What are PCB Relays?

PCB Assembly 
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Editorial Team

Oct 20, 2022

Printed circuit board (PCB) relays are small relay devices used for power management in control system designs. A PCB Relay protects the printed circuit board from high-voltage ESD or large current surges. These relays have high voltage ratings, and because of their size and weight, they are mounted to a printed circuit board as through-hole components. 

PCB Relays are available in variable types and some of the prominent ones include: 

  • High-voltage: These are made to operate at very high currents and voltages.
  • Overcurrent: This kind of relay opens to prevent high currents from flowing around a circuit in the wrong direction.
  • Semiconductor: Electronically switched solid-state relays.
  • Time delay: These relays activate for a set amount of time.
  • Thermal: When the temperature reaches a certain point, these relays will trip.
  • Differential protection: This kind of PCB relay can activate when the voltages or currents in two different circuit components differ.
  • Protection against frequency fluctuations: When the frequency of an AC is too high or too low, these solid-state relays trip.

A PCB Relay consists of an armature, a coil, and contacts. Relays function as switches, so the terminology used for switches also applies to relays. The common terminal of the PCB Relay is the movable armature which is connected to the external circuit. The armature moves once the coil is energised and connects to the normally open contact until the current is flowing through the coil. When it loses energy, it returns to its initial position. 

internal structure of relay


 There are three modes of operation in which the PCB Relay functions.

  1. When the relay is turned on, the NO or normally open contacts complete the circuit, and when the relay is turned off, they deactivate.
    How Relay works in NORMALLY OPENED condition 
  2. When the relay is turned on, NC or normally closed contacts disengage the circuit. Additionally, it is known as "Form B contact" or "Break contact."
    How Relay works in NORMALLY CLOSED condition 
  3. CO, also known as Change-Over, is implied as DT (double-throw). The contact regulates two circuits, one of which has an open contact and the other a closed contact.

Most PCB relays are made to operate quickly by design. It lessens noise in low-voltage applications and restricts arcing in high-voltage ones. They are also used in computer circuits to perform mathematical and arithmetic operations. PCB relays are broadly utilized in motor control systems, robotics, medical devices, multiplexers, home appliances and data acquisition systems etc.

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