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(Image courtesy of Sensata-Crydom.)

Relays are electrically operated switches that control high-power devices using low-power signals. Relays control one circuit by switching contacts in a control circuit, usually not directly switching the load. Control signals are typically in the range of 3 – 32 volts DC. Relays also provide protection by detecting electrical abnormalities such as overcurrent, undercurrent, overloads and reverse currents to prevent equipment damage.

There are different types of relays, such as machine control relays, reed relays, electromechanical relays (EMRs) and solid-state relays (SSRs). In this article, we examine the differences between these types of relays, and understand seven solid reasons to use solid-state relays.

Types of Relays

Machine control relays are operated by a magnetic coil. These heavy-duty relays are usually used to control starters and other industrial components. They are more durable, but also more expensive, than general purpose relays such as electromechanical and solid-state relays.

Reed relays have a switching design with one contact that is normally open, which provides a fast operating switch. Reed relays are hermetically sealed in a glass envelope, so their contacts are protected against contaminants, fumes and humidity. This design ensures reliable switching and longer contact life.

Electromechanical relay contacts are operated by a magnetic force. This relay type is robust, but bigger and slower than solid-state relays. EMR operation time is in the range of 5 – 15 ms, which is insufficient for some applications. EMRs also contain moving parts, which are the main reason for their shorter life span. However, EMRs are available in a wide range of switch configurations and they are affordable and easy to replace.

A typical EMR consists of a heavy-duty frame that supports all parts of the relay; a coil, wound around a metal core, which creates an electromagnetic field; contacts to close or open the circuit; the armature, a moving part that opens and closes the contacts; a spring, to return the armature to its original position; and a yoke, which provides a low reluctance path for magnetic flux.

The main parts of an EMR. (Source: Wikimedia.)

Solid-State Relays

SSR technology continues to displace EMRs in many general-purpose applications. The main difference between SSRs and EMRs is that SSRs provide completely electronic switching and do not contain any moving contacts. Electronic devices such as silicon-controlled rectifiers enable this electronic current switching. SSRs can be fabricated with SCRs (silicon-controlled rectifiers), TRIACs (triodes for alternating current) or switching transistors, but MOS transistors are commonly used as the switching element.

SSRs are designed to ensure complete electrical isolation between input and output. When SSRs are switched off they have a very high resistance, and when they are conducting they have a very low resistance. SSRs can switch both AC and DC currents. SSRs can provide a wide range of current depending on the application, rating from microamps to hundreds of amps. SSRs provide a voltage range of 3 VDC to 32 VDC, making them useful for most electronic circuits. The SSR control signal input circuit consumes less power than EMRs. Additionally, the switching time of SSRs is much shorter compared to EMRs.

Connection of SSRs to the control circuit and load. (Image courtesy of Sensata-Crydom.)

An example of solid-state relays is the Sensata-Crydom Series 1 panel mount SSRs. These are available in a wide range of current and voltage ratings (10 to 90 A at 24 to 530 V AC), making them suitable relays for many industrial applications such as motion, power, heating and lighting control.

SSR Configuration

SSR design includes three main parts: an input circuit, a control circuit and an output circuit.

Equivalent circuit block and wiring diagrams of SSRs. (Image courtesy of Sensata-Crydom.)

The input circuit has the same function as the coil in EMRs. This circuit is activated when it senses a voltage higher than the relay’s specified pickup voltage. The input circuit is deactivated when the applied voltage is lower than the relay’s specified minimum dropout voltage. The control component is connected to this relay part.

The coupling between the input and output circuits determines when the output component should be energized or de-energized. The output circuit has the same function as the mechanical contacts in EMRs, switching the load. SSRs usually have only one output contact.

Seven Advantages of SSRs

The absence of movable parts in SSRs enables numerous important advantages compared to EMRs. These advantages can be grouped into seven key factors: design simplicity, long life, low power consumption, fast switching, quiet operation, low EMI noise and fitness for harsh environments.

1. Design simplicity: The circuit board footprint and total volume of SSRs are much smaller than EMRs of similar specifications. SSRs can also be lighter than EMRs by up to 70 percent, depending on the power. The size and weight advantages make SSRs highly desirable for embedded systems in order to save valuable installation space.

SSR operation is also position insensitive, so they are suitable for mounting in either a vertical or horizontal position. Some SSRs, such as the Sensata-Crydom Series 1 panel-mount SSRs, have housing with anti-rotation barriers. Although they are smaller in size, SSRs are not less powerful than EMRs. Optical coupling completely isolates the circuits of the relay, eliminating the failure caused by high voltage.

2. Long life: Since SSRs do not include any moving parts and contacts, there are no issues of arcing or mechanical wear. Consequently, the expected lifetime of SSRs is 50 times longer than EMRs, making them an ideal solution for applications that require frequent operation.

3. Low power consumption: SSRs do not need to energize a bulky coil and open and close contacts like EMRs do. This means that SSRs use significantly less power to operate than EMRs. The input power of SSRs must only be enough to drive an optical coupler LED, which is very low energy consumer. EMRs require input power in the range of hundreds of milliwatts to a few watts, while SSRs need an input power of microwatts to a few milliwatts.

4. Fast switching: SSRs provide much faster switching when compared to EMRs. SSRs switch on/off faster because there are no physical parts to move. The switching time depends on LED switching on/off time which responds to a control signal almost instantaneously (less than 100 µs). The average switching time of EMRs is from 5 to 15 ms.

5. Quiet operation: SSRs use electronic circuits to provide switching. Since they do not have moving parts, they have completely silent switching operation. This is a highly desirable feature in various commercial and medical applications.

6. Minimum EMI noise: Low noise SSRs provide both zero-voltage switching on and zero-current switching off, reducing the electromagnetic interference (EMI) noise to a negligible amount. The zero-crossover switching feature is one of the most important advantages of SSRs. This feature enables switching off the AC loads when the sine load current is zero, eliminating issues such as the arcing and electrical noise. Even when the input control signal is removed, the switching devices continue conducting until the current drops below its threshold value.

Zero-crossover switching.

This is why SSRs will never switch off the load in the middle of a sine wave peak, which is especially important in the case of inductive loads—otherwise large voltage spikes can appear. The zero voltage turn-on and zero current turn-off feature provides the minimum electrical disturbances generated by SSRs. These zero-switching relays are the most widely used relay type.

7. Ideal for harsh environments: In industry, harsh environments are characterized by the following factors: temperature, dust, humidity, vibration and mechanical stress. Since SSRs have no moving parts and are entirely enclosed in housing, they are well-suited for harsh environment applications. In addition, SSR operation does not cause sparking, making SSRs suitable for combustible environments. External magnetic fields have negligible effects on SSRs as well. Sensata-Crydom asserts that SSRs should be EMC compliant to operate reliably in harsh environments. Their Series 1 panel mount SSRs are EMC compliant to Level 3.

Solid-state relays are the way to go for many applications. To learn more about a wide range of SSR products that offer the advantages we’ve discussed in this article, visit Onlinecomponents.com.