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From: rsd@sei.cmu.edu (Richard S D'Ippolito)
Newsgroups: sci.electronics
Subject: Re: Variable speed motor control ... need help.
Message-ID: <8165@aw.sei.cmu.edu>
Date: 10 Jan 89 20:01:59 GMT
References: <288@heurikon.UUCP> <15880003@hpficad.HP.COM>
Sender: netnews@sei.cmu.edu
Reply-To: rsd@ae.sei.cmu.edu.UUCP (Rich D'Ippolito)
Organization: Carnegie-Mellon University, SEI, Pgh, Pa
Lines: 23

In article <15880003@hpficad.HP.COM> stu@hpficad.HP.COM (Stu Bell) writes:

>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.


Rich
--

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You can lead a horse to water,                              RSD@sei.cmu.edu
but you can't make him fish.
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