A Solid State Relay (SSR) can be either Normally Open (NO) or Normally Closed (NC), depending on its internal circuit design, but most commonly, SSRs are Normally Open. This means that when no control voltage is applied to the input side, the output circuit remains open and no current flows through the load. Once a control signal is applied, the relay switches ON electronically, allowing current to pass through the output side.
In an NO (Normally Open) SSR, the load circuit activates only when the input signal is present, which makes it ideal for most automation and control applications. This type is widely used in heaters, motor control, and industrial machinery, where the circuit should remain OFF until a specific command is given. On the other hand, an NC (Normally Closed) SSR works in the opposite way — it stays ON when no control signal is applied and turns OFF when a control signal is received. However, NC types are less common because most industrial systems are designed to operate with NO switching logic for safety and control reasons.
Unlike traditional mechanical relays, an SSR has no moving parts. It switches electronically using semiconductor components such as triacs, thyristors, or transistors. This design gives it several advantages — silent operation, fast switching, long lifespan, and resistance to shock and vibration. Because it uses electronic switching, it also produces less electrical noise and requires very little maintenance compared to mechanical relays.
An example of a reliable Solid State Relay is the Omron SSR, which is available in various models and current ratings to suit different industrial needs. These relays are built for precision, high switching speed, and long service life. They are widely used in applications such as temperature control, packaging machinery, and semiconductor equipment.
To put it simply, most SSRs are Normally Open, designed to close the circuit only when an input control signal is applied. NC versions exist but are used in special applications where the load must stay active until a signal turns it off.