I became curious recently about the max current per pin on the Pi. I thought I might share some of the helpful references I found. I read this discussion on the Foundation's forum:
Two great references were linked from that thread:
GPIO pads control
documentation by Gert which concludes:
all the electronics of the pads are designed for 16mA.
Explains relationship between voltage, current and impedence. It summarizes:
A maximum of 16mA per pin with the total current from all pins not exceeding 51mA.
When the Pi was designed, they used a figure of 3mA per GPIO pins in determining if the regulator could supply enough current. Hence the simple 3mA limit mentioned before. However, if not all the pins are supplying current then you can divert those pin’s share of the current into the ones you are using. Hence the total recommended current limit of 17 * 3 = 51mA.
BTW, I noticed that Gert has posted other useful Pi technical documents on Scribd: