>-----Original Message-----
>From: Russell McMahon [mailto:apptech@PARADISE.NET.NZ]
>Sent: 09 July 2004 10:37
>To: PICLIST@MITVMA.MIT.EDU
>Subject: [EE:] Design Challenge - Lowest cost PWM

>A 555 MAY be able to do most of this by itself. Prove it :-)
>

Many years ago I designed a motor speed controller using 556, one timer
configured as an fixed fequency astable, triggering the other timer
configured as a monostable with variable pulse width (using the control
voltage input).  This drove a car starter motor via a bank of about 20
2N3055's (and a high power driver stage, suitable MOSFETS were very
expensive at the time, and 2N3055's were dirt cheap on the surplus market).

Linearity won't be good with the standard monostable due to the capacitor
being charged by a resistor, and compared to the reference.  Replacing the
resistor with a simple constant current source (maybe a jfet or constant
current "diode") should improve this considerably.  Getting 0% and 100% may
be trickier!

You might also look at the retriggerable monostables available in the cheap
logic families, perhaps using a simple voltage controlled current source to
charge the timing cap.

Regards

Mike

=======================================================================
This e-mail is intended for the person it is addressed to only. The
information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
person. Please contact us immediately to tell us that you have
received this e-mail, and return the original to us. Any use,
forwarding, printing or copying of this message is strictly prohibited.
No part of this message can be considered a request for goods or
services.
=======================================================================

--
http://www.piclist.com hint: To leave the PICList
mailto:piclist-unsubscribe-request@mitvma.mit.edu