In an electric circuit, a relay is a component used to switch electrical contacts. Specifically, a relay is an electromagnet containing an internal spring-loaded lever. The spring, or coil, energizes the relay by conducting either an alternating current (AC) or direct current (DC) around the core. This small current in turn magnetizes the core which attracts the lever to switch a large electrical current between contacts. The initial current can be sourced from a few places, those of which include another switch, the circuit board the relay is on, or another electrical circuit. To further understand the applications of a relay, this blog will explore the difference between alternating current and direct current as they function to control current through a relay coil.
A relay can be found in a number of applications ranging from daily use applicances to aviation and industrial applications. For example, a starter in a car uses a combination relay and solenoid, or another electromagnet. When the key switch is activated as the car is started, the small current activates the relay to direct the large battery current to the starter motor. The main difference between a relay that relies on alternating current and a relay that relies on direct current is the fact that AC relays are susceptible to chatter due to the constant switching of the direction of electric current.
Chatter is a term that refers to the constant cycle of the lever being pulled back and released within an AC relay. This occurrence is due to the fact that alternating current switches direction continuously, about 50 to 60 times per second. Each time the current switches direction, it builds until it reaches a peak, and it then zeroes out in order to reverse direction. It then immediately starts building in the other direction. The magnetism in the core, being generated by the current in the coil, also follows in this back and forth cycle. Each time the magnetism of the core/relay reaches zero, the lever is released, but it is immediately pulled back once the current builds again in either direction. Since this happens quickly and continuously, the cycle produces a noise similar to buzzing or chattering.
Direct current is steady in one direction, so it requires only a single coil of wire wound around the iron core to make the electromagnet. On the other hand, a relay that uses alternating current has two coils to make a transformer to keep the core magnetized. The primary coil is of a traditional shape, while the secondary coil is shaped like the letter D. Some of the magnetism from the primary coil produces current inside the copper ring, and it is slightly delayed in comparison to the primary coil. This keeps the core partially magnetized at all times so the lever is never fully released, and it therefore cannot chatter against the core.
Overall, a direct current relay is a less complex mechanism with a steadily magnetized core, while an alternating current relay requires additional fail-proof measures. Despite their differences, both types play unique and important roles in electronic devices. While AC relays are used in applications where currents travel in both directions across a circuit, DC relays should be used for applications where currents travel in one direction across a circuit. Both types of currents are common in day to day appliances; for example, any device that runs off battery power or relies on a USB cable for power runs on direct current, while most home wiring systems run on alternating current. Understanding the role of relay coils allows for a better understanding of electricity as a whole, so regardless of your specific application, this knowledge is vital to anyone working with electric circuits.
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