Tuesday, December 17, 2013
How Does a Mechanical Relay Work
A mechanical relay is actually only half mechanical, with the second half being electrical. A relay is a switch that uses a low-current power supply to control a high-current power supply. Relays are often used in automotive applications to control headlights, fuel pumps, starters and electric cooling fans, as these devices require a much higher current than a normal switch would be able to sustain under long term use without failing. Your relays will often be located within the engine or passenger compartment fuse box and occasionally behind the dash or center console.
Relay Design
Most relays have either four or five terminals, depending on their design and purpose. Four terminal relays control a single circuit and five terminal relays can control two circuits; In this scenario, one circuit is always closed and one circuit is always open. Relays will vary in color, but commonly resemble a small cube or rectangle depending on application and use. If a relay looks similar, they are likely interchangeable, but you must verify the terminal numbers on the backside of the replacement relay, using the diagram on the side of the relay, match that of the original relay.
The Need for Relays
The need for relays rose when normal switches could no longer sustain the high amperage required by a vehicles headlights and electric starters. In order for a switch to withstand high amperage feeds, it has to be built a certain way which normally leads to the switch being too big and unstylish. When the amperage of a circuit, or the draw of the components attached to the circuit exceed the rating of a switch, it creates heat that normally leads to a melt down and failure of the switch.
How They Work
Your relay houses an electromagnet connected to the low current or control, circuit. A high current circuit runs through the opposite end of the relay. This circuit is normally open, meaning that the circuit is not complete, until the relay is activated. When the electromagnet is energized, an arm is pulled down to close the connection between the high current circuit. However, five-prong relays will always have power going to one circuit (the fifth terminal), until you complete the control circuit which will transfer the same current to the normally open circuit.
Relay failure
Relays, like all other mechanical and electric components, will fail from time to time and need replacement. Common belief is that something without many moving parts is less likely to fail; however, relays can easily fail if the electromagnet -- the relays weakest point -- overheats and melts. Overheating is normally kept under control through a fuse placed in-line before the relay. The fuse will blow and prevent as much damage as possible if too high of a current draw travels to the relay. Replacing the recommended fuse with a fuse with a higher amperage fuse, can result in overheating the relay. Relays also fail as they wear and dirt collects on the high-current contact inside the relay. After excessive use or long periods of high current travel, the contact on the end of the high-current arm can wear down and cause an intermittent connection. Intermittent connections can also occur, if the relay casing has become damaged, allowing dirt to build up on the contact.
