Solid state relays are the choice for many applications

       Relays are electrical switches that use low power signals to control high power devices. Relays control a circuit by switching contacts in the control circuit, usually without directly switching the load. Control signals are typically in the range of 3–32 VDC. Relays also provide protection by detecting electrical anomalies such as overcurrent, undercurrent, overcurrent and reverse current to prevent equipment damage.
       There are different types of relays such as machine control relays, reed relays, electromechanical relays (EMR) and solid state relays (SSR). In this article, we’ll look at the differences between these types of relays and discover seven good reasons to use solid-state relays.
       Machine control relays are controlled by magnetic coils. These heavy duty relays are commonly used to control starters and other industrial components. They are more durable than general purpose relays such as electromechanical relays and solid state relays, but are also more expensive.
       Reed relays have a switch design with one normally open contact, providing fast operation of the switch. Reed relays are enclosed in a glass housing, so their contacts are protected from dirt, smoke and moisture. This design ensures reliable switching and longer contact life.
       The electromechanical relay contacts are magnetically controlled. This type of relay is durable, but it is larger and slower than solid state relays. The EMI response time is in the range of 5 – 15 ms, which is not enough for some applications. EMRs also contain moving parts, which is the main reason for their short lifespan. However, EMRs are available in a variety of switch configurations that are affordable and easy to replace.
       A typical EMR consists of a heavy frame supporting all the relay components; a coil wrapped around a metal core that generates an electromagnetic field, making or breaking a circuit; moving part that opens and closes contacts; ; a spring that returns the armature to its original position and a yoke that provides a magnetic flux path with low resistance;
       SSR technology continues to replace EMR in many general purpose applications. The main difference between SSR and EMR is that SSR provides fully electronic switching and contains no moving contacts. Electronic devices such as silicon rectifiers provide this electronic current switching. The SSR can be made using an SCR (Silicon Controlled Rectifier), TRIAC (Transistor Alternating Current) or a switching transistor, but MOSFETs are commonly used as the switching elements.
       SSRs are designed to provide complete electrical isolation between input and output. When SSRs are off they have very high resistance and when they are on they have very low resistance. SSRs can switch AC and DC current. SSRs can deliver a wide range of currents, with ratings ranging from microamps to hundreds of amps, depending on the application. SSRs are available in a voltage range from 3 to 32 VDC and are suitable for most electronic circuits. The input circuit of the VTR control signal consumes less energy than EMR. In addition, the switching time of the SSR is much shorter than that of the EMR.
       An example of a solid state relay is the Sensata-Crydom Series 1 Panel Mount Solid State Relay. These relays are available in a variety of current and voltage ratings (10 to 90 A at 24 to 530 VAC), making them suitable relays for many industrial applications such as motion, power, heating and lighting control.
       The input circuit performs the same function as the coil in the EMR. This circuit is activated when a voltage greater than the specified relay operating voltage is detected. When the applied voltage is below the minimum drop voltage specified by the relay, the input circuit is turned off. The control part is connected to this relay part.
       The connection between the input and output circuits determines when the output component should turn on or off. The output circuit performs the same function as the mechanical contacts in the EMR that switch the load. Solid state relays usually have only one output contact.
       SSR has no moving parts and therefore has several important advantages over EMR. These advantages can be divided into seven key factors: simple design, long service life, low power consumption, fast switching, quiet operation, low electromagnetic interference and suitability for use in harsh environments.
       1. Simple design. The PCB area and overall volume of the SSR is much smaller than that of an EMR with similar specifications. SSRs can also be up to 70% lighter than EMRs, depending on power. Size and weight advantages make SSRs ideal for embedded systems, saving valuable installation space.
       SSRs are also position independent, so they can be installed vertically or horizontally. Some solid state relays, such as the Sensata-Crydom Series 1 panel mount solid state relay, have an anti-rotation barrier in the housing. Although the SSR is smaller in size, its functionality is no less powerful than that of the EMR. The optocoupler completely isolates the relay circuit, eliminating faults caused by high voltage.
       2. Long service life: Since the SSR contains no moving parts or contacts, there are no problems with sparking or mechanical wear. As a result, the SSR has a life expectancy that is 50 times longer than an EMR, making it an ideal solution for applications that require frequent use.
       3. Low Power Consumption: SSR does not require powering huge coils and opening and closing contacts like EMR. This means that the operating power consumption of the SSR is significantly lower than that of the EMR. The input power of the SSR must be sufficient to drive the optocoupler LED, which consumes very little power. EMR requires input power ranging from hundreds of milliwatts to several watts, while SSR requires input power ranging from microwatts to several milliwatts.
       4. Fast switching. Compared to EMR, SSR provides faster switching speed. Solid state relays switch faster because there are no physical parts to move. The switching time depends on the switching time of the LED, which responds almost instantly to the control signal (less than 100 µs). The average EMR switching time is between 5 and 15 milliseconds.
       5. Quiet Operation: SSR uses electronic circuitry to achieve switching. Since they have no moving parts, they operate completely silently. This is a highly desirable feature in various commercial and medical applications.
       6. Minimum level of electromagnetic interference. The low-noise SSR provides zero-voltage on and off functions, reducing electromagnetic interference (EMI) noise to negligible levels. The zero crossing feature is one of the most important benefits of SSR. This feature turns off the AC load when the load’s sinusoidal current is zero, eliminating problems such as arcing and electrical noise. Even if the control input signal is removed, the switching device continues to conduct current until the current falls below its threshold value.
       This is why the SSR will never turn off a load in the middle of a sine wave peak, which is especially important with inductive loads, otherwise large voltage surges can occur. Zero voltage on and off functions minimize electrical noise generated by the solid state relay. These zero switching relays are the most widely used type of relay.
       7. Ideal for harsh environments. In industry, harsh environments are characterized by the following factors: temperature, dust, humidity, vibration and mechanical stress. Because the SSR has no moving parts and is completely enclosed, it is ideal for use in harsh environmental conditions. In addition, the SSR does not generate sparks during operation, making the SSR suitable for use in flammable environments. The influence of external magnetic fields on solid-state relays is also insignificant. Sensata-Crydom states that SSRs must meet EMC standards to operate reliably in harsh environments. Its Series 1 panel mount solid state relays meet Level 3 EMC standards.
       Solid state relays are the choice for many applications. To learn more about the various SSR products that offer the benefits we discuss in this article, visit OnlinecomComponents.com.