How to keep fans running after power-off ? - diyAudio
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Old 23rd September 2013, 09:45 AM   #1
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Default How to keep fans running after power-off ?

I want to keep my 5v (x2) heatsink fans running after all power is disconnected.
Maybe for 60 seconds or-so.

Would this chip + the super-capacitors work ?


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http://www.linear.com/docs/41966

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The LTC3122 is a synchronous step-up DC/DC converter with true output disconnect and inrush current limiting. The 2.5A current limit along with the ability to program output voltages up to 15V makes the LTC3122 well suited for a variety of demanding applications. Once started, operation will continue with inputs down to 500mV, extending runtime in many applications.

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Last edited by redrabbit; 23rd September 2013 at 09:48 AM.
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Old 23rd September 2013, 10:04 AM   #2
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Far too complicated. I would use a 3.6v Lithium Ion rechargeable battery and a blocking diode from you fan supply. If you use the blocking diode in series with your fan, feed the battery with a 3k3 resistor, from the diode blocked fan feed, then feed the fan through another diode linked from the fan supply, that will give you your fan timer.
So when you power up your amp, the fan works normally and the battery receives a charge through the resistor. When the power is off, the battery feeds the fan with 3volts, (place the other diode reverse bias across the resistor} allowing cooling at a slightly slower speed for the duration of the batteries charge. The voltage from the battery does not feed anything else because of the blocking diode placed before the fan feed.
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Old 23rd September 2013, 11:22 AM   #3
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Why?
The junctions can only cool down from their operational temperatures.
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Old 23rd September 2013, 11:30 AM   #4
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Why keep the fans running at all. Once the amp is turned off the chips will no longer be producing any more heat and they will cool naturally.
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Old 23rd September 2013, 03:24 PM   #5
johnr66 is online now johnr66  United States
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With fans, there should not be too much heat if they're doing their job. At power off, a slow cool down would be less stress anyway.
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Old 23rd September 2013, 03:31 PM   #6
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The heatsinks will cool down naturally without the fans after the amp is turned off.

Last edited by KatieandDad; 23rd September 2013 at 03:34 PM.
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Old 23rd September 2013, 04:02 PM   #7
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Quote:
Originally Posted by JonSnell Electronic View Post
I would use a 3.6v Lithium Ion rechargeable battery and a blocking diode from you fan supply.
Aren't lithium-ion batteries destroyed if you fully discharge them? And dangerous to leave them charging for hours on end via a simple series resistor?
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Last edited by richie00boy; 23rd September 2013 at 04:07 PM.
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Old 24th September 2013, 08:53 AM   #8
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Some PC psu does this also, but for the same reason I don't understand what is the use of it. No sense.
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Old 24th September 2013, 09:39 AM   #9
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Perhaps it is not necesary.
I may be thinking too much in engine (auto) theory....where the fan keeps running (or in some engines the oil keeps circulating after shut-off).
Some digital projectors also keep the fan running onto the bulb even if it accidentally get unplugged.
The above cases must be that...it's the movement of air that is the main heat sink.
I have enough mass in my hsinks.
I'm over-designing again.
thx.
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Old 24th September 2013, 09:39 AM   #10
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I think it is a misunderstanding, a hangover from automobile technology.

With a water cooled car, thrashing the engine then suddenly switching off CAN cause the water to boil and escape. Even worse with belt driven mechanical fans (do they still exist?) since the cooling effect at idle is very poor.

This happens because the heat from combustion stored in the block does not instantly transfer to the coolant fluid, so temperature can actually RISE after switch off!
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