I have a USB fan of about 0.5 A draw.
For fun, I want to try to add some modes, where the ran ramps up and down, and makes pauses (externally controlled with a MCU, not planning to build it in). Why is this useful? Well, it might be useless, I just want to try it and see. If you keep a fan constantly blowing towards you, your body adjusts to this. Once you remove the fan, the part of your body where the fan was blowing suddenly feels warmer than the rest, for a little while. If the fan is not constantly blowing, perhaps your body will not have as much time to adapt, so you might feel that the air is "colder".
The other thing is, I want to do this as cheaply as possible (I already have a clone of Arduino Uno, but no power transistors, and connecting 10-20 in parallel seems impractical, unstable - e.g. one might hog more current and heat up - and more expensive).
The cheapest relay I could find is about $0.3. But a relay might not be practical. Cheapest power transistors I could find are these:
"B0083 50pcs NPN power transistors BD139 TO-126". One costs about $0.05 or less if you order 50.
PNP variants are slightly cheaper, but it should be easier to work with NPN.
One buyer tested one on Aliexpress, and got the following data from the tester:
BJT-NPN
hFE=233
Ic=2.1mA
Vbe=668mV
Fairchild's datasheet (
https://www.fairchildsemi.com/datasheets/BD/BD139.pdf) shows the following characteristics:
Absolute maximum ratings:
Collector Current (DC) 1.5 A
Collector Current (PULSE) 3.0 A
Base Current 0.5 A
Do you think this transistor would be sufficient to drive that load? Obviously, I'm not considering any commercial products (just want to test this out for a few minutes and see how it works) so I don't really need reputable / high quality parts, it's more important that the cost is minimal.