Why use transistor semiconductor relays for switching DC currents on and off?

Why use RSR88 solid state relays?

Both electromagnetic and semiconductor relays are widely used in industrial automation, and the choice of a specific type depends on the requirements of a particular application. 

Solid state relays (SSR) excel in situations where speed, durability and reliability are important. Their key advantages are:

• No mechanical components – no contact wear, high resistance to shock and vibration, significantly longer service life.
• Fast switching – response in fractions of a millisecond, ideal for applications requiring high switching frequencies.
• Silent operation – no noise or sparking, allowing for use in environments with high acoustic requirements.
• Galvanic isolation – effective separation of the control circuit from the load (up to 2,500 Vrms), increasing system safety.
• Compact design – miniature dimensions and mounting on a board or in a GZMA socket facilitate integration with control systems.
• High operational durability – no sparking or contact wear translates into a much longer service life than in electromagnetic relays.

These features make Relpol semiconductor relays an excellent choice for applications in automation, robotics, testing systems, DC motor control and monitoring systems.

RSR88 NC – solid state relay, NC contact normally closed

Solid state relays with a NC contact normally closed are a very rare solution on the market, and the operation of such a relay differs from that of a standard solid state relays. These devices, thanks to a specially built-in transistor circuit, have a closed output circuit in the idle state, i.e. when the control signal is not active. When the control voltage is applied, the relay disconnects the circuit.

Benefits of using an NC version RSR relay

• System safety and protection

In many industrial applications, it is important that the device remains active in the event of a power failure, control signal failure or communication interruption.
The NC version ensures that the load is powered by default until the control system decides otherwise.
This is particularly important in systems such as:

- emergency power supply
- ventilation or cooling of critical components
- safety systems that need to remain operational until a fault is detected.

• Energy savings and simplified control

In applications where the device is to be switched on most of the time, the use of the NC version reduces the energy consumption of the control circuit.
The relay does not require a constant control voltage to maintain the ‘on’ state — voltage is only needed to disconnect.
This reduces power losses during long-term operation, especially in systems with multiple relays.

• Greater design flexibility

The NC version allows you to build systems with reverse logic.
It allows designers to adapt the control system to the requirements of the process — e.g. so that a fault signal activates the power disconnection, and the absence of a fault keeps the circuit closed.
This makes it easy to combine RSR88 NC and RSR88 relays into a single, logically consistent control architecture without the need for additional logic components.

• Reliability and certainty of operation in emergency situations

The NC version minimises the risk of unwanted power disconnection in the event of a control signal failure, which is crucial in systems requiring continuous operation.
This makes the RSR88 NC ideal for systems where interruption of operation could cause loss or danger (e.g. production lines, industrial air conditioning systems, fan or valve control systems).

Where to use RSR88 series relays:

• measuring devices
• industrial automation systems
• PLC input/output modules
• photovoltaic installations and energy storage facilitie
• medical and laboratory equipment
• telecommunications and electronic systems emergency power supply

The RSR88 and RSR88…NC series solid state relays from RELPOL are modern and functional solutions for DC applications in industrial automation. Choosing SSRs allows you to take advantage of the benefits of no moving parts, fast switching, quiet operation and high reliability.

The RSR88...NC version – with a normally closed contact – offers additional valuable solutions in systems where default circuit conduction is required in the absence of a control signal or in an emergency situation. This allows designers to achieve a higher level of safety and reliability. If your applications involve the control of DC motors, auxiliary power supplies, monitoring systems or require a fail-safe condition, the RSR88…NC relay is a fully justified solution.