The relay board is what powers the switching mechanics on your electronic devices. It contains a power supply circuit, regulatory circuitry and of course the relays that you need to turn parts or all of your device (or devices) on and off – or to switch them between states.
Normally speaking, the relays on your relay board will be solid state, unlike the mechanical relays that you may see in larger electrical applications (like, for example, the switch you throw to turn off a phased power relay, or the breakers in your home’s circuitry). Solid state relays are reliable over a longer term than electro mechanical relays thanks to the simple fact that they have no moving parts (hence the name solid state), which means of course that nothing can seize up or break off and prevent successful operation.
The SSR on your relay board will contain a transistor or a number of transistors, through which the switch of state is made. Early solid state relays suffered a drop in voltage across their transistors, but modern SSRs are capable of carrying switching loads with much higher currents.
The terminology with which your relay board will be described and/or discussed in manuals, advice and literature is the same terminology that you will find for the discussion of any electronic or electric switches. The basic function of the relays on your relay board is to flick between one or several poles. This is done by throwing contacts between them (“pole” and “throw” are the electrical switching terms we’re interested in here).
Most of the terminology describing the state of the contacts in your relay board is compressed into acronymic form: NO (Normally Open); NC (Normally Closed); and Change Over or Double Throw (referred to either as CO or DT, both of which mean the same thing).
A Normally Open contact is one that is usually disconnected – so in its resting or inactive state the contact is open. A Normally Closed contact has the opposite characteristic: in its resting state the circuit is connected and active because the relay has closed on the contact.
A Change Over or Double Throw contact connects two linked circuits – one Normally Open and the other Normally Closed. The switch on your relay board throws the pole between the Normally Open and the Normally Closed circuit when the switch is activated.
The functions of the switches on your relay board depend (naturally) on your application. In general terms they are commonly used to increase power where there is a weak power source (amplification, usually of a signal); to isolate a controlled and controlling circuit from each other; to switch to a standby power state; to implement a time delay; or to control logic within your electronic circuitry. A series of Normally Open contacts on a relay board is equal to AND; a parallel connection of Normally Open contacts becomes OR.