A relay is an electrically operated switch, a component that uses an electromagnet to control whether or not electrical current flows from one element of a machine to another. Any relay switch, mechanical or electrical, includes a bridging element that allows or breaks the current flow.
When the bridging element is open, the current is broken; when it is closed, the current flows. Relay switches have just two positions: on or off. As such, they are small, simple devices, though they can be used to control large amounts of current and voltage. Here is a generalized illustration of a very simple relay switch in its off and on positions:
Relay switch off
Relay switch on
Relays can be purely electromechanical (like a circuit breaker in a fuse box), or they can be entirely electronic, without moving parts (like a transistor in a computer). Electromechanical relays have been in use for about a hundred years, and are some of the most common components in electronic devices.
Most consist of four parts: an electromagnet, an armature, a spring, and a set of electrical contacts. One contact is at the end of the armature; the other is on the other side of the break. The spring is attached to the armature and normally holds the switch open. The electromagnet is located on the other side of the armature from the spring (that is, if the spring is above the armature, the magnet will be below it).
When the relay is activated, a tiny amount of current energizes the electromagnet, which attracts the armature, causing it to swing down and allow the contacts to touch. The current then flows through the switch. When the relay turns off, the electromagnet stops working and the spring pulls the contacts apart. Viola, no more current.
Not all electromechanical relays are exactly like this, however. In the switch described above, the rest position is “off;” the relay is “normally open” (NO). Some electromechanical relays, however, are meant to be on all the time, except in cases of electrical overload. These are “normally closed” (NC). The most familiar NC relays are a special type called protective relays, for example, circuit breakers.
Protective relays are designed to turn off the electricity if it threatens to damage the system. A circuit breaker can do this in two basic ways: by causing a strip that is bimetallic (made of two metals) to bend when it gets overheated, thus causing the circuit to trip (open); or the circuit breaker can use fuses, which contain a metal filament that melts and breaks when overheated.
Most of us have had to change a fuse in our car or home fuse box at one time or another. A circuit breaker is the simplest of protective relays; there are many different varieties of protective relays, some highly specialized and used for monitoring tools, alarms, and the like.
Relays are common in home appliances and other everyday machines. Non-protective relays are often cascaded; that is, one relay provides the current for another relay, which provides the current for another, on down the line. Many automobiles, for example, use this type of electrical system. If you have central heating and air conditioning, an interesting relay can be found inside the thermostat.
Many thermostats respond to your heating/cooling instructions by balancing a small tube of mercury on a bimetallic coil. The mercury tube has three metal wires in it: one that runs through the whole tube and two open wires on each end. The complete wire is electrified, and the glob of mercury is big enough to bridge the open switches on the ends. When the specified temperature is exceeded, the coil expands and unwinds a bit, tilting the mercury tube to the right.
The mercury runs down to the right end of the tube, bridging the circuit there and turning on the A/C. When the temperature falls below a certain point, the coil contracts and tightens, and mercury runs in the other direction, breaking the circuit. Too low a temperature and the coil tightens more, tips the mercury further left, and throws the switch for the heater.
Transistors and other “solid state relays” (SSRs) are purely electric, with no moving parts at all. As such, they can be very small; indeed, as technology advances, we’ve learned how to put increasing numbers of transistors on a single silicon chip. This has allowed computer power to grow at an exponential rate.
Like any other relay, an SSR can do only two things: it can let current flow, or it can shut it off. Transistors have three terminals: the source, the gate, and the drain. The gate is a metallic electrode insulated from the rest of the transistor. When the gate is “closed,” it allows electricity to flow from the source to the drain.
As anyone who has worked with a car battery knows, electrical current can be separated into positive charges and negative charges. Put simply, this is how a transistor works: the silicon chip the transistor is on is made with an excess of positive charges. The source and drain have a negative charge. When a small current is applied to the gate, it becomes positive, pushing the positive charges in the silicon chip away (because like repels like, as with magnets).
Negative charges in the silicon are attracted to the gate, creating an “electron channel” between the source and drain, and current flows. When the charge to the gate is cut off, the transistor relay is broken. In this case, the gate can be “opened” and “closed” without moving at all.