I'm trying my best to understand and utilise questions that already exist as well as other websites, however I just wanted to check the circuit I've come up with (which is a very simple switch circuit) will work.

I plan on using a BD139 transistor, although I haven't purchased anything yet, so can change to anything really.

The fans are each 0.09A at 12V. I've calculated that R1 should be 360ohms. The 5V control will be coming from an Arduino. I believe it's within the specs of the transistor. Ic max is 1.5A.

Does the circuit look correct? Are there any changes I should make?


For saturation you should assume hfe to be 10. This gives:

(5V − 0.85V) / ((0.09A ⋅ 3) / 10) = 154 Ω

Consider using a 150-220Ω base resistor instead. And since a fan is an inductive load, don't forget the flyback diode from the collector to the +12V rail. A 1N4000 series diode should be fine for this small a load.

  • Where did you get that value for hfe? Nov 11 '15 at 11:30
  • That is a safe assumption for normal (i.e. non-darlington) transistors. It's used by EEs the world over. Nov 11 '15 at 11:32
  • The datasheet specifies 25 (minimal) for Hfe. Using 150Ω would result in about 32mA of current, which is above the recommended 20mA for a digital pin.
    – Gerben
    Nov 11 '15 at 15:58
  • @Gerben: Are you sure you didn't confuse the Vbe and Vce on the datasheet? Vbe(sat) according to the chart is about 0.85V at Ic=0.3A. Nov 11 '15 at 16:05

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