🚀 go-pugleaf

In article <288@heurikon.UUCP>, lampman@heurikon.UUCP (Ray Lampman) writes:
> I need a circuit that will provide variable speed control of a DC motor. The
> operator control should be via some device with a knob attached. Would this
> be a variable resister? The control should be roughly linear, let me explain.
> 
>          |             /     When the control knob is set half way between
> control  |           /       minimum and maximum, the motor should run at a
> setting  |         o         rate half way between the rate at the minimum
>          |       /           knob setting and the maximum knob setting.

	In general, with either a series or shunt wound DC motor, using just
a variable resistor you don't have a prayer of a chance of achieving a linear
transfer function between the control setting and effective RPM - even at
no load.  Loading the motor makes the matter significantly worse.  Of course,
you could always have a custom-wound non-linear variable resistor :-) - but
this is not what you are seeking.

	Now, if the DC motor is permanent magnet or has a field winding that
is separately excited at constant current, the transfer function between
a linear variable resistor setting and effective RPM gets "straighter" -
but by no means is it perfectly linear.  Should there be any change in
load upon the motor, this transfer function will go to hell in a handbasket
real quick-like. :-)

	Unfortunately, it is really not possible to achieve the _linear_
relationship which you seek without having an electronic motor control
circuit that utilizes feedback through back-EMF sensing and/or a tachometer
measurement.  Such motor control circuits are not that complex, but they
are going to require transistor or SCR control.  Under these circumstances,
the variable resistor would be a 3-terminal voltage divider providing
only a low-current control voltage to the motor control circuit, say,
0 to 10 volts for 0 to full RPM.  Even under these circumstances the
transfer function will not be perfectly linear - but you can get it pretty
straight with clever circuit design.

	Now as to a source for an appropriate circuit, I would suggest
perusing application manuals from semiconductor manufacturers such as
National, RCA, Signetics, etc.

> I have an appropiate DC power supply for the system. Should I be able to
> locate parts for this circuit at a Radio Shack store?

	Yes, believe it or not. :-)

<>  Larry Lippman @ Recognition Research Corp., Clarence, New York
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