The Solid State Relays

Solid-State Relays

The Solid-State Relay (SSR) is an electronic switch that has no moving parts and offers low current control and a high current load. SSRs are designed to switch either AC or DC in a wide variety of loads from heating elements to motors and transformers and are increasingly taking the place of electromechanical relays (EMRs) in many applications.



While the EMR currently takes a large majority share of the relay market today, especially in terms of units, the SSR is fast increasing its penetration. This is been largely because of improvements in switching capabilities and also reliability of power semiconductors as well as the decreasing price differential. SSRs are perhaps double the cost of EMRs at lower amp ratings and potentially three times at higher ratings. And naturally this can be a major limitation in many markets such as transportation vehicles for example. But in manufacturing and process automation, relays are not a relatively significant expense whereas downtime for machinery can be costly. The higher-cost SSR with its higher reliability, longer life and relatively smaller size, among many other advantages, therefore offers an edge over EMRs in these types of applications.


Life and Reliability

SSRs employ semiconductor-switching elements such as thyristors, triacs and MOSFETs to switch load current. Initially SSRs and EMRs offer similar levels of reliability, but as the SSR has no moving parts it will over time gain the edge. Unlike the EMR, where life is dependent on the switching load and number of cycles – typical ratings are 100,000 to 500,000 operations – SSR reliability is mainly determined by the time in operation. Assuming usage within manufacturer specifications, the MTBF can be from 2-4 million hours and typical estimates for the number of operations can range from 50 million up to 500 million under normal operating conditions and depending on the application.


Switching, Power and Control

SSRs will turn on between 20 microseconds and 10 milliseconds after an input signal is applied and within half of an AC cycle after it is removed (for DC-control versions). EMRs, depending on type and ratings, can only switch at a maximum of 10 to 20 times a second; and the higher the switching frequency of the application, the lower the MTBF. When used in heating applications, the fast cycling of a SSR can dramatically improve the life of the heater by reducing thermal stress. SSRs also allow the switching of large loads with very low input power; a 15mA low-level logic signal for example can activate a relay switching as much as 125A. In addition, due to the fast switching time of SSRs, another advantage is phase-angle control. Essentially, more current can be provided to the load by increasing the portion of each AC half cycle when the relay is in the ON-state – a type of control that is not possible with EMRs.



In EMRs, contact bouncing causes short-term interruptions and degrade contact life, which does not occur in SSRs as they do not have contacts. Contact bounce is particularly unwanted in applications where relays are used for pulse counting. In addition, no contact erosion derating is required for SSRs, whereas EMR manufacturers need to specify relays in terms of maximum switching capacity, usually expressed in Volt-Amps or Watts. EMRs substantially derate with regard to maximum voltage or current capabilities and often derating is applied beyond manufacturer recommendations to extend contact life. In fact, this derating will often place the actual load handled by an EMR within the operating range of an SSR (see figure 1, courtesy of Crydom).



Shock and Noise

SSRs are not as sensitive to physical shock, vibration and acceleration, as again they do not have moving parts. In testing, SSRs have demonstrated functional shock resistance more than 10 times that of EMRs. A further advantage is quiet operation, which is highly beneficial in medical applications, environmental controls, lighting controls and other applications, whereas the EMR will make an acoustic noise during switching.



However, there are limitations for SSRs beyond cost, when compared with EMRs. Thermal dissipation for example: SSRs have residual voltage in the ON state, which in conjunction with the current flow in the triac, generates heat that needs to be dissipated usually via the use of a heatsink. Although in many cases, depending on factors such as load, ambient temperature and panel material, it is often sufficient to mount the relay to the metal panel housing.

Another limitation is leakage current in the OFF state. A low residual current circulates in the load, which can be harmful in applications controlling very low loads such as in small solenoid valves. Other limitations can be avoided by the use of various protection devices such as snubber circuits to handle dV/dt during switch-on or from mains interference, or devices to protect against overvoltage and transients from either the mains or components such as motors and solenoids.



RS stocks an extensive collection of SSRs and accessories including filters, heatsinks, adaptors, covers and mounting kits with products from leading manufacturers such as Omron, Panasonic and Siemens. A highly popular brand of SSRs is the Crydom range, which includes a wide range of AC- and DC-output SSRs in industry-standard panel-mount, PCB-mount and DIN-rail packages as well as a range of solid-state contactors, which are widely used in three-phase AC heater and motor control applications.

Products recently added to the Crydom SSR line-up include an expanded AC-output 22.5mm DIN-rail-mount CKR series, which offers zero voltage for resistive loads and instantaneous turn on for phase control or motor switching. The CKRA products are rated at 110 to 280V(AC) input and the CKRD models complement the series rated at 4 to 32V(DC) input.