Re: Pulse Width Modulation Help

From: Tim Wescott (tim_at_wescottnospamdesign.com)
Date: 10/18/04 

Date: Mon, 18 Oct 2004 08:32:11 -0700

Mike wrote:

>>If the 12V is modulated an NPN transistor should
>>do the trick.
>
>
> Al, yes it is 12V modulated on 3 different motherboards I tested. I
> also did find one interesting bit of information while testing this
> out.
>
> It seems there is a capacitor before the header, as when I was
> measuring voltage on the header after turning it off via software, the
> voltage took a good 3 minutes to settle at around 0.3V.
>
> So I have 3 questions:
> (please be patient as I have no formal education in electronics
> engineering or prior experience :-] )
>
> 1.) I'm assuming that the capacitor before the header will make the
> single transistor approach impossible, unless a load is presented on
> the header. Resistor? Is that a good approach? And how would I
> calculate watts from ohms? Given voltage.
>
> 2.) I found an old NPN TIP120 transistor laying around and tested it.
> I'm getting 420 ohms between the emitter ad the collector and 1.6k
> ohms between collector and emitter. Is it dead?
>
> 3.) If it is, would a new TIP120 be good for what I'm trying to do?
> How many watts (ballpark) would it be able to handle on a small
> heatsink inside a sealed project box?
>
> Thanks for your help!

Is the circuit one that drives the motor straight off of a PWM
transistor, or does it make a switching amplifier out of it with a
filter coil and capacitor to spare the motor? If it's filtered (which
the presence of a cap would indicate) then the simple transistor circuit
we're giving you won't suffice; you'll need something to turn the analog
voltage back into a PWM duty cycle.

You should check: load the output (with a resistor or a real fan). If
you have an o-scope then look at the voltage to the motor -- this will
be good because the simple circuit given is also sensitive to frequency;
above 1kHz you need to actually worry about the transistor speed. If
all you have is a DVM then all you can do is check to see if there's
significant AC on the pin, and hope that it's slow enough for your
transistor circuit. 

-- 
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

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