From: John Fields
Organization: Austin Instruments,Inc.
X-Mailer: Mozilla 4.01 [en] (Win95; U)
Subject: Re: Quieting all these fan motors
NNTP-Posting-Date: Fri, 13 Sep 2002 12:12:58 CDT
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Date: Fri, 13 Sep 2002 17:12:58 GMT
George Gonzalez wrote:
> I'm surrounded by an embarrassment of riches- lots of older
> (1960-1980) test equipment.
> These nice old doohickeys are just right for my relatively modest
> needs-- just enough performance,
> nice big knobs, when they go sick, they're mostlty easily serviced.
> But most of them have one annoying design quirk---
> THEY MAKE A LOT OF NOISE. From their fans, mostly.
> If I turn on the Tek 76xx rack-mount scope, I get a buzzy hiss.
> I usually use the silent 5000-series scope, but if I need more than 2MHz
> of bandwith, I need to power-up the buzzmaster-7000.
> Then I need to check a capacitor labeled "120". Is it 12pf, or 120?
> Only the old HP cap meter can tell, and it adds a more "wooo-wooo" to the
> Then I need to blow up a shorted tantalum in this Tek 475 scope,
> (well, actually, it turns out to have FOUR shorted caps), so I need
> to turn on the 0-40V, 0-10Amp power supply. It adds a rattly hiss to the
> On rare occasions I need to also measure a frequency to many digits,
> so I turn on the 5360A computing counter. I don't begrudge it having a big
> fan, as it must contain 300+ TTL IC's (and not LS TTL either). But that fan
> would blow off my soon-to-be-needed toupee, it's just too much to stand for
> any length of time.
> I'll soon be trying to resuscitate a PDP-8/E, which IIRC has at least three
> whiny fans.
> Any one of those fans, I could stand for a short time, but all of them is a
> bit much.
> I can try concentrating on a fuzzy schematic, but with my disposition,
> as much as I try to ignore them, after about 30 seconds, I want to yell
> "QQUUUIEEETTT!!!". Not that it would help, but it sure makes me feel
> Turning them off is one obvious option, but I hate putting these older
> thingys thru any unnecessary power and thermal cycles. What to do?
> I see that a lot of new PC power supplies have a simple thermistor
> the little 12VDC fan in there. Is there a source for these thermistors?
> How amenable are they to controlling your basic 1/100th HP shaded-pole
> Any hints appreciated.
+V +V +V ACHOT
| | | |
| | [10K] [FAN]
| | | |
[RTH] | +---[1M]---+ +----+ |
| | | +-----|+ |--+
+---[1K]-----+---|+\ | +--|- |--+
| | | >---+ | +----+ |
[R1] [POT]<---|-/ | SSR |
| | U1 | |
GND GND GND NEUT
RTH is an NTC thermistor, Digi-Key's got lots of them. Choose something
with around 10K R at 25°C, and make R1 = RTH
Make the pot about 10K. The SSR is any solid state relay capable of
handling the fan load and being turned on by V+ through the 10K pullup.
U1 is a comparator like half of an LM393, and +V is pretty much anything
you have around from about 3V to 36V DC.
As you crank the pot toward V+ the temperature at which the fan turns on
will be increased.
Professional circuit designer