Path: utzoo!attcan!uunet!lll-winken!lll-lcc!ames!pasteur!ucbvax!hplabs!hpfcdc!hpficad!stu
From: stu@hpficad.HP.COM (Stu Bell)
Newsgroups: sci.electronics
Subject: Re: Variable speed motor control ... need help.
Message-ID: <15880004@hpficad.HP.COM>
Date: 16 Jan 89 19:53:23 GMT
References: <288@heurikon.UUCP>
Organization: Hewlett Packard CICD
Lines: 32

> 
> >I use the circuit below for model trains (I have left out the direction
> >switches):
> >
> > [deleted]
> >
> >Make sure you heat sink the PNP transistor -- If you are pulling a couple of
> >amps through the motor at ~10 volts, the PNP will be dissipating 20 Watts!
> >This is plenty enough to get (VERY!) warm.
> 
> Nope.  The motor will -- the transistor sees only the difference between the
> supply voltage and the motor voltage. Maximum dissipation occurs when both
> the motor and transistor see v_supply/2.
> 

You're right, of course.  Still, that heat sink gets PLENTY hot.  I know, as
I've burned my fingers more than once.

For everyone that gave a PWM solution, your solution will work BUT...  some
motors (especially the expensive Japanese "can" (or "micro") motors) are
reportedly sensitive to the pulses and tend to burn up with even moderate
(100 HZ - 1 KHZ) frequency pulses.  I have no experience with this, but I'm
not willing to sacrifice an $80 motor just to find out.

Stu Bell
HP Colorado "Chips 'R' Us" Division

Thot for the Day:  What if I didn't give a Disclaimer and nobody sued?
                   ^^^^^^^-(NOT an HP advertisement!)

Disclaimer:  The above may have been generated with line noise.  Hewlett
Packard Company certainly knows nothing about this,  Please don't sue me!