Strange USB power behavior for a fan

Thread Starter


Joined Oct 27, 2017
First of all, thanks for reading this post.
For a project I am using an arduino nano, a 2A-5V-10W power source and a small computer fan rated for 12V, 0,11A.
I also use a TIP31C transistor to use a digital pin to turn on the fan from the power source. The fan connected directly to the power source uses about 40mA, which enables a fine speed for the application. When the USB power is used however things become strange: when I power the arduino from my computer USB port, the fan uses about 30mA (acceptable, but I can't power it from the computer for my application) and when I power the nano from the external power source with a working usb cable the fan draws about 21mA. Why is this and how can I maximize the current and therefore the speed of the fan?

I have tested the used USB cables before cutting them in halve and both worked fine.

I just don't understand why there is a difference as 2A is more than enough for the fan and the arduino...

I also have a BS170 transistor laying around, would that do a better job?

Thanks in advance for answering and I hope someone can tell me what is happening.