Once the LED is turned off, the photons stop knocking electrons across the barrier and so the current in the secondary side stops. As we have seen earlier in this article, we have the simple normally open relay. This means the load of the secondary side is off until the circuit is complete on the primary. We can use this for example to control a fan by using a bimetallic strip as a switch on the primary side.
The bimetallic strip will bend as it increases in temperature, at a certain temperature it will complete the circuit and turn the fan on to provide some cooling. We also have a normally closed relay. This means the load on the secondary side is normally on.
We could for example control a simple pump system to maintain a certain water level in the storage tank. When the water level is low, the pump is on.
But, once it reaches the limit we require, it completes the primary circuit and pulls the contactor away, which cuts the power to the pump. In a standard, normally open, relay, once the primary circuit is de-energised, the electromagnetic field disappears and the spring pulls the contactor back to its original position. Sometimes, we want the secondary circuit to remain live after the primary circuit is opened. For that we can use a latching relay. For example, when we press the call button on an elevator, we want the light on the button to remain on, so that the user knows the elevator is coming.
So, we can use Latching Relays to do this. There are many different designs for this type of relay, but in this simplified example, we have 3 separated circuits and a piston which sits between them.
The first circuit is the call button. The second is the lamp and the third is the reset circuit. When the call button is pressed, it completes the circuit and powers the electromagnet, this pulls the piston and completes the circuit to turn the lamp on. A signal is also sent to the elevator controller to send the elevator down.
Once the elevator car reaches the lower floor, it makes contact with the off switch. This powers the second electromagnet and pulls the piston away, cutting the power to the lamp. Once activated, they will remain in their last position without the need for any further input or current. Relays can have single or double poles. The term Pole refers to the number of contacts switched when the relay is energised. This allows more than one secondary circuit to be energised from a single primary circuit.
We could for example, use a double pole relay to control a cooling fan and also a warning light. Both the fan and lamp are normally off, but when the bimetallic strip on the primary circuit gets too hot, it bends to complete the circuit. This creates the electromagnetic field and closes both contactors on the secondary side, this provides power to the cooling fan as well as the warning light. This refers to the number of contacts or connection points. A double throw relay combines a normally open and normally closed circuit.
A double throw relay is also called a changeover relay, as it alternates, or changes, between two secondary circuits. In this example, when the primary circuit is open, the spring on the secondary side pulls the contactor to terminal B, powering the lamp. The fan remains off because the circuit is not complete. When the primary side in energised, the electromagnet pulls the contactor to terminal A and diverts the electricity, this time powering the fan and turning the lamp off.
So, we can use this type of relay to control different circuits depending on an event. Here we can see a DPDT relay. The red LED and the indicator light are energised. Something we need to consider when working with electromagnets is the back EMF, or electro motive force.
When we power the coil, the electromagnetic field builds up to a maximum point, the magnetic field is storing energy. When we cut the power, the electromagnetic field collapses and releases this stored energy very quickly, this collapsing field continues to push the electrons, which is why we get the back EMF.
To overcome this, we can use something like a diode to supress this. The diode only allows current to flow in one direction, so in normal operation the current flows to the coil. Save my name, email, and website in this browser for the next time I comment.
The Engineering Mindset. Home Electrical How Relays Work. Electrical Relays. For example, a triple throw switch can be connected to one of three contacts instead of one. In an electromechanical relay, a small circuit has the ability to switch a larger circuit on or off through contacts by using an electromagnet.
When charged, the electromagnet creates a magnetic field that attracts the armature and closes the contacts. Some contacts come in different configurations depending on the use of the relay. Electromechanical relays can be broken down into the following distinct categories: general purpose relays, machine control relays and reed relays.
General purpose relays are electromechanical switches that typically function via a magnetic coil. Additionally, they can command currents ranging from 2AA. These relays are sought after due to them having a multitude of switch configurations and being cost-effective. Like general purpose relays, machine control relays are operated by a magnetic coil. Typically used to control starters and other industrial elements, these relays are robust.
While this gives them greater durability, it also means that they are less economical than general purpose relays.
However, with additional accessories and functionality, they have an advantage over general purpose relays. Reed relays consist of two reeds, which can open or close when controlled by an electromagnet. These small relays can operate up to eight reed switches, which are typically found inside of the electromagnetic coil. When the magnetic force is removed, the reeds return to their initial open position. Since the reeds are only a short distance apart from each other, reed relays work rather quickly.
There are many benefits of using a reed relay, as their hermetic seal prevents the passage of contaminants. Additionally, this seal enables reed relays to have dependable switching.
Relays are highly versatile components that are just as effective in complex circuits as in simple ones. They can be used in the place of other forms of switches, or they can be specifically designed based on factors such as required amperage. One of the most common situations that require the use of a relay occurs when an application needs to switch from high to low current or vice versa within the same circuit. For example, the temperature sensors that power HVAC units require levels of amperage that vastly exceed the capacity of their wiring.
Relays provide the necessary amplification to convert a small current into a larger one. Relays are not limited to transforming single inputs into single outputs at single points in the circuit. In other applications, a single relay can activate multiple circuits, allowing one input to initiate many other effects. Similarly, relays can be used in combination with one another to perform Boolean logic functions that, while possible to enact using other components, may be more cost-effective when implemented using relays.
Time-delay relays, to name just one category, allow systems to run only for a set period of time or to start only after a set period of time. This introduces more sophisticated possibilities for constructing electronic systems.
Relays can reduce the need for high-amperage wiring and switches, which are expensive and take up space. Therefore, switching to relays in your electronic systems can reduce the size or weight of a casing, for instance, or allow manufacturers to fit more functionality into a space of the same size. Relays differ in their size, capacity, and corresponding uses. However, although they may differ in these respects, all relays function in essentially the same way: one circuit is used to power another.
The specific manner in which this occurs depends on whether the relay is normally open NO or normally closed NC. Most relays are normally open; that is, the second, larger circuit is in the off position by default. In a normally open relay, power flows through an input circuit, activating an electromagnet.
This generates a magnetic field that attracts a contact to join with the second, larger circuit, allowing current to flow through. When the source of power is removed, a spring draws the contact away from the second circuit, stopping the flow of electricity and turning off the end device.
The fundamentals of an NC relay are the same as an NO relay: there are two circuits, with the second being larger, and an electromagnet moves a physical contact between two positions.
But in the case of an NC relay, the default states are reversed. When the first circuit is activated, the electromagnet draws the contact away from the second circuit. As such, NC relays keep the larger circuit in the on position by default. Though generally reliable, relays can fail like any mechanical component. To do so, you must first locate where the circuits enter and exit the relay, an area typically marked by pins.
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