A relay is a device that closes and opens an electrical circuit. It affects the operation of other devices in the circuit, performing the "on-off" function.
Relays are very commonly used mainly in industry, but they are also used in construction or plumbing, among others.
The most popular industrial relay is the electromagnetic relays.
The electromagnetic relay consists of such components as:
relay coil
changeover contacts
normally open contact (NO contact - normally open, normally closed switch)
normally closed contact (NC contact - normally close, a normally open contact)
Contacts of electrical switches are classified as normally open or normally closed, referring to the state of opening or closing under "normal" conditions – before applying voltage to the coil.
In order for the contacts of the relay to open or close, the coil must be driven, that is, a voltage must be applied to it. The current that flows in the coils of the coil excites an electromagnetic field. Thanks to it, a metal jumper is attracted to open or close the corresponding switch contact. When the input voltage of the coil is removed, the return spring pushes the contacts away from each other. This causes them to open, thus disconnecting the circuit.
A normally closed (NC) relay is an electrical switch that allows current to flow through it in its normal state. Applying voltage (driving the coil) causes the contact to open.
A normally open contact does not conduct current when no voltage is applied to the coil (it is not energized) and closes (conducts current) when the coil is energized.
NO is the symbol used to designate the switch contact normally open.
We deal with this type of contact in everyday life - a normally open relay is, for example, a light switch. When activated, it closes the circuit, allowing current to flow to the serially connected bulb. This normally open (NO) contact changes its state to closed when it exceeds a fixed switching point.
We deal with this type of contact in everyday life – a normally open relay is, for example, a light switch. When activated, it closes the circuit, allowing current to flow to the serially connected bulb. This normally open (NO) contact changes its state to closed when it exceeds a fixed switching point.
Pressure switches with normally open contacts are most commonly used because they close the circuit when the switching function is activated, and thus can directly activate the follow-up process.
The contacts in a NO switch remain open when the pressure is within the permissible range and close when the pressure falls outside the permissible range.
Pressure transmitters with a switch function open or close an electrical circuit when a certain pressure level is reached. They are used for many safety applications. By type, they are used in:
mechanical pressure transducers – due to their sufficient switching current, used to control larger loads such as motors or pumps
electronic pressure transmitters – monitored by PLCs or a central control system. The PLC performs the safety function via a separate circuit.
When the "normally open" pressure transducer detects that the pressure limit has been exceeded, it closes the circuit. If, in addition to the transducer, a safety system is connected, such as an emergency stop system or an alarm sounder, it will be activated by the transducer. In this way, the normally open switches, in an indirect way, performs control functions.