How switches work and their structure
The switches are used to facilitate or interrupt a current flow in an electrical circuit. Depending on their design, these switches can be push buttons or retention switches.
• In the case of a push button, the selected switching position is only maintained for as long as the push-button is operated. The buttons are used in the stamps, for example.
• In the case of a holding switch, both switching positions (ON / OFF) are mechanically closed. Each switching position is maintained until the switch is activated again. The light switches in the houses are an example of a hook switch in use.
Another classification and selection criterion for switches is their switching status in normal position (ie deactivated).
1.Normally open contacts
In the case of a normally open contact (or N/O contact), the circuit is interrupted when the push button is in its normal position. Actuating the push button closes the circuit and supplies the consuming device with current. When the push button is released, spring force returns it to its normal position and the circuit is interrupted once more.
2.Normally closed contacts
In the case of a normally closed contact (or N/C contact), the circuit is closed by spring force when the push button is in its normal position. Actuating the push button interrupts the circuit.
The changeover switch combines the functions of an N/C contact and an N/O contact in one device. They are used to close one circuit and open another one with a single switching operation. Both circuits are briefly interrupted during the changeover.
Structure of a relay
A relay is an electromagnetically-actuated switch where the control circuit and the controlled circuit are electrically separated from each other. It essentially consists of a coil with an iron core (see (3)(1) in Figure), an armature as a mechanical actuating element (4), a return spring (2) and switch contacts (6). When a voltage is applied to the solenoid coil, an electromagnetic field is generated. This causes the movable armature to move towards the coil core. The armature acts upon the relay contacts that are either closed or opened, depending on the arrangement. If the flow of current through the coil is interrupted, a spring returns the armature to its initial position.
A relay coil can be used to switch one or more contacts. In addition to the relay type described above, there
are also other designs of electromagnetically-actuated switches, for example the remanence relay, the time
relay and the contactor.
Applications of relays
Relays are used in electropneumatic control systems to:
• multiply signals,
• delay and convert signals,
• link information,
• separate the control and main circuits.
They are also used in purely electrical control systems to separate the DC and AC circuits.