- Yes, yes. I understand that. My question is if Q0-Q7 are using 70mA, does the 74hc595 also needs 70mA or less ?
by saying 0-7 are using 70 ma I assume you mean total, then you would need 70ma + whatever the logic in the chip requires
- I understand there is a fuse, set to 500mA if using an USB? Does it mean that it can actually supply 500mA? What changes if I don't use USB but some external power supply?
most USB ports can supply 500ma, the way its suposta work is it only provides 250ma until requested, depends on who made the usb port, what changes on an external supply is a whole different ball of wax as your now using a external higher voltage regulated on board to 5 volts. what it can do is dependent on how much voltage is going into it, and how much current is being drawn from it, vs how hot is it getting. 9 volts in, 350ma draw, may be warm, 12 volts in 350ma may be hot.
- I'll be using GND pin on arduino, not I/O. Is the same story, about 500mA fuse, with GND pin also?
ground is typically not fused, but its safe to assume you should not sink more than you can source