Path: utzoo!attcan!uunet!lll-winken!ames!pasteur!ucbvax!hplabs!hpfcdc!hpfclm!hpfcdj!myers
From: myers@hpfcdj.HP.COM (Bob Myers)
Newsgroups: sci.electronics
Subject: Re: Variable speed motor control ... need help.
Message-ID: <16750011@hpfcdj.HP.COM>
Date: 13 Jan 89 19:36:31 GMT
References: <288@heurikon.UUCP>
Organization: Hewlett Packard -- Fort Collins, CO
Lines: 56

>... not to mention what the inductive backlash can do to your *transistor*!

>The appropriate fix is a clamp diode around the motor, I suppose, but that
>raises another question: Does the motor behave differently when it's allowed
>to backlash, instead of being clamped?

>And another question I just thought of: What exactly does placing a clamp
>diode around the *transistor* do?  It seems to me that the motor would try
>to backlash through the power supply.  I often see these extra diodes as part
>of Darlington packages, etc.


Hmmm; I'm getting in a bit late on this, so forgive me if the answer below
doesn't address the specific circuit in question.  You don't say exactly
what "around the transistor" means, so here's a generic answer on clamping
transistors/inductors:

"Inductive backlash" results from the fact that an inductor doesn't like to
see changes in the current passing through it, which is what the formula
V = L * di/dt  means.  If you attempt to interrupt the current through an
inductor suddenly (by switching of a transistor, for example), the voltage
across the inductor increases drastically; the "polarity" of the voltage across
the inductor is reversed, too, as the inductor goes from being a "sink" to
a "source". (We're suddenly sourcing current from the inductor, as the field
collapses.  An inductor is, after all, simply a device which stores energy in
a magnetic field.)  If this causes the voltage across the switching transistor
to exceed that transistor's rated breakdown voltage, it's good-bye
transistor; so a hefty diode is commonly connected "backwards" across the
transistor (cathode to collector, anode to emitter - assuming an NPN device)
in order to limit the peak reverse voltage to the forward drop of the diode.

Now, as you may have realized, this doesn't do diddly for the case of switching
an inductive load in series with the collector; the inductor current still
has to go *somewhere* without breaking down the transistor.  For this reason,
a diode is typically connected "backwards" across the inductive load,
letting the energy stored in the inductor's field dissipate harmlessly.
(Harmlessly, that is, assuming a hefty enough diode on a good heat sink!)

There is another "clamp" diode which is often associated with transistors; 
this is a diode connected from base to collector, and often referred to as
a "Baker clamp".  This serves a different purpose entirely, that of keeping
the transistor out of saturation.  If the forward drop of the diode is less
than the forward drop of the base-collector junction, then the transistor
will never truly enter saturation (which requires both the B-E and B-C
junctions to be forward biased).  Instead, current is diverted around the
B-C junction, and the transistor is kept *just* out of saturation.  This
"speeds up" the transistor's switching time, as there is no charge build-up
in this juction during the "on" time.  A Schottky diode is typically used,
due to its lower forward drop as compared to a silicon P-N junction.

Hope this helps!


Bob Myers  KC0EW   HP Graphics Tech. Div.|  Opinions expressed here are not
                   Ft. Collins, Colorado |  those of my employer or any other
{the known universe}!hplabs!hpfcla!myers |  sentient life-form on this planet.