Current-sense AC over a threshold

Why not just use a thermal switch such as this?

Very cheap, very simple, will only switch on the fan when it is needed. They come in various temperatures.

Gareth.

I've sent you a pdf which might do the trick. Let me know if you don't receive it.

Arfa

Now why didn't I think of that? I suppose even an el-cheapo furnace thermostat might work too. Thanks for that idea.

Now why didn't I think of that? I suppose even an el-cheapo furnace thermostat might work too. Thanks for that idea.

One thing to watch out for is the hysteresis in these devices. The device I pointed out will turn the fan on at 30 degrees, but it won't go off again until the temperature drops to 20. If your room is warmer than that it will never turn off!

Gareth.

Could build something. I recall seeing some temperature controller circuits that used a diode as the sensing element. Should be easy to adjust the hysteresis by selecting component values.

Well this is the range of those devices:

Note that the opening and closing temperatures on some of them are the wrong way around, but I reckon that the normally open switch that will switch on the fan at 55 degrees C and turn it off at 35, part no. 1006853, would be ideal. 55 degrees is not going to cook any electronics and it is unlikely that room temperature is ever going to get to 35. Case solved, one component, no design work required, and extremely reliable.

Gareth.

Probably why I've seen the sensor directly in front of the fan. when the fan is operating it is getting cooled faster than the rest of the cabinent and thus actualy turns off at a higher temperature.

OTOH, unless the fan noise is a problem, does it matter. Fan just stays on until power to unit is removed, aka it might lert you to the fact that te entertainment cabinent is still using power when not in use.

If you do go down that route, I think that the sensor needs to be inside whichever piece of your equipment generates the most heat (with the attendant potential electrical safety issues which that may cause on a piece of kit with a switch-mode power supply ...) because if the temperature is going to reach 55 deg inside the glass-doored cabinet, then it's going to be a whole bunch hotter than that inside the equipment, and trust me, that *is* electronics 'cooking' temperature. Sort of electrolytics on toast if you like ... :-)

For me, your original power slave switch idea was the better option.

Arfa

Why not just use a USB port to energise a relay or FET or whatever to turn on the fans. The thermostat idea is better but more complicated, I guess it comes down to your expertise or lack thereof :-)

There are probably other temperature switches without such a wide hysteresis, like central heating room thermostats for example, or one from an old room heater / electric fire.

I am very much of the opinion that the best solution to a problem is the simplest, most reliable, and cheapest. A single switch fulfills all these requirements admirably. That is all you need. There is not even the need for a clean power supply for any electronics - the fan could just use a cheap Wall Wart - that is a grand total of 3 components and a bit of wire.

Finding a much more complex solution, building, debugging, testing, and re-designing it may well be a lot more fun but will ultimately always be the second best solution to a really very simple problem.

Gareth.

I encountered a similar heating problem when putting an LCD TV in a cabinet. I came up a fix using a $3 PICAXE chip that is programmable in BASIC and a $5 temperature sensor that gives the actual temperature (in degrees C). The sensor is mounted above the hottest point of the TV (nothing inside any component). It's powered by a "wall wart" supply, so all the wiring is low voltage. The PICAXE programming software is a free download from

You can set the on and off temperatures (in the program code) to whatever works best for you. A more sophisticated version would use the PWM capability of the PICAXE chip to control fan speed, running the fan(s) only fast enough to maintain an acceptable temperature rise.

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Yes, I think I agree - when I first saw pictures of those thermal switches, the first thought that occurred to me was: "Now where am I going to bolt one of these onto my components..." A bit too invasive, I think, at least for me, but it is a great idea otherwise.

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You're absolutely right, of course, but then I did mention that I suffer from tinkering disease... Thanks very much.

Excellent idea - however I don't believe any of my components provide USB ports. Thank you.

Oh, now this is nice! I've shied away from programmable microcontrollers in the past, but this looks like something even I could deal with, and a lot of fun. Thanks a lot for posting this reply!!

Yes, I think I agree - when I first saw pictures of those thermal switches, the first thought that occurred to me was: "Now where am I going to bolt one of these onto my components..." A bit too invasive, I think, at least for me, but it is a great idea otherwise.

You don't need to bolt them on any of your equipment, you want them to react to the air temperature. As the air gets hot, so does the switch, you can put them wherever in the cabinet you like.

Gareth.