Different classifications of relays
Relays are of many kinds but two major specifications you need to know are poles and throws. A pole is simply the input, or the control line, to the relay and the throw is the output. Now a relay can have multiple inputs and outputs, or more appropriately, multiple poles and throws and you can recognise that from the type or the category of the relay itself.
Relays are named with appropriate specifications so that you know what kind of device it is even before you look into its details. SP stands for single pole, DP stands for double pole, and so on. Similarly, ST is for single throw and DT is for double throw. So, now you can easily understand that a SPST relay has one pole and one throw whereas a SPDT relay has one pole and two throws.
How you can use relays
Now with the advent of electronics and integrated circuits, you can control your relays by a variety of methods. The most popular implementations are obviously usb controlled relays and ip controlled relays. Relays have myriad implementations, but as you might imagine, it is still used as a controllable switch most of the time. For instance, if your robotics project involves using electric motors, you can use usb controlled relays or ip controlled relays as a motor driver.
Where relays excel
Relays are quite efficient at being used to provide voltage or current that the microcontroller cannot provide natively from its pins. Sensitive integrated circuit components have limited tolerances about the voltages they can sustain or the current they can drive. Therefore, it makes sense to connect the relay to the microcontroller. The microcontroller can provide enough voltage to operate the relay which then can route high current from the power source to the electric motor unit.
Complicated relay applications
Now you can obviously see that using a relay as a motor drive is not a very versatile arrangement: you can only control the speed and not the direction. However, even direction control can be achieved with some creative design and a bit more complex circuitry that will utilise more than one relay.
Relays are also used if you are using any large arrays of LEDs, which cannot be directly powered by the microcontroller, or if you want normal state operations where you want the external device to be in a state of operation even if the relay is off. Relays can operate in normally open or normally closed mode which means a logic 1 trigger can be used to open the circuitry and power the externally connected device down.
Further details about using relays
You can prototype your circuitry using bread boards and when you are satisfied with the results, you can send in the circuitry for final integrated fabrication. You can also purchase relay cards that are basically chips with multiple relays printed on them that can used in a modular fashion. It is entirely up to you.
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Article Added on Thursday, June 23, 2016
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