A relay is an electromechanical switch: it is necessary when, with small power available, you need to control a secondary circuit that can have higher voltage, amperage, or even different types of current, such as alternating or direct current.
The drive circuit can be closed by a simple manual switch, but also by various kinds of analog or electronic equipment.
The controlled circuit, on the other hand, can power any kind of electrical device, from electric motors to light bulbs.
In this video we will show you how relays work, in order to understand how these small devices are essential for many electronic equipment.
JAES, for over a decade has been providing its customer with the best solutions for their supply chain and in their catalogue you can find every type of relay from the leading manufacturers.
Let’s now see how the most common relay is made: the normally open “single pole, single throw” electromagnetic relay.
As we can see, the drive circuit is connected to the electromagnet which is composed of a solenoid coil of electrical conductor wire, usually in copper, wrapped around a core of ferromagnetic material.
When the electric current passes through the electromagnet, it forms a magnetic field that attracts the armature to which the movable contactor is connected. The latter will change position closing the controlled circuit.
When the electric current of the drive circuit is interrupted, the contact returns to its open position thanks to a small spring.
Very important for the protection of the components and the correct operation of the relay is the presence of a flyback diode. The moment which the switch opens and the circuit is interrupted, the relay inductance will try to hold the current. Adding a diode to the circuit will create an easy path for this current to circulate until it is depleted.
If you are interested in knowing how the diode works, watch our previous video.
Relays can have different types of contacts, let’s see them one by one.
- The “single pole, single throw” (SPST) version is the one we have just seen, the relay behaves like a simple on-off switch.
This type of relay can be made in the “normally open” (NO) or “normally closed” (NC) configuration.
In the "normally open" configuration the contacts are disconnected, and they connect only when the electromagnet is energized; while in the "normally closed" configuration the contacts are always connected, and when the electromagnet is energized they disconnect.
- There is also another version, called "double pole, single throw" (DPST), it is a type of relay that is equivalent to two "single pole, single contact" controlled by a single electromagnet, which can also be made in the “Normally open” (NO) or “normally closed” (NC) variants.
The "single pole, double throw" (SPDT) type works as a simple multiway switch. It has a movable contactor in common with two fixed contacts, which can be connected to either one or the other. There is also the version "single pole, centre off” (SPCO), very similar but with the moving contact that can occupy a central position since it doesn’t connect to any fixed contact.
- Finally, the "double pole, double throw" (DPDT) type is equivalent to two "single pole, double throw" controlled by a single electromagnet.
So far we’ve seen classic electromagnetic relays, made from moving components that physically change position in order to open / close the contacts or break the circuit when required. A solid state relay (SSR), by contrast, has no mechanical or moving parts so it has the same functions as electromagnetic relays without any internal physical movement.
The solid state relay is very common and consists of an opto-isolator, an electronic device that transfers a signal between two circuits while maintaining a galvanic isolation between them. It is usually made by optically coupling a LED with a photosensitive element. In this way, when the LED is turned on, the photosensitive element detects its brightness, deciding whether to close or to open the circuit.
Their cost is high but since they do not wear out, they are used where high reliability over time is required.
There are many other types of relays:
- such as the " analogue switch” similar to integrated circuits;
- The reed relay, a type of relay where the contacts are sealed in a glass tube filled with inert gas.
- “Mercury-wetted relay ”, whose mobile plate is wetted with mercury, a conductive liquid metal.
But the most common ones still are electromagnetic relays, widely used in different fields, like public lighting, in the railway sector and especially in vehicles: on headlamps, auxiliary headlamps, horns, and so on.
If you are interested in the vehicles, don’t miss our playlist about cars engineering.
Now let’s do a practical experiment.
We have a "single pole, double throw" (SPDT) relay, and we’ll use it to divert the electrical current first to one bulb, and then to the other one.
As we can see in this case the contact arrangement is symbolized on the relay: the spiral symbol symbolizes which contacts are connected to the electromagnet and therefore which to be used for the drive circuit; then we can notice that the two fixed contacts are placed on the opposite side; finally we can see that the moveable contactor, starts from the side pin, and continues until its rest position on the fixed contact placed on the lower right; when the coil is crossed by electric current, the situation is reversed and the movable contactor moves to the fixed contact on the upper right.
A new circuit with a battery and a switch was created on this electromagnet.
To the two fixed contacts was instead connected a light bulb for each one, and the two circuits, (after passing through a battery), they both close on the pin of the movable contact.
When establishing or breaking the electrical continuity of the drive circuit, the relay diverts the current to the second controlled circuit or the first, respectively. Therefore , the bulbs light up alternately.
If you want to find out how small electronic devices around us work, check out our playlist about the electronic world.
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