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I'm not 100% sure if this is the right form to post in, and I'm not the best writer... so please let me know if I make no sense.

So I'm doing a little project and was wondering on how I could power two 3.3V fans. The wires end in copper... so I don't know how to proceed. Here are the two fans:

I'm wondering if there is a way I can just power these off of a single USB port, but that's just preference. I want this powered independently of the Raspberry Pi, unless it's through a USB port. I really don't want another power outlet (for a power supply) taken up as well (I don't want a mess of wires). Thoughts?

Greenonline
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Henry L
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  • Have you tried plugging them into the VCC and GND pins on the GPIO? Or you could cut off a USB cable and connect them to the VCC and GND wires on that – Alex Jul 13 '15 at 03:52
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    I can't see how this is relevant to the Raspberry Pi given you say "I want this powered independent of the Raspberry Pi, unless it's through a USB port". – joan Jul 13 '15 at 12:13
  • @AlexYan - I can unfortunately not use the GPIO because I may not actually be using it on the Pi. Thus why I said in my description I don't know where to put this post... b/c it doesn't fully apply to Raspberry Pis. I felt as if people in this section would have the "smarts" to help me out though. Not trying to abuse any system. – Henry L Jul 13 '15 at 20:55

2 Answers2

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You have two options, which differ in the amount of current that they can supply:

  • Connect the fans to the USB ports (current is limited by the current supplied by the external PSU - for later RPi models)
  • Connect the fans to the 3.3V line of the GPIO port (current is limited by internal fuse to approx 1A)

Using USB

You need to connect the wires to pins 1 and 4 of a standard A connector. Which way round you connect the copper wires will determine in which direction the fan spins (or maybe it will spin in one direction, but not in the other, depending if reverse polarity is allowed, or blocked by an internal diode).

USB Standard A

"USB" by Simon Eugster – Simon / ?! 19:02, 7 January 2008 (UTC) - Own painting/graphic. Licensed under CC BY-SA 3.0 via Wikimedia Commons - https://commons.wikimedia.org/wiki/File:USB.svg#/media/File:USB.svg

See the answers to the question How much power can be provided through USB?. The value will depend upon the model of RPi that you have. Early models can only supply 100mA, later can supply more as the onboard fuses are bypassed by the USB.

Using 5V USB power lines with 3.3V fans

You would need to reduce the voltage from 5V to 3.3V by using a Voltage Regulator (such as a LM1117 or LT1086, plus many others), or a potential divider.

A linear regulator takes 5V, outputs 3.3V and expels the rest as heat.

Linear regulator

Source: Nut & Bolt

Using a regulator, you need to connect the regulator's Vin to USB pin 1, GND to USB pin 4. Then connect the regulator's Vout to one of the wires of the fan and the other wire to GND.

As you point out, you could use a breadboard, and a MB102 Power supply, as these have onboard regulators, and both 5V and 3.3V pins. Here is a photo of mine (ignore the optocouplers and the LCD displays):

Breadboard and MB102

If used in conjunction with a breadboard, that would be a perfect solution. They are reasonably priced. Then you can just poke the wires from the fans into the breadboard holes, and no soldering is required.

Using GPIO

Connect the bare copper wires to the 3.3V and the GND pins (Pins 1 and 6 respectively for both a RPi 1 and RPi 2).

Raspberry PI 1 GPIO Raspberry PI 2 GPIO

The answer to this question, Plug a 5V fan to GPIO 1 and 2, may also help you.

NOTE: Do NOT connect the fans to the IO pins of the GPIO port, as they can supply only a very limited current and you will end up frying your RPi

The current available, via the GPIO Power pins, is stated here, on GPIO +5v max current?

According to the schematic there is a 1A fuse and from what others have said the Raspberry Pi uses about 700mA so 200mA is what your left with to use for USB and anything over the +5v GPIO pin.

So yes you could use it on the +5v GPIO pin but then there would be nothing left for the USB.

Also because it is so close to the fuse limit any slight change could cause the fuse to engage.

See also the answers to the question How do I supply power through the GPIO?

Either way, you should make sure that your PSU to the RPi can supply sufficient current. Any PSU capable of 2 to 3 Amps would be good. Unfortunately, the specifications of the fans do not state their current draw, so it is better to plan for more current draw than less, else the RPi may end up behaving erratically and/or crash, see RPI 2 B freezes when Wi-Fi dongle is inserted and doesn't detect it.

You can measure the current draw by using a USB voltmeter between the RPi and the fans. These are available on eBay for a couple of dollars.

USB Voltmeter

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Greenonline
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  • Hmm. So I can't have it powered through the GPIO. Since the required voltage for the fans is locked at 3.3v, can't I not use a USB port? They provide 5v. I have never seen that voltmeter before, and it seems extremely helpful. Is there some sort of "regulator" (I don't know the word), that can bring the USB voltage down to 3.3v? Thanks for the beautiful answer, I really appreciate that :) – Henry L Jul 13 '15 at 20:52
  • I just added the second fan on there. Idk why it didn't show up earlier – Henry L Jul 13 '15 at 20:59
  • Have any suggestions off hand? And what is the best way to attach the wires to the v regulator? – Henry L Jul 13 '15 at 21:03
  • I've been rather busy so I just had some time to browse some more products. Would something like this work? http://www.amazon.com/dp/B00XF4J57W?psc=1 and how would I use it? It looks promising... – Henry L Jul 17 '15 at 05:30
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It is not recommended to run a bare DC fan directly from GPIO port power pins.

The DC fans/motors emit back EMF that could induce hundreds (or even thousands depending on inductance) of transient volts back into the power supply. If the power supply output is not protected, the power supply will be damaged.

You can use a DC fan if a proper back-EMF protection circuit (example: TVS) is applied between the GPIO Raspberry Pi power pins and the DC fan. It is better to power the fan from another wall wart or a hub.

For a 3.3v Fan you can take power from Raspberry Pi USB port and convert the voltage to 3.3v using a small 3 pin VRM like this:

VRM

What is back-emf: counter-electromotive force?

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Chetan Bhargava
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