Comparison of Electromechanical Relays and Solid State Relays

Comparison of Electromechanical Relays and Solid State Relays

Although both electromechanical relays (EMRs) and solid state relays (SSRs) are designed to provide a general switching function, they both achieve the end result in different ways.

Basically, EMRs provide switching by using electromagnetic devices and sets of contacts, while SSRs rely on electronic devices such as silicon controlled rectifiers (SCRs), triacs, and MOSFETs to operate without contacts switch below. The following figure graphically illustrates a simple example of EMR and SSR with input circuit and load circuit.

Simple comparison of EMR and SSR

Simple comparison of EMR and SSR with input and load circuits.

Types of Solid State Relays

There are basically four solid state designs that dominate the control market: direct control, transformer isolation, optical (LED) isolation, and hybrid solid state relays. Each will be discussed because each provides similar but distinct operational characteristics.

direct control

The following figure shows in block diagram form a direct control or contact closure type relay used to switch an AC load. In this SSR, a set of external switch contacts, connected to the same AC voltage source as the load to be controlled, serves as the control circuit. A triode AC semiconductor (triac) or a pair of back-to-back silicon controlled rectifiers (SCR) can be used as load switching devices.

Solid State Relay with Triac

Solid state relays with triacs can form direct control or contact closure type relays for switching AC loads.

When the switch contacts are closed, the triac conducts and applies the AC supply voltage to the load. Opening the external contacts closes the triac, removing the AC supply voltage from the load. To prevent accidental opening of triacs due to transient voltage surges, transient protection networks are included.

When direct control of a DC load is required, the solid state relay configuration shown below is used. In this circuit, DC power transistors are used as electronic switching devices. As shown in the circuit diagram for controlling an AC load, the external switch contacts are used to turn the power transistor on and off. Alternatively, a second source voltage can be used in place of an external contact to control the operation of the power transistor. It is important to note that when an external contact is used to control the operation of the SSR, the source voltage appears on the external control contact. Therefore, they must be properly protected to ensure the safety of users.

Solid State Relays with DC Power Transistors

Solid state relays with DC power transistors form direct control or contact closure type relays for switching DC loads.

Optically isolated SSR

Optically isolated solid state relays are equivalent to SPST standard relays. Isolation is optically provided by using light emitting diodes (LEDs) and photodetectors as shown in the figure below. The LED accepts the relay control voltage and converts this power into light energy through the LED. This light is collected by a photodetector that controls a triac (or MOSFET) gate trigger circuit. When the specified control voltage is reached, the gate is triggered due to sufficient light energy being transmitted to the photodetector. Removing or lowering the control voltage reduces the light output and stops the triggering circuit. The DC voltage required to operate an LED can be a specific voltage, such as 5 VDC, or typically falls within the range of 3 to 32 volts. The characteristics of LEDs allow control circuit designs to accept a wide range of input voltages.

Operation of Optically Isolated Solid State Relays

Operation of optically (LED) isolated solid state relays.

The input and output isolation of such relays can reach 10 billion ohms. The breakdown voltage is typically 1500 V RMS 50/60Hz. This isolation can only be provided up to a certain point, depending on the ratings of the components used. These ratings can be found on most manufacturers' data sheets. Once these ratings are exceeded, transients can be introduced into the control circuit.

Based on a 10% reduction in light output, the life expectancy of the optocoupler is over 50,000 hours. It starts in microseconds, is immune to shock or vibration, has no bounce, and can be driven directly from MOS or TTL gates.

On/off state control of the photodetector allows the output of the triac gated logic state. Optocoupled designs typically have zero-voltage turn-on of the triac. This means that whenever the input control voltage is applied, the triac will not turn on until the source voltage is on the order of 15V. This reduces EMI at turn-on to less than one percent of EMR and about one-fifth of EMR.

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