Relays are switches that open and close circuits electromechanically or electronically. They can be found nearly everywhere from printers and shredders to cars and telephones. Relays control one electrical circuit by opening or closing contacts in another circuit. When a relay contact is normally open (NO), there is an open contact and the relay is not energized. When a relay contact is normally closed (NC), there is a closed contact when the relay is not energized. Whether the relay is normally in an open or closed position, applying electrical current to the contacts will change their state.
Relays are typically used to switch smaller currents in a control circuit and are not generally utilized to control power consuming devices with the exception of small motors and solenoids that draw low amps. That being said, relays still have the ability to “control” larger voltages and amperes due to their amplifying effect. A small voltage applied to a relay’s coil results in a large voltage being switched by the contacts. In this blog, we will be covering two main types, electromechanical relays (EMR) and solid-state relays (SSR), their distinctive properties, their differences, and the most common types. Deciding between the two is dependent on your application’s electrical requirements, expected service life, and cost constraints.
Electromechanical relays consist of a heavy-duty frame that supports the various components within the relay. Wiring is wound around a metal core which generates an electromagnetic field. There are three types of EMRs: general purpose relays, machine control relays, and reed relays .
A general purpose relay is usually operated by a magnetic coil. Moreover, they use an AC or DC current with voltages such as 12V, 24V, 48V, 120V, and 230V and have the ability to control currents ranging from 2A-30A. Additionally, these relays are cost-effective, easy to replace, and allow for a variety of switch configurations. Next, machine control relays are another such type that is also operated by a magnetic coil. These relays typically control starters and other heavy-duty industrial components. While these are more expensive, machine control relays are more durable. In addition, by adding accessories such as poles, convertible contacts, and latching control and timing attachments, you may increase the relay’s functionality. The last such type is a reed relay. This type of relay is small, compact, fast, and operates with one NO contact. Reed relays are sealed in a glass envelope which protects the contacts from contaminants, fumes, and humidity, resulting in an extended service life. The ends of the contact are usually plated with gold or another low resistance material which allows for increased conductivity, and they are drawn together or closed by a magnet. Moreover, they consist of two reeds. The ends of the reeds assume polarity when a magnetic force is applied. If the magnetic field is strong enough, the attracting force of the opposite poles draws the reeds together. As the magnetic force is removed, the reeds go back to their original, open position.
Solid-state relays consist of an input circuit, a control circuit, and an output circuit. The input circuit is connected to the control component and is activated when a voltage higher than the relay’s specified pickup voltage is applied to the relay’s input. The input circuit is deactivated when the voltage applied is less than the specified minimum dropout voltage of the relay. One main advantage of utilizing SSRs is that they do not have to energize a coil or open contacts so less voltage is required to turn them on or off. Additionally, their design is equipped with a residual electrical resistance and/or current leakage regardless of whether the switches are opened or closed. There are four main types of solid-state relays: zero-switching relays, instant ON relays, peak switching relays, and analog switching relays.
Zero-switching relays turn ON the load when the control voltage is applied and the load’s voltage is close to zero. When turning OFF a load, the control voltage is removed and the current in the load is close to zero. Next, instant ON relays are fairly simple in that they operate by turning on the load as soon as the pickup voltage is present. Peak switching relays turn ON the load when the control voltage is present, and the voltage of the load is at its peak. Additionally, peak switching relays turn OFF by removing the control voltage and current in the load is near zero. The last type is an analog switching relay which is equipped with a synchronizing circuit that controls the amount of output voltage. In addition, they can be turned OFF when the control voltage is removed and the current in the load is close to zero.
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