From: Winfield Hill
Subject: Re: 12V computer fans run from 27 to 38V, speeding up at high temperature
Date: 22 Oct 2002 06:37:18 -0700
Organization: Rowland Institute
References: <firstname.lastname@example.org> <email@example.com> <firstname.lastname@example.org>
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> Winfield Hill wrote:
>> . 25 - 42V
>> . IN _____ ______buck inductor
>> . ---| |--MMMMMM-----------+------>-- 12V uP sink/fan
>> . | | 12V zener | +
>> . |_____|--+----|>|--+------+--||--gnd
>> . | +---/\/\--'
>> . gnd | ____thermistor
>> . +--/\/\--|____|-,
>> . '--/\/\---------+-- gnd
> Win, we've used fan cooling widely, and have the burnt fingers
> to show for it. Bitter experience says that the thing which
> will cause overtemp is not neccessarily too much dissipation in
> the heatsink, it is more likely to be a failure of the cooling.
> Fan bearings, blocked airstream (crud, books, you name it).
> That thermistor really ought to earn its keep by sensing the
> case of the device (LM12?) and warning of overtemperature,
> even shutting down if required.
Excellent advice Tony. I was hoping you'd chime in.
The power-amplifier module I'm cooling has an internal thermal
shutdown, which I've linked to an LED overtemp FAULT indicator
on the front panel. Also, all computer heat-sink/fan sets these
days have a 3-pin connector. The extra wire is a pulsing output
that can be used to verify fan operation or measure its speed.
I'm thinking of tying this signal into my FAULT indicator, or
to use something like Microchip's TC670 fan-failure indicator.
The thought process that began this journey was triggered by the
nice fan ICs offered by Microchip. For example, the TC650 is a
PWM supply chip that varies the fan speed as a function of chip
temperature, and it includes an overtemp alarm. Others like the
TC642 use external thermistors. Sadly, these chips operate from
5V supplies and PWM a fan operating on a 12V supply (100% = full
speed), so I'd be required to add two more regulators and waste
5 to 10W more power as well, unless the 12V reg was a switcher.
But at this point one thinks, one switcher IC should be enough!