History
Electromagnetic relays were once the main ingredient in automated machinery. Factories used to control everything from conveyors to robots with huge panels filled with hundreds of relays clacking away, each in turn. This method had several drawbacks, but for years it was the only method available. Recently, Programmable Logic Controllers (PLCs) have replaced banks of relays for automation needs. Essentially a small computer with (relatively) high-voltage outputs used to control real-world equipment, a PLC consumes less power, has fewer moving parts, and is easier to re-program and troubleshoot than banks of relays. If the machine stops working, how do you know which relay failed? Long hours of trial and error.
Relays are still used in small applications where a PLC would be overkill. They come in several varieties to suit a wide range of applications.
Relays have a huge number of uses, but a few very common ones constitute the vast majority. Holding circuits are used to hold power on until the connection is broken by another signal. This is achieved by connecting one of the relay's own contacts to its coil — once the relay is turned on, it stays on. Double Throw connections (see below) are used to ensure that only one or another connection, but never both, can have power. Relays are also useful for allowing one signal to switch connections at two or more different voltages since the contacts are isolated from each other. But most often, they are used to switch connections that are at different voltages than the control power.
In many cases, control power and signals generated by sensors are generated at low voltages. This is for reasons of safety and efficiency. Low voltage signals, however, are inefficient for doing high-wattage work, so a relay is used to allow the low voltage signal to switch a higher-voltage connection to do work, such as pull in a large solenoid, run a motor, or activate an alarm siren.
Solenoid
Magnetic Field
The solenoid, or coil, is the part of the relay that activates the switch. When the correct voltage is applied to the coil, it creates a magnetic field which moves the contacts, making or breaking connections. Relay coils come in a variety of voltages to suit a the varies control power standards used in different places: sometimes 5, 12, 24, or 48 Volts DC; sometimes 24, 120, or 240 Volts AC; and others are used as well.
Relay coils are rated at those voltages, but it actually takes much less than their rated voltage to activate them. This is to desensitize them to voltage fluctuations in control power — common in large factories (large motors starting up can cause voltage dips in other electrical systems connected to them).
AC vs. DC
Hooking up DC power to an AC coil may or may not work, depending on voltage and solenoid design. Hooking up AC power to a DC coil on the other hand is generally very bad for the relay. Instead of creating a stable magnetic field, the alternating current creates a field that reverses direction at 60Hz (50Hz in Europe), opening and closing the contacts rapidly and producing a buzzing noise. Relays are not designed to take that abuse for long.
Potential Problems
Solenoids are prone to failure by "burning out." That is, too much current flows through them and destroys the coil of wire. When this happens, the magnetic field cannot be generated and the contacts will not change position. Coils can also burn out after being used for very long periods of time under normal conditions.
Contacts
Normally Open vs. Normally Closed
A relay's solenoid will open or close one or more sets of contacts. These are the switching elements in the relay - the parts that make or break electrical connections in the circuit. Contacts can be normally open (N.O.) or normally closed (N.C.), indicating what state they are in when no power is applied to the solenoid. Closed means there is an electrical connection and current can flow, open means the connection is broken and current cannot flow.
Potential Problems
The contacts of a relay are somewhat more prone to failure than the coil. Since the contacts are doing the actual work in the relay, both due to being moving parts and to passing large amounts of current to the device they are powering, they are under more stress than the coil. The most common contact problem is corrosion. If the contacts get dirty or corroded, they become less effective at passing power through themselves. Hermetically sealed relays are available which all but eliminate this problem.
One of the most dangerous things that can happen to a relay contact is putting too much current through it. When this happens, it can either destroy the contact by vaporizing the metal or weld the contact shut so it can never open again. Relays must be protected by circuit breakers or fuses to protect them from this.
Poles and Throws
Relays rarely have only one switch. This would be called a Single Pole Single Throw (SPST) relay, and would operate similar to a light switch. It would make or break one electrical connection when turned on. Some other combinations for contacts are Single Pole Double Throw (SPDT), Double Pole Single Throw (DPST), and Double Pole Double Throw (DPDT). The number of poles indicates how many separate sets of contacts there are. The number of throws indicates whether the contact only makes a single connection in one state (on or off) or if both states have a connection. For example, a DPDT contact might look like the following:
__________________
| 1 2 3 4 |
| | | | | |
| +--\---O---\---+ |
| | | | | |
| 5 A B 6 |
When power is applied to terminals 5 and 6, the coil (O) is energized and creates a magnetic field. This switches the contacts from connecting A and B to 1 and 3 (the Normally Closed position) to connecting A and B to 2 and 4 (the Normally Open position). The two poles are always electrically isolated from each other and from the coil, which prevents the mixing of different sources of electrical power. It is very common to see a 24 Volt DC coil switching 120 Volt AC contacts. This allows safe, low voltage control power to switch more dangerous, higher voltage power that does work.
Single throw relays are only connected in one position. While relays are limited to being single throw or double throw (having only two states to choose from, on or off), they can have any arbitrary number of poles. When there are more than 2 poles, we switch from using the SPDT / DPDT format to #PDT, where # is the number of poles (4PST for example).
Make Before Break vs. Break Before Make
Furthermore, double-throw contacts can be designated Make Before Break or as Break Before Make. Break before make is more common, and indicates that there is a fraction of a second in which there is no connection at all during the switching process. Make before break indicates that the connection to the second pole is made before the connection to the first pole is broken. Sometimes break before make is preferred, such as when making first would create a short circuit. Other times make before break is preferred, in cases where a connection must be maintained because another part of the circuit cannot be allowed to lose power. In many cases, it doesn't matter either way.
Timer Relays
Ordinarily, relays do their switching immediately upon application or removal of power form their coils. Sometimes, however, it is useful to introduce a time delay in the switching process. In the past this was achieved with an R-C circuit attached to the coil — the capacitor would store a charge which would delay turning the relay on, off, or both and a variable resistor was used to adjust the time delay. Modern relays use transistor circuits and clock oscillators, making them cheaper, more flexible, and more reliable.
The term "timer relay" encompasses a large number of such time delay devices that perform different functions. Off delay timer relays are the most common, maintaining the connection to their normally open contact for a given time after the coil has lost its signal. On delay timer relays wait for a given time before activating the switch. Cycle timer relays turn on and off repeatedly at set intervals as long as they have power. There are many other variations as well, such as whether the switch deactivates immediately upon the loss of coil signal or if it finishes its timing function.