JoeN:
If you read one of the reviews on the board at that site:I just received an 8 channel and a 2 channel relay module today. It took me a while to figure out that the pins have to be driven low, not high, to cause the relay to activate. Not only that, but the current sink is less than 3 mA. I don't know what the "15-20mA Driver Current" is all about.
Maybe that current is the amount of current required to switch the relay because it does seem high for an optoisolator. I think you are good to go with this board.
No, the input section of a opto is an LED, and 15-20 ma is the nominal maximum forward current spec for most leds. However just like most LEDs there is significant light generated even at 3ma and in this case enough to activate the opto's output stage. What we probably have there is a board that they can sell in either a 5vdc, 12vdc, or 24vdc versions just by stuffing it with the proper coil voltage relays. So they probably sized the external LED series limiting resistor such that one resistor value would work no matter what voltage relay version they sold it as. At least that's my theory as it fits how many solid state AC relays work with their wide 3-32vdc input operating spec, as that voltage feeds an internal current limiting resistor and optoisolator to activate the thyristor device.
Lefty