How a relay Works

The relay is an electromechanical device; it is, in other words, a mechanism that is operated thanks to the electric current. A relay can be compared to a switch, with the difference that, instead of being manually operated, its command takes place automatically through a special circuit.
As can be seen in the animation of Figure 1, the relay is composed of a coil of copper wire coming from terminals B and B , wound around a ferromagnetic material core, and a series of electrical contacts, referred to as S, C and D, that can be operated through the special bracket ST .
When the coil is traversed by current, its core is magnetized and attracts the bracket ST ; the bracket, rotating around the point R , moves the central reed of the contacts, the one indicated as C . The C reed, which was initially in contact with the S reed, comes consequently in contact with the reed referred to as D . A switchover is then performed: the central contact C is switched from S to D.

Control circuit and user circuit

The relay allows to control users of any power and any supply voltage, via a circuit, usually electronic, powered by its own supply voltage (tipically of a few volts).

Figure 2 shows the light sensitive circuit described on the photoresistors page. The circuit, that detects the brightness level through the photoresistor, works with a 12v power supply and, through the transistor TR, activates the relay by sending current in the terminals B1 and B2 of the coil.
The Relay Contacts act on the high voltage circuit, i.e. they turn on and off the 230 V powered outdoor lights.
Two completely independent circuits are realized: a 12v control circuit and a user circuit operating with the mains voltage. For everything to work properly, you need to choose a relay with the right characteristics.

Technical specifications of a relay

By choosing a relay, it is necessary to analyse its operating characteristics: on one side the characteristics of the control coil, on the other the characteristics of the contacts intended to control the utilities.

The most common operating voltages for the relay coils are 5, 12 or 24 V. Typically a 5 V coil has a resistance of some tens of Ohm and can absorb up to 100 mA; A 12 V coil can have a resistance of several hundred ohm and absorb currents from about 15 to 40 mA.
The coil must be fed with the right voltage; by providing a lower voltage, even if you hear the shot it may happen that the contacts do not really get in touch with sufficient pressure, and this can give rise to twinkler.
Due to the current absorbed, it is evident that it is not possible to control a relay by connecting it directly to the output of a logic IC, like, for example, an operational amplifier, but it is necessary to interpose a transistor capable of handling this current.

The contacts of a relay can considerably vary by characteristics and by number of ways. First of all, being each contact made up of two blades that get in touch, it is dimensioned to work with a certain maximum current; with greater currents the contact is quickly consumed. Even small relays, however, can safely endure currents of 4 A or higher.

Depending on the type of contacts, the relay is defined by a different code, as seen in Figure 3:
SPST is the simplest configuration: it is a single contact that opens or closes
DPST is equivalent to two distinct simple contacts, each one opening or closing
SPDT means a switching contact
DPDT indicates two distinct switching contacts
There are, in addition, configurations with multiple contacts, which are used in particular cases.

The previously described relays, which are the most common, are called "monostable" since they have only one resting position; feeding the coil, they switch to the other position, but only remain there until the coil is fed.
In Figure 4 we can see the image and the diagram of the simplest relay: in the specific case it is a "spst" type, with a single ON-OFF contact. The contact, which heads to the pins Na1 and Na2, is defined as "normally open" because with the relay in resting position it is equivalent to a switch in the off position (i.e. it does not pass current). The pins B1 and B2 allow to feed the coil.

When using a relay inside an electronic circuit, it must always mind that its coil, consisting of a conductor wired around a nucleus, constitutes in all respects an inductive element. It is known that when the current is interrupted in an inductance, high extratensions are created, with a sign opposite to the supply voltage; Then it becomes necessary to place a diode in parallel to the coil, in order to short-circuiting such extra voltages. An example is shown in the diagram in Figure 2, where the diode is indicated by D.

The self-powered relay

A particular vay of using the relays is defined as "self-overed relay". In this type of circuit, the relay is activated by pressing a button; subsequently, the relay remains active thanks to one of the its own contacts.
Figure 5 illustrates this type of operation; two distinct circuits are shown: on the left the control one, which includes the relay, and, on the right, the user circuit, which operates a 230 V alternating current motor. By pressing the P button, the coil gets powered and the relay trips, closing both the a-a and the u-u contacts. These contacts actuate the motor, while we observe that the contacts a-a result in parallel to the P button; being now closed, the a-a contacts continue to power the relay coil even when the button is released.
To stop the engine, we need to disconnect the power supply; if power supply is then supplied again, the motor can not restart, since the relay contacts are in open position. You must then press the P button again.
Such a mechanism is used, for example, as a safety device in the power supply in powering machine tools: if for any reason the voltage gets off and the machine stops, it prevents it from starting alone when the voltage returns, may be while someone is, for example, inspecting the machine itself. Naturally for this type of use it is necessary to use a relay that has at least two contacts (type DPST), as one of them is used for self-supply.