Mechatronics, Industrial Control & Instrumentation

Solid-state Relays (SSR) Features and Operation

A solid-state relay (SSR) is a purely solid-state device used for turning ON and OFF AC loads such as motors.

Features of a Solid-state Relay

Let’s consider the figure below:

Figure (a) SSR being used to drive a motor
Figure (a) SSR being used to drive a motor

Physically, the SSR is packaged in a box with four electrical terminals: two input terminals and two output terminals.

The input or control voltage of the SSR is typically 5 Vdc, 24 Vdc, or 120 Vac. The 5 Vdc models are designed to be driven directly from the TTL digital logic circuits. On the output side of the SSR you can notice the load is placed in series with the 120 Vac or 240 Vac power. The output current can be as high as 50 A in some models.

A large number of SSRs incorporate a feature called zero-voltage switching: The line current is switched ON at the precise time that the AC voltage is crossing the 0 V. This eliminates sharp output voltage-rise times and hence minimizes electromagnetic interference noise (EMI).

Operation of a Solid-state Relay (SSR)

Let’s consider the SSR circuit below:

Figure (b) SSR Circuit

Figure (b) shows a block diagram of the interior of a solid-state relay. The input voltage drives LED, and the light from the LED turns ON the phototransistor, which in turn, turns on the triac. The LED electrically isolates the input and output sections of the solid-state relay. This is significant as it allows the control electronics to have a separate ground from the powerlines and it prevents high-voltage spikes in the power circuit from working their way back upstream to the more delicate control electronics.

The hybrid solid-state relay is similar to SSR but uses a low-voltage, fast acting reed relay instead of an LED on the output triac. Employing the reed relay provides good electrical isolation and may work better than the SSR in some circumstances.

Related: Optoelectronics

Advantages of Solid-state Relays

  • They have no moving parts which means that, they are not prone to wear, this makes them practically immune to shock and vibration.
  • Because of the inbuilt electronics, they can be driven with a low-voltage source such as the transistor-transistor logic (TTL) signal regardless of the output-current capability.

Disadvantages of Solid-state Relays

  • They can be triggered by electrical noise.
  • Even when ON, the output resistance is not exactly 0 ohm, so there is some voltage drop resulting in power loss within the relay, and when OFF they may have alarming levels of leakage current.
  • Contact arrangements are limited, so they may not work  for all relay applications.
  • Even though, they are long-lasting, unlike electromechanical relays (EMR), they do not fail predictably.
John Mulindi

John Mulindi is an Industrial Instrumentation and Control Professional with a wide range of experience in electrical and electronics, process measurement, control systems and automation. In free time he spends time reading, taking adventure walks and watching football.

View Comments

Recent Posts

How AI and Machine Learning are Enhancing Electrical Control Processes

Electrical control processes form the backbone of industries, enabling seamless operations across sectors like manufacturing,…

2 days ago

Standard Process Signals for Industrial Instrumentation

Industrial measurement and control processes employ standard process signals that are used throughout all the…

2 weeks ago

Top 5 Benefits of Combining CCTV Cameras with Biometric Systems

The integration of advanced technologies in security systems has become imperative for ensuring safety and…

2 weeks ago

Sources of Power Quality Problems

Power quality may be affected by a number of issues. Our discussion in this article…

2 weeks ago

Common Terms Used When Describing Power Quality Problems

Power quality has become an important issue to electricity consumers at all levels of consumption.…

2 weeks ago

What to Expect from PCB Assembly Services in China

The importance of printed circuit board (PCB) technology has escalated throughout the years with the…

3 weeks ago