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11 posts


# 109533 21-Sep-2012 14:12
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I’m very much a newbie at this sort of thing, so please excuse any apparently stupid questions / assumptions.

We have a media pc that lives inside a cabinet.  Things have been heating up in there so I hooked up an 80mm fan, at the back of the cabinet, to push out the hot air.  At the moment the fan is powered by an old 12V power adapter that I had lying around, but everything seems to be working well.  Unfortunately with this setup, any time we want to use the media pc, we need to remember to turn the fan on and it would be really great if it could come on automatically.

I had considered powering the fan off the pc power supply but I’ve discounted this because it would mean having wires sticking out of the case and would be harder to disconnect in the event of pc maintenance.

I’m now playing around with the idea of triggering the fan by way a USB port.  This is where my ignorance comes in, but it seems to me that if I had a relay across the powered USB port, it could switch on the external fan.  It doesn’t need to be anything flash – just when the computer comes on, the fan comes on.

If this is viable, what components would I be looking at?  Is it as simple as finding a 5V relay (e.g. and bridging the USB with it, or do I need something a bit more complicated (e.g., or am I looking at having to develop a full-on circuit to support the relay?

Thanks in advance.

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543 posts

Ultimate Geek


  # 689455 21-Sep-2012 14:27
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The relay you linked to is breadboard able I have used them before.

But given that it would need to have the usb wires coming into it I would say it would be better to make your own board.

I have made an eagle footprint in the past for it. I have not used it on a working board yet. But I can send you the file if it helps and your willing to give Eagle CAD a try.

You could use the Voltage pins on the USB but they will need to be ones that do not supply voltage all the time. Otherwise it sort of defeats the point.

Geoff E

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Uber Geek


  # 689468 21-Sep-2012 14:32
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geocom: You could use the Voltage pins on the USB but they will need to be ones that do not supply voltage all the time. Otherwise it sort of defeats the point.

The way I read it, the OP wants the fan to start whenever the PC is powered on.  So long as the USB port in question does not have 5V on the power supply lines when the PC is in standby, it should be fine.

Contact rating of the relay is 1 Amp, so as long as your fan doesn't take more than that, it will do the job.  Just connect the 5V and 0V lines from the USB port across the coil of the relay, and hook the contacts in series with the fan.  A nice thing to do would be to put a spike suppressor diode backwards across the coil of the relay, so it doesn't spike out your USB port when it turns off.  Just a precaution that I always use when connecting relay coils to transistor or FET circuitry.


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  # 689489 21-Sep-2012 14:59
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Buy a 5V fan and power it from the USB port.
I had a cooling pad powered this way with a pass-thru USB connector

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  # 689717 21-Sep-2012 21:54
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Ah, sweet - thanks for the replies. It's reassuring to know that I'm kind of on the right track and will try hacking something together with a breadboard, in the first instance. :)


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  # 689810 22-Sep-2012 10:39
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If you have a spare expansion slot bracekt space on the case then just get one of the many molex passthrus for e-sata drives and run the fan off that.


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Wannabe Geek

  # 701087 14-Oct-2012 19:05
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This might be a little bit more hi-tech and expensive than you planed, but with those two things:

You would be able to cool down your cabinet when temperature raises only (no annoying fan noise unless venting is really necessary). This would require some basic programming skill though.

You might want to check the competition as well:

Btw powering a 5v fan directly from the USB bus would work, but it's not recommended since you are not supposed to draw more than 500mA from the USB bus.

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