I was horrified to find that many of the people writing here do not understand the principle of operation of ... a relay. So I'll try to fix it.
A relay is an electrically operated switch. The relay is controlled by applying power to its coil. This makes it possible to switch the working contact (or contacts) (which can conduct significant currents) with a relatively low control current. The relay can be normally open (normally open working contact), normally closed (normally closed working contact) or changeover. The working contact terminals (pins) are described as COM (common terminal), NC (terminal normally closed to COM) and NO (terminal normally open to COM). By "normally" is meant the relay is in idle state, ie with no power to the coil. In addition, there are signs (they also apply to mechanical switches / switches):
SPST (Single Pole, Single Throw) - a single-circuit relay with a normally open contact,
SPDT (Single Pole, Double Throw) - single-circuit relay with a changeover contact,
DPST (Double Pole, Single Throw) - two-circuit relay with a normally open contact,
DPDT (Double Pole, Double Throw) - two-circuit relay with a changeover contact.
Typical automotive relays have a coil adapted to a 12V or 24V supply and a single working contact with a current carrying capacity of 30A. They are available in "4-terminal" and "5-terminal" variants. 4-terminal relays are normally open relays (SPST), and 5-terminal switching relays (SPDT), but note: you can meet 5-terminal normally open relays, where two terminals are short-circuited with each other. The coil of the "automotive" relay takes a current of about 150mA (i.e. about 2W), it is not much compared to the current that the relay can switch, but quite a lot when it comes to the quiescent current drawn from the battery (typically the whole car consumes
A relay is an electrically operated switch. The relay is controlled by applying power to its coil. This makes it possible to switch the working contact (or contacts) (which can conduct significant currents) with a relatively low control current. The relay can be normally open (normally open working contact), normally closed (normally closed working contact) or changeover. The working contact terminals (pins) are described as COM (common terminal), NC (terminal normally closed to COM) and NO (terminal normally open to COM). By "normally" is meant the relay is in idle state, ie with no power to the coil. In addition, there are signs (they also apply to mechanical switches / switches):
SPST (Single Pole, Single Throw) - a single-circuit relay with a normally open contact,
SPDT (Single Pole, Double Throw) - single-circuit relay with a changeover contact,
DPST (Double Pole, Single Throw) - two-circuit relay with a normally open contact,
DPDT (Double Pole, Double Throw) - two-circuit relay with a changeover contact.
Typical automotive relays have a coil adapted to a 12V or 24V supply and a single working contact with a current carrying capacity of 30A. They are available in "4-terminal" and "5-terminal" variants. 4-terminal relays are normally open relays (SPST), and 5-terminal switching relays (SPDT), but note: you can meet 5-terminal normally open relays, where two terminals are short-circuited with each other. The coil of the "automotive" relay takes a current of about 150mA (i.e. about 2W), it is not much compared to the current that the relay can switch, but quite a lot when it comes to the quiescent current drawn from the battery (typically the whole car consumes
