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From: nagle@netcom.com (John Nagle)
Subject: Re: Any cheap linear amps?
Message-ID: <nagleDD65GI.K58@netcom.com>
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References: <DD3qCK.LDp@poly.edu>
Date: Fri, 11 Aug 1995 23:05:05 GMT
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jguthy@poly.edu (John Guthy) writes:
>	I'm looking for a cheap solution for linear amplification (for DC
>	motors).  Ideally, I want high power op-amps that can handle 12-24vDC
>	at 2-5 amps.  I have checked places like digikey, and only found op-amps
>	that can handle 2 amps (for 30-40$!).  

>	I would rather not use PWM. BTW, how do you model a PWM amp if the
>	switching frequency is not high enough to ignore? 
>	Thanks for any help,

      There's a good reason you don't see linear amps for motor control
much.  You have to be prepared for big losses in the drive transistors,
with corresponding heat dissipation.  In the power range you're talking
about, though, linear transistors with suitable dissipation are 
available.  Motorola has several lines of power transistors
in the 100-150W range you need.  Prices run under $2 each from
Newark.  You can probably put together a drive circuit using an op-amp
and one of these transistors; see "The Art of Electronics" for some
suggested circuits.  

      This stuff is forgiving.  Just get an op amp, a big power transistor,
a solderless breadboard, and try some reasonable circuits.
Remember to use a heat sink.  And a fuse.

					John Nagle
