Features and Applications of Solid State Relays

Features and Applications of Solid State Relays

Solid State Relays (SSRs) function the same as electromechanical relays, but they are non-moving, non-contact devices that can switch voltages to hundreds of Vac for hundreds of thousands of times or more... which makes them useful for switching heating elements, Electric motors and transformers that require frequent and high-speed switching.

Electromechanical relays and solid state relays use different technologies to perform essentially the same function. The fast switching of the SSR makes it suitable for countless high power loads.

In contrast to electromechanical relays, subassemblies in SSRs are entirely electronic:

1. The input circuit of the SSR (such as the coil of an electromechanical relay) is connected to the system control. As the voltage into the SSR changes (typically 3 to 32 Vdc), it prompts the input circuit to act. In a common variant, the circuit activates when any voltage in the range above the relay's activation voltage value is applied...and deactivates when the input drops below the relay's dropout voltage value.

2. The SSR's coupler transmits power-up and power-down commands to the relay outputs - acting as a middleman between the input and output circuits. Note that this part of the SSR is specifically designed to ensure that the input circuit is galvanically isolated from the output circuit...so high power (output) load currents are isolated by the coupled part of the relay. This reliably prevents load current from flowing to the relay input - even during system failures.

The coupling part of the SSR uses several techniques:

The most common type is the optocoupler SSR, which uses an LED or infrared light source on the input circuit to communicate with a photosensitive semiconductor on the output switch side. In contrast, a transformer-coupled SSR uses a dc-ac converter to generate an output that is magnetically coupled to the output through a low-power transformer.

3. Next, the trigger or drive circuit of the SSR connects to one of the following designs:

• Silicon Controlled Rectifiers (SCRs) for high-speed switching of (usually) short "on" periods

• Back-to-back thyristors called triacs - short for AC triodes

• Metal Oxide Semiconductor Field Effect Transistor (MOSFET) or Darlington Transistor (for DC)

• Insulated Gate Bipolar Transistor (IGBT) for DC

Here, zero-switching operation (most common for SSRs), as well as peak switching, DC switching, and momentary switching are options for adjusting relay action based on the type of load the system is driving. For example, analog switches use a synchronizing circuit to make the output voltage track the input voltage and allow a variety of possible output voltages within the allowable range of the SSR. These excel in the soft-start design of the drive motor.

4. In addition, the output (power) circuit of the SSR is connected to the load being controlled. In addition to switching, it may include a snubber circuit (in some cases, a reverse-connected diode) or a zero-crossing detector to reduce spikes and transients and electromagnetic interference (EMI) during switching. This is a problem because the SSR switches the load current through an additional inductive load - and (according to Faraday's law) interruption of the current causes the voltage to rise. Any such rise above the maximum ratings on the SSR may result in damage.

Where SSR excels

SSRs are compatible with a wide variety of control systems and are immune to magnetic noise; their solid-state nature means they can be mounted in different orientations...and SSRs are immune to severe vibration. It's true that SSRs are more expensive than some of the alternatives, but the most sophisticated can provide exceptional longevity.

Consider some motor applications for SSR:

• On large conveyor belts or motors on assembly lines that may clog

• Industrial motor blowers for commercial ovens risk overworking if the door is ajar

• On motors with overcurrent conditions or incorrect starting current

• On neglected motors and those attached to excessively worn mechanical parts

• On general purpose motors in high temperature environments

Such motor-driven machinery can include protective relays (electromechanical relays and SSRs) on its power supply to detect any such overheating and shut down the motor to prevent damage. Durable SSRs are very common in such applications because they have no moving parts that would degrade precision life...in fact, they are often more durable than the equipment on which they are installed.

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