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I came up with this simple circuit to drive a couple of PC fans ON and OFF from a Raspberry Pi's GPIO pin:

TIP122_as_fan_switch

Probably I need a flyback diode somewhere, but at this point I don't know where and how to choose it (and if it's really necessary).

Any hint? Thanks.

EDIT: (if may help) I noticed that the fans are "brushless"... Does it mean I can go without diode?

dentex
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    you need a diode between the +12V rail and the collector, with the anode towards the +12v. If these are simply case fans, you will not need a diode, as the fans will have them built in. –  Jul 12 '16 at 16:17
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    @tuskiomi I would still add a diode: it's a cheap insurance, and it doesn't hurt anything. Use a fast Schottky diode. For a moderately sized PC fan, I use B340 (or some such Schottky diode rated for 2A to 3A). Don't use 1N4001, because they are too slow for a flyback diode. – Nick Alexeev Jul 12 '16 at 16:29
  • Thanks guys. In addition, I noticed that the fans are "brushless"... Does it mean I can go without diode? [updating the question] – dentex Jul 13 '16 at 07:24
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    If you are only making a small number, e.g. one I'd add the diode it costs almost nothing and does no harm. – Warren Hill Jul 13 '16 at 07:32
  • We communicate circuits with schematics here, not cartoons. – Olin Lathrop Jul 13 '16 at 11:11
  • @OlinLathrop oohhh... I'm really sorry if I might have hit your sensibility. – dentex Jul 13 '16 at 13:18

1 Answers1

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You need a flyback diode?
The answer is yes.

The equivalent (simplified) circuit of a BLCD motor is:

enter image description here

You have a inductance and thus you need a flyback diode.

Usually you need to put a diode across the coil, as tuskiomi said in comment.

Some useful links:

  1. This is exactly your circuit
  2. A question that explain diodes in this circuits
  3. Another similar question
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