• DC relays can control relatively large currents.12 volt relais, relay image by Sascha Zlatkov from Fotolia.com
  
  A DC relay is an electromechanical switch able to handle larger currents and higher voltages than most electronic components. It has a coil that energizes with a relatively modest current. When it turns on, the coil magnetically pulls on a spring-loaded lever connected to a set of electrical contacts. When the current turns off, the lever snaps back to its original position, connecting another set of contacts. Using a relay, you can control powerful motors, lamps, and other devices with small, inexpensive circuits.
  
  

Coil Voltage

• The relay's coil needs a certain voltage to turn it on. This makes different relays suited to particular kinds of circuits, such as digital or analog. You can find many DC relays rated for 5, 12 or 24 volts, with other voltages also available.
  
  

Contact Rating

• A relay's contacts have maximum current and voltage ratings. This is the most current you can expect the relay to switch for you. Generally, the larger the relay, the more current it can carry at a given voltage. A typical printed circuit-mounted relay can switch up to 5 amps. Subminiature reed relays are rated for a fraction of an amp. If you try to switch larger currents and voltages than what the relay is rated for, excessive sparking and heat will melt the contacts, quickly destroying the relay.
  
  

Speed

• Since the relay is a mechanical switch, the time it takes to turn on is typically several milliseconds. For an electronic circuit, this is a long time. Larger relays, because their parts have more mass and inertia, take longer to switch than small relays. In addition to the inertia of parts, when the relay turns on, the contacts bounce. While the bouncing motion is too small and fast to see with the naked eye, it too affects sensitive electronic circuits. So the true time for a relay is how long it takes the contacts to touch and finish bouncing (settling time). Reed relays have been designed to switch relatively quickly, a little over a millisecond. More typical relays take about 15 milliseconds.
  
  

Life

• Even under ideal conditions, relays wear out after a number of opening and closing cycles. The contacts corrode and become dirty, and friction and flexing deteriorate the mechanism. Because engineers want to know how long a part will last, every relay manufacturer publishes their lifetime ratings. Depending on the model, a relay's life can run from hundreds of thousands to billions of on-off cycles.