The relay is an electrical device consisting of a coil and a set of contacts. This device can operate in one or more circuits. The coil and the contacts are isolated from each other which mean there is no electrical connection between the coil and the contacts. This is one of the few devices that is both a load and a switch. The coil is the load and the contacts are a switch.
The coil is a coil of wire which generates a magnetic field when it is energized. This field closes a contact which is normally open or opens a set of contacts which is normally closed.
In electrical diagrams the contacts are shown with the coil de-energized.
Normally open contacts are the equivalent of an open switch. Remembering back to the basics when a switch is open there is no movement of electricity from the switch to the load. Normally open describes the position of the contacts when there is no power on the coil, or the coil is “de-energized”.
Normally closed contacts are the equivalent of a closed switch. Closed switches allow the movement of electricity from the source to the load and this work the opposite of the normally open contact. When the power is applied to the coil, or the coil is “energized” the normally closed switch opens.
The relay coil is the portion of the relay that is the load. The load, or coil, is supplied with power, or “energized” and this forms a magnetic field that closes or opens contacts.
Every relay coil has a rating that tells you how much control voltage can be applied to the coil. This rating is normally stamped on the side of the coil and is usually 24vac, 120vac, 240vac, or 440vac.
It is critical that the control voltage you are applying directly matches the coil rating. Failure to make this match will cause the coil to burn out or just not work.
The contacts are rated for the amperage they are controlling. The contacts control two types of loads. These are either inductive loads which are motors, or resistive loads like electric heaters or light bulbs.
It is important to realize that motors draw higher amperage when starting while heater amperage remains constant.
The purpose of the contact ratings is to make sure that the contacts can handle the amperage of the load they operate. Best way to do this is to check the maximum amperage
Relays normally have 1, 2, 3, or more contacts. Some are normally open and some are normally closed. The relay will always have a label on it giving the contact configuration and operation.
When the coil has no power applied the contacts are positioned as shown in a schematic diagram. The normally open contacts are open and the normally closed contacts are closed.
The normally closed contacts allow current to pass. When you check across these contacts with a voltmeter the meter will show 0 Volts.
The normally open contacts will not allow current flow. When you check across these contacts with a voltmeter the meter will show source voltage.
When the coil has power applied the contacts will reverse position from what is shown on the schematic diagram. The normally open contacts are now closed and the normally closed contacts are now open.
The normally open contacts allow current to pass. When you check across these contacts with a voltmeter the meter will show 0 Volts.
The normally closed contacts will not allow current flow. When you check across these contacts with a voltmeter the meter will show source voltage.
A circuit with a relay will look like the circuit in figure 19. The contacts are labeled #1 and #2 in this diagram. The coil is labeled as #3. In this diagram the relay is a 120V relay coil as it is on the line voltage side, along with the contacts.
In the next section we will discuss control voltage circuits in more detail and you will see a relay coil using control voltage to control a line voltage circuit.
Any electrical component has a sequence of operation that you will need to know. This sequence is a verbal, or written, description of what happens in the circuit. You will find the sequence of operation is very important when you are speaking with technical support or a service manager while you are in the field working on a piece of equipment that you can not figure out.
The sequence of operation for the relay in figure 19 would be:
1. When the circuit is energized current flows through the normally closed contact of C1 and then B1 is energized with 120 Volts.
2. When SW1 is closed the coil of C1 is energized with 120 Volts. This opens the normally closed contact of C1 and de-energizes B1. The normally open contact of C1 closes and energizes B2 with 120 volts.
In the next shop project you will build and wire a circuit similar to the one in Diagram 5.1 and then use your volt meter to check the voltages and prove the above sequence of operation.
Relay: A switching device and a load that controls one circuit from another. A device that can be used to isolate one circuit from another.
Contact: A mechanical switch that can be normally open or normally closed.
Coil: A magnetic coil of wire that pulls in the armature or contacts when energized.
Normally Open Contacts: Contacts that are open when the coil is de-energized.
Normally Closed Contacts: Contacts that are closed when the coil is de-energized.
Coil Voltage: The voltage rating of the coil. Coils are usually 24 Volts, 120 Volts, or 208/230 Volts.