Maybe a PIC or dsPIC plus some dip switches or keypads
is better than a stepper motor + potentiometer.
I think it is cool to key in voltage values like 29.3V
from keypads. We have a HP6800A and it uses keypads.
But it is so expensive and so expensive to repair.

To me it is also odd to use linear pre-regulator and
switch-mode output regulator. What not use a isolated
flyback AC-DC converter + switch-mode DC-DC output 
regulator? You can drop the AC voltage to 48V first
and then run a buck converter from there to get 1 to 30V
(1V may be a bit difficult though).

Regards,
Xiaofan

-----Original Message-----
From: Philip Pemberton [mailto:philpem@dsl.pipex.com]
Sent: Saturday, September 03, 2005 8:08 PM
To: piclist@mit.edu
Subject: [EE]: Digital control of a MAX724/726 (voltage controlled
resistor?)


(For some odd reason my messages don't seem to be reaching the PICLIST - I
sent this at 9AM and it hasn't appeared yet, so here's a resend...)

Hi,
  I'm trying to design a bench power supply, seeing as all the ones I've
seen
on the second-hand market are either too expensive or a load of junk. I
wanted something with a voltage output of 1-30V with a maximum load current
of around 5A. It's going to use a combination of linear and switchmode
technology - linear pre-regulator, switchmode output regulator.

  The problem I'm having is with the switchmode regulator - a MAX724 (the
726
is the same chip with a 2A current limit instead of 5A). It uses a resistor
divider to set the output voltage, which is great for normal analogue
control
with a potentiometer. Problem is, I want to control it digitally.
  All the digital pots I've seen either have a maximum voltage of 5V between
both ends of the wiper. That rules out using a digipot because at 30V out
there's going to be (30-2.2)=27.8V between the end of the track and the
wiper.

  Can anyone suggest a way to control this thing? So far the only idea I've
come up with is a stepper motor and a potentiometer...

  Ideally I'd like 0.05V resolution for the voltage. I'm going to do current
limiting with a current shunt, a bunch of opamps and an 8-bit DAC, which
gives a theoretical resolution of 0.01A. I was thinking of something similar
for the voltage control (but with a 12- or 16-bit DAC), but couldn't think
of
a way to convert the voltage output from the opamps into a resistance.

Thanks.
-- 
Phil.                              | Acorn RiscPC600 SA220 64MB+6GB 100baseT
philpem@philpem.me.uk              | Athlon64 3200+ A8VDeluxe R2 512MB+100GB
http://www.philpem.me.uk/          | Panasonic CF-25 Mk.2 Toughbook
... Hey!  Your Trakball is upside down!
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