Home      Projects     Experiments     Circuits     Theory     BLOG     PIC Tutorials     Time for Science

13 March 2010
Author: Giorgos Lazaridis
How Relays Work

Relay Symbols and Characteristics

The relay symbols are unlimited. Each manufacturer can make his own symbol for a specific relay that has different internal connections and characteristics, carrying out a specific task. I will illustrate the most basic types of relays:

Relay Characteristics

A sample of a label from a relay

The things that characterizes a relay are the followings:

• The coil voltage: This is the voltage that the coil can actuate the armature. This value should indicate also if the current is AC or DC

• The coil current: This value indicates the current that the coil will draw when it is powered with the indicated coil voltage. Very important characteristic to take into account when designing the driver of the relay. The current that goes through the driver must be enough to actuate the armature.

• The off-voltage: This characteristic shows the minimum voltage that the armature is pulled by the electromagnet. If the voltage goes bellow this value, the spring will overcome the magnetic force and the relay will change state.

• The number/type of contacts: Is it an SPST? An DPST? DPDT? Or what?

• The power for the contacts: This characteristic indicates the maximum power that the contacts can handle. Some manufacturers will use the voltage and the amperes, some others the voltage and the kilo-watts, while some others will indicate all three values.

• The operating temperature: The temperature that the relay can operate without problems

• Switching frequency: The maximum make-break frequency

• The package: Last but not least is the package. Some packages (like the octal type) comes with an appropriate base, while some others are directly soldered/connected to the PCB/electric cabinet.