[sci.electronics] Fan for my amp revisited

ritz@msb.com (Chris Mauritz) (04/17/91)

Someone was kind enough to send me a message saying that controlling
the speed of an AC fan was not as simple as adding a pot.  OK,
how about I use a DC fan and siphon off power from the power supply
for the amp?  The power supply seems to be hefty.  Would there
be any danger that a small DC muffin fan would draw enough current
to affect the amplifier in any negative way?

Another question, I've opened the amp to see what kind of DC 
power comes out of the power supply.  It says +17v, but my
trusty little voltmeter says +20v.  Would I need to get a
20v fan or can I use a 12v fan with some sort of resistance
to chop the voltage.

Any help would be greatly appreciated.  As you can see, I'm
a bit confused. :-)

Cheers,

Chris

-- 
------------------------------------+---------------------------------------
Chris Mauritz                       |People are strange
ritz@msb.com                        |when you're a stranger.
Copyright (C) 1991                  |The Doors-

kludge@grissom.larc.nasa.gov ( Scott Dorsey) (04/17/91)

In article <1991Apr17.022927.7094@msb.com> ritz@msb.com (Chris Mauritz) writes:
>Someone was kind enough to send me a message saying that controlling
>the speed of an AC fan was not as simple as adding a pot.  OK,
>how about I use a DC fan and siphon off power from the power supply
>for the amp?  The power supply seems to be hefty.  Would there
>be any danger that a small DC muffin fan would draw enough current
>to affect the amplifier in any negative way?

   The fan will put noise on the rails of the supply.  Use a seperate
supply for the fan if you can.  Power amp supplies have to deliver a 
large amount of power very quickly in order to track transients, but they
also have to have very low noise to avoid affecting quiet passages.  Power
supply design is the real secret to good sounding amps, and if it works
well, don't touch it.
--scott

jon@inqmind.bison.mb.ca (Jon Chesnut) (04/18/91)

ritz@msb.com (Chris Mauritz) writes:

> Someone was kind enough to send me a message saying that controlling
> the speed of an AC fan was not as simple as adding a pot.  OK,
> how about I use a DC fan and siphon off power from the power supply
> for the amp?  The power supply seems to be hefty.  Would there
> be any danger that a small DC muffin fan would draw enough current
> to affect the amplifier in any negative way?
> 
 
Before you go any further, I'm concerned (slightly) about adding a fan.  
Solid State amps run better warm.....  why did you want to add the fan in 
the first place?  Rather than adding a DC fan, why not attach a thermostat 
to the heat sink.  YAMAHA standard says 50 degrees celcius (sorry, I'm lousy 
in Imperial measures!) Is the ideal, so their thermostats are set for cut IN 
at 60 degrees, and OUT at 45 degrees.
 
If this is a ROCK 'N ROLL type amp, such a circuit would be great, and much 
simpler to do, and probably better for the amp than tapping off it's supply.
 
If it's a home amp, the cut-ins and cut-outs would probably get annoying...
 

AARGGHH.... I can't find a farenheit conversion table around ANYWHERE!
 
Later,
Jon Chesnut

gt0869a@prism.gatech.EDU (WATERS,CLYDE GORDON) (04/18/91)

In article <1991Apr17.022927.7094@msb.com> ritz@msb.com (Chris Mauritz) writes:
>Someone was kind enough to send me a message saying that controlling
>the speed of an AC fan was not as simple as adding a pot.  OK,
>how about I use a DC fan and siphon off power from the power supply

This should work, but I see 2 potential problems:
1. The voltages in the supply are really high(generally 60v or so) for 
motors. You would have to build a voltage divider, usually to below 35v,
and then use suitable regulators to get down to 12v or so, for most motors.
There are regulators around which should easily handle enough current- I 
can't see this carrying more than 500ma.(try 7812 or 7912- positive or 
negative 12v)
2. A motor hooked up this way has the potential to backfeed brush noise
to the supply and the amp. Use fairly large filter caps in the motor 
circuit bypassed with small caps (big: typically 3000u, small; typically
1u or so.)One of each is a good starting point.Although this should 
take care of noise, further methods may have to be employed in some cases
(see filtering in many EE text or handbooks)
Hope this helps.
Gordon.

-- 
WATERS,CLYDE GORDON-Georgia Institute of Technology Atlanta Ga 30332 
******<LANGUAGE IS A VIRUS! - Laurie Anderson-Home of the Brave>******* 
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hbg6@citek.mcdphx.mot.com (04/22/91)

>ritz@msb.com (Chris Mauritz) writes:
>
>> Someone was kind enough to send me a message saying that controlling
>> the speed of an AC fan was not as simple as adding a pot.  OK,
>> how about I use a DC fan and siphon off power from the power supply
>> for the amp?  The power supply seems to be hefty.  Would there
>> be any danger that a small DC muffin fan would draw enough current
>> to affect the amplifier in any negative way?
>> 

All of the DC muffin fans I have used contain a bunch of digital control
and analog drive circuitry ( tiny SMT parts under the cap at the non-moving
end of the motor ).  This stuff drives a brushless DC motor and probably
would not take kindely to much less than their rated voltage. I'll see if
there is one in our scrap pile to 'smoke' test.

John

.....................................................................
John Schuch - Motorola Inc., Computer Systems Division (602)438-3008
All opinions expressed are mine and not Motorolas,        their loss.
.....................................................................

hbg6@citek.mcdphx.mot.com (04/26/91)

In article <12462@qisoff.phx.mcd.mot.com> I wrote:
>>ritz@msb.com (Chris Mauritz) writes:
>>
>>> the speed of an AC fan was not as simple as adding a pot.  OK,
>>> how about I use a DC fan and siphon off power from the power supply
>
>All of the DC muffin fans I have used contain a bunch of digital control
>and analog drive circuitry ( tiny SMT parts under the cap at the non-moving
>end of the motor ).  This stuff drives a brushless DC motor and probably
>would not take kindely to much less than their rated voltage. I'll see if
>there is one in our scrap pile to 'smoke' test.
>

I found a fan around here rated for 6 -> 15 volts DC. There IS quite a
range of fan speed based on voltage. It starts to spin at around 4 volts,
moves a lot of air at 15volts, makes an interesting hovercraft or
gyroscope demonstration at about 48Vdc, and gets mad and won't play
anymore at around 65Vdc.  :-)

John