Choosing a RELAY

Choosing a relay

You need to consider several features when choosing a relay:

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.
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.
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:

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

• Switch ratings (voltage and current)

 The relay's switch contacts must be suitable for the circuit they are to control. You will need to check the voltage and current ratings. Note that the voltage rating is usually higher for AC, for example: "5A at 24V DC or 125V AC".

• Switch contact arrangement (SPDT, DPDT etc)