benhewitt97
Posts: 16
Joined: Thu Apr 18, 2013 5:26 pm

PSU Case

Fri Jun 14, 2013 4:34 pm

Hi all,
I'm working on a small project involving making a case for my pi, including a fan and LEDS, out of an old PSU case. I'd like for the fan to come on when the Pi goes over 45 degrees c, but if i plug it straight into GPIO, it's very slow due to the 3.3v GPIO outputs. Is there any way i can boost the output voltage to say 12v? maybe using a UL2003 chip?

6677
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Re: PSU Case

Fri Jun 14, 2013 6:21 pm

benhewitt97 wrote:Hi all,
I'm working on a small project involving making a case for my pi, including a fan and LEDS, out of an old PSU case. I'd like for the fan to come on when the Pi goes over 45 degrees c, but if i plug it straight into GPIO, it's very slow due to the 3.3v GPIO outputs. Is there any way i can boost the output voltage to say 12v? maybe using a UL2003 chip?
If you boost the voltage you drop the current. With the inefficiencys of boosting you will end up with a slower fan than you started with.

Also, YOU SHOULD NEVER RUN ANY MOTOR DIRECT FROM THE GPIO! Motors draw more current than the GPIO can supply (and this is the real reason your fan is so slow, it has no current to get it turning at any decent speed). Attempting to draw more than the GPIO can supply CAN PERMANENTLY DAMAGE YOUR PI!

You have to use a higher current power source for the motor alone and then use either a motor driver or transistor to control it from the pi GPIO, but not power it.

If you are using a powered USB hub then you may be able to tap power from one of the USB ports for that (which will give 0.5A at 5V), alternately the 5V pin on the pi GPIO can be used as a supply, not a huge one still. The current on this port is approx: [input power]-[approx 500-700mA/0.5-0.75A]-[power drain from USB devices]. It is not recommended to use this pin either really. External supply or nothing.

jfornango
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Re: PSU Case

Mon Jul 29, 2013 8:16 pm

The transistor is a good idea. A micro-relay might also work.
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F1p
Posts: 20
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Re: PSU Case

Mon Aug 05, 2013 2:08 pm

You will need two power supplies, a 5V one for your Pi and a 12V one to power your fan.

Here is an example of powering a motor with a TIP120 transistor from an Arduino, although would work the same with the Raspberry Pi GPIO header:

Image

Using a transistor also gives you the advantage of PWM control of the fan if you desire (maybe something for the future).

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RaTTuS
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Re: PSU Case

Mon Aug 05, 2013 2:13 pm

the fan will probably heat up more than just leaving it fanless [just saying]

.... YMMV .... depenign on what else you are doing

your RPi is probably idle and running at 50C and you will not be able to cool it below
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