cost works out economically in the long run.
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.