# Arduino & 74hc595 question

Here is my problem. I want to drive 5 74hc595, and 4 of them will drive 8 LEDs ( 32LEDs in total). Since supply current for hc595 is 70mA, it gives about 8.5mA PER pin with 360ohm resistors.

1. How much current does a 74hc595 need? 20mA or 70mA?
2. If 70mA, can 5V arduino pin supply 350mA? What is the maximum current draw from arduino?
3. Can arduino sink all the current from LEDs? 32*8.5mA =~ 275mA?

I plan on using it with USB, and later with some 9V adapter.

1. How much current does a 74hc595 need? 20mA or 70mA?

the 595 itself needs very little to function, most can handle around 35ma per output pin max, but a max of 70ma for the entire package

1. If 70mA, can 5V arduino pin supply 350mA? What is the maximum current draw from arduino?

Input / Output pins that run though the chip can handle a max of 40ma, 200ma for the entire package (maybe 250 I dont remember). The voltage supply could supply 350ma, but you would be getting close to the edge.

1. Can arduino sink all the current from LEDs? 32*8.5mA =~ 275mA?

if your running it though the i/o no, same as above, if your running it direct to ground yes

1. Yes, yes. I understand that. My question is if Q0-Q7 are using 70mA, does the 74hc595 also needs 70mA or less ?
2. 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?
3. I’ll be using GND pin on arduino, not I/O. Is the same story, about 500mA fuse, with GND pin also?
1. 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

1. 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.

1. 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