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Post: Hi Steve The maximum current you can draw from each of the MCP23017 IO chips is 25mA per channel or 125mA in total so the overall current for the IO Pi should not exceed 250mA. The external power input is only normally needed if you stack several boards together on a Raspberry Pi, with one or two boards it should be safe to use the 5V pin on the GPIO header to power the boards. For an external power supply, any stable 5V supply should do the job as long as it can provide enough current. If you use a separate supply for the Raspberry Pi then you need to make sure both supplies share a common ground otherwise you can get unexpected voltages between the two supplies. For driving relays from the IO Pi, I would recommend using a transistor or Darlington array to act as a buffer between the IO Pi and the relay. We have a knowledge base article on [url=/kb/article/1052/driving-relays-or-higher-loads-with-the-io-pi-plus]driving relays from the IO Pi[/url] and we also have the [url=/kb/article/1060/relay-board-for-the-io-pi-plus-2]schematics and PCB files for a relay board[/url] which can be used with the IO Pi Plus.