The article covers electromechanical switches and relays, explaining different types of mechanical switches such as toggle, push-button, rocker, and slide switches, along with their configurations and applications. It also discusses electromechanical relays, their working principles, advantages, and disadvantages, and provides an example of relay-based circuit design.
Mechanical Switches
Mechanical switches are devices that make-or-break contact in electrical circuits. There are a variety of mechanical switches available, including toggle, push-button, rocker, slide, and others (see Figure 1).
Figure 1 Mechanical switches (a) toggle, (b) pushbutton, (c) rocker, and (d) slide
Toggle switches are specified in terms of their number of poles and throws. Poles refer to the number of circuits that can be completed by the same switching action, while throws refer to the number of individual contacts for each pole.
Figure 2 shows four different configurations of toggle switches. In Figure 2(a), a single-pole, single-throw (SPST) switch is shown, which is the configuration of basic switches (such as on-off switches and mechanical contact limit switches). In Figure 2(b), a single-pole, double-throw (SPDT) switch is shown. Figure 2(b) configuration is commonly used in the residential wiring of rooms that have two switches to operate a light fixture, and Figure 3 shows an example of such a circuit that uses two SPDT switches. Note that the SPDT switch is commonly known as a ‘three-way switch.’ Figure 2(c) shows a double-pole, single-throw (DPST) switch, which is equivalent to two SPST switches controlled by a single mechanism. Figure 2(d) shows a double-pole, double-throw (DPDT) switch configuration. This configuration is commonly used in the construction of electromechanical relays.
Figure 2 Different configurations of toggle switches
Figure 3 Wiring circuit for a light bulb using two SPDT switches
A DPDT switch that is internally wired for polarity reversal applications is commonly called a four-way switch (see Figure 4). Such a switch has only four wires coming out of it (instead of six) and can be inserted between two SPDT switches to enable the wiring of a single light bulb using three switches.
Figure 4 DPDT switch wired as ‘a four-way switch’
Toggle switches are known as ‘break before make’ type, which means that the switch pole never connects to both terminals in SPDT or DPDT switch configuration. Push-button switches have the symbol shown in Figure 5. They can be either of two types: normally open (NO) or normally closed (NC). Normally open or normally closed refer to the state of the switch before it is activated. Pushbutton switches are widely used as reset switches and doorbell switches.
Figure 5 Push-button switch (a) normally open and (b) normally closed
One possible disadvantage of mechanical switches is switch bouncing. Since the switch arm is typically a small flexible element, the opening and closing of mechanical switches cause the switch to bounce several times before settling at its desired state. Figure 6 shows a typical pattern in closing a switch. Note that each of the contacts during the bouncing interval, which is typically about 15 to 25 ms long, may register by a processor as separate switch action unless means were incorporated to address this issue. The most common approach to solve this problem is to provide for each switch a debouncing circuit that makes use of flip-flop circuit elements.
Figure 6 Switch bounce pattern for switch closure
Electromechanical Relays
Many computer interfacing applications rely on electromechanical relays, which are electrically actuated switches utilizing a solenoid to establish or disrupt the mechanical contact between electrical leads. Figure 7 illustrates the schematic of a single-channel relay. In addition to the coil leads responsible for actuating the solenoid, a relay typically features three other leads. One of these leads is called COM (or COMMON), and it serves as the connection point for the power to the load. Another lead is designated as NO (or normally open), while the third lead is labeled NC (or normally closed).
When the coil of the relay is energized, it causes the pole connected to the common lead to switch from the NC contact to the NO contact. In this energized state, the NO lead establishes electrical continuity, while the NC lead becomes disconnected. Conversely, when the coil is de-energized, the NC lead restores its contact, and the NO lead breaks the connection.
Figure 7 Schematic of a single-channel relay
Figure 8(a) shows the connection diagram of a typical small power relay. An example of using this relay to switch ON and OFF a 24 VDC motor is shown in Figure 8(b). When the coil circuit is closed (terminals 1 and 16), the solenoid will move the two poles that contact terminals 6 and 11 (NC terminals) to contact terminals 8 and 9 (NO terminals), respectively. For the motor example, closing the coil circuit will cause the 24 V to be applied to one lead of the motor and the motor will start rotating. The switch configuration in this relay is an example of the double-pole, double-throw switch configuration. Some of the important characteristics of this relay are listed in Table 1. This relay can be used to switch up to 60 W (or 62.5 VA) using a coil (solenoid) that requires 5 VDC at 100 mA to operate. This current input value is beyond the current output limits of digital output ports, so a current amplifying component (such as a transistor) is normally used to interface the digital output port to the coil terminals of the relay.
The advantage of a relay is that the input circuit is electrically isolated from the output circuit. So, any noise-induced voltages in the output circuit have a minimal impact on the input circuit. The second advantage is that a small coil current can be used to switch a much larger load current.
Figure 8 (a) G5V-2, a typical small power relay and (b) example wiring diagram
Table 1 Characteristics of the G5V-2 OMRON Relay
|
Coil Rating |
Rated Voltage |
5 VDC |
|
Rated Current |
100 mA | |
|
Coil Resistance |
50 Ω | |
|
Contact Rating |
Rated Load |
0.5 A at 125 VAC, 2 A at 30 VDC |
|
Max. Switching Voltage |
125 VAC, 125 VDC | |
|
Max. Switching Current |
2 A | |
|
Operating Characteristics |
Operate Time |
7 ms max |
|
Release Time |
3 ms max | |
|
Max. Operating Frequency |
Mechanical: 36,000 operations/hr Electrical: 18,000 operations/hr |
One disadvantage of electromechanical relays is their relatively long switching time. For the previous relay, the maximum operate-release cycle time is 10 ms, and the maximum mechanical switching frequency is 10 Hz. This is in contrast to solid-state transistors, which have nanoseconds switching time.
Electromechanical Relay Example
A circuit consists of two SPST switches, A and B, and a load. Design a circuit using two single-channel relays to control the load so that it receives power when either switch A or switch B is closed and turns off only when both switches A and B are open.
Solution
The circuit to solve this example is shown in Figure 9. Notice how the load voltage is applied to the COM lead on both relays. Also, notice that the NO leads on both relays are connected with the load. If either switch is closed, power is delivered to the load through the NO lead. If neither switch is pressed, no power is delivered to the load since the NO open leads will be disconnected from the load voltage.
Figure 9 Circuit for Relay Example
Electromechanical Switches and Relays Key Takeaways
In conclusion, electromechanical switches and relays play a crucial role in various applications, providing reliable control and automation in electrical circuits. Mechanical switches, including toggle, push-button, rocker, and slide switches, are widely used in household, industrial, and electronic systems due to their simplicity and effectiveness. Electromechanical relays, on the other hand, enable the safe and efficient switching of high-power loads using low-power control signals, making them essential in automation, motor control, and protection circuits.
