Brigadoon - An Occasional Corner on the Internet
Relay Boards
Designed by Mark Little
Relay boards provide a convenient way to control electrical loads by an Arduino, Raspberry Pi or similar microcontrollers. Most of the readily available boards can be driven by a digital output bit. The relays in common use can control DC or AC loads.

Like most relays, the  contacts have a rated current, but the voltage that it is capable of switching depending on whether it is DC or AC.
As can be seen on the photo to the left, the relay is rated at 10A at 250VAC, but only 10A at 30VDC.

This is because when the contacts open, a minute arc is generated that allows a current to flow while the plasma is there. With an AC supply, the arc will be extinguished when the voltage passes through 0V as part of the AC waveform.
When switching DC, the voltage across the contacts does not disappear and above 30V, there is enough voltage to allow the plasma to be maintained, allowing current to flow.
The current flowing in the plasma may be enough for the load to continue operating, and it will eventually destroyed the contacts, causing the relay to fail.
The relays can be obtained a single units, or in blocks of 2, 4. 8 or 16 relays.
The advantage of these modules is that they provide a driver to protect the output pin pf the microcontroller driving them, and provides back EMF protection when the relay coil is turned off.