GPIO pins lose voltage when connected

Question

I am still a beginner in raspberry pi world. I have a 3 B+ and a small water pump that can work with 3V to 12V, and a multimeter that I use to measure voltage.

First when I measure voltage between pins 1 & 6, I can see a clear 3.3V, then when I measure voltage between pins 3 & 6, I also see a clear 3.3V. So far so good.

Then when I connect the water pump to pins 1 & 6, it works and I can see it move water from its input to output, but when I connect the pump to pins 3 & 6, it does not work. The weird thing is when I measure voltage between pins 3 & 6 while the pump is connected, the multimeter reads only 0.4V which is not enough to power the pump. What's wrong here and why there is such inconsistency?

Thanks!

Answer 1

Pin 1 is 3.3V power, and can supply up to 800mA (although it is inadvisable to connect a pump to the supply - particularly without any protection).

Pin 3 is a GPIO - if programmed as an output it can supply a maximum of 16mA. By default it will supply ~2mA (because it has a 1.8kΩ pullup) - neither is capable of running any kind of motor. Also connecting any inductive load without any protection risks damaging the Pi.

PS I hope you are NOT poking at the pins with a multimeter probe. Accidentally touching the adjacent pins 1 & 2 will instantly kill the Pi!