I'm trying to directly power a Raspbery Pi 3 by wiring it to the 5 volts rail of a power supply (5v to pin 2 and GND to pin 9), in this case a peripheral 4 pins connector. However, it never manages to boot (green LED blinking 3-4 times).
An ampmeter shows a current of 0.1A, which seems rather low, so that's probably the matter. I've tried connecting it to the motherboard ATX 20 pins and the floppy connector, but current is always limited to 100 mA. I thought that the problem could be that the PSU may not start properly without a big enough load, but even connecting it to a running PC hasn't changed anything.
Does anybody have any advice?
EDIT: after another test (I've removed the 5 volts fan from the raspberry) the red and green LED are costantly glowing)