Basics of Relays


A relay is an electrically operated switch. Relays are a remote control electrical switch that can be switched using low current to control a high current this session we are going to discuss about basics of relays.


Current flowing through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts. The coil current can be on or off so relays have two switch positions and they are double throw (changeover) switches.


Relays allow one circuit to switch a second circuit which can be completely separate from the first. For example a low voltage battery circuit can use a relay to switch a 230V AC mains circuit. There is no electrical connection inside the relay between the two circuits; the link is magnetic and mechanical.


The coil of a relay passes a relatively large current, typically 30mA for a 12V relay, but it can be as much as 100mA for relays designed to operate from lower voltages. Most ICs (chips) cannot provide this current and a transistor is usually used to amplify the small IC current to the larger value required for the relay coil.

The relay’s switch connections are usually labeled COM, NC and NO:

  • COM = Common, always connect to this; it is the moving part of the switch.
  • NC = Normally Closed, COM is connected to this when the relay coil is off 
  • NO = Normally Open, COM is connected to this when the relay coil is on
  • Connect to COM and NO if you want the switched circuit to be on when the relay coil is on
  • Connect to COM and NC if you want the switched circuit to be on when the relay coil is off 



Choosing a relay

You need to consider several features when choosing a relay:

  1. Physical size and pin arrangement

If you are choosing a relay for an existing PCB you will need to ensure that its dimensions and pin arrangement are suitable.  You should find this information in the supplier’s catalogue.

  1. Coil voltage

The relay’s coil voltage rating and resistance must suit the circuit powering the relay coil. Many relays have a coil rated for a 12V supply but 5V and 24V relays are also readily available. Some relays operate perfectly well with a supply voltage which is a little lower than their rated value.

  1. Coil resistance

The circuit must be able to supply the current required by the relay coil. You can use Ohm’s law to calculate the current: Relay coil current =supply voltage coil resistance

For example: A 12V supply relay with a coil resistance of 400 passes a current of 30mA. This is OK for a 555timer IC (maximum output current 200mA), but it is too much for most ICs and they will require a transistor to amplify the current.


Advantages of relays:

  • Relays can switch AC and DC, transistors can only switch DC.
  • Relays can switch high voltages, transistors cannot.
  • Relays are a better choice for switching large currents (> 5A).
  • Relays can switch many contacts at once.

Disadvantages of relays:

  • Relays are bulkier than transistors for switching small currents.
  • Relays cannot switch rapidly (Except reed relays), transistors can switch many times per second.
  • Relays use more power due to the current flowing through their coil.
  • Relays require more current than many chips can provide, so a low power transistor may be needed to switch the current for the relay’s coil

calibration of pressure switch

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