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From: Tony Williams
Subject: An interesting little prob.
Date: Wed, 30 Oct 2002 09:34:07 +0000 (GMT)
NNTP-Posting-Date: Wed, 30 Oct 2002 09:34:06 +0000 (UTC)
User-Agent: Pluto/1.14i (RISC-OS/3.60)
We use quite a number of programmable dc supplies in our
equipment, both Volts and Amps being programmable. These
are basically switchers producing Volts, so they are
excellent as voltage sources, but when used as current
sources (especially at low currents) there are problems.
These problems mainly stem from the big smoothing cap
on the output of the switcher....... it results in a
current source that is sluggish to programme, and which
is useless with a varying or non-linear load.
So I'm reversing the logic.... design a programmable
analogue current source (say 0 to 500mA dc), with a
programmable shunt voltage limiter (say ? to 20V dc).
As per the block-schematic below;
Unregulated dc+>---[ V-controlled ]--|>|-+-----+---->
[Current-Source] | |
/|\ | |
| [V-controlled] |
[12-bit]--------+ [ Shunt-reg ] | Iout
[ DAC ] /|\ | _|_
| | /_\
[12-bit]--------------------+ | |
[ DAC ] | |
The unregulated DC will be about 25 to 35V. The two 12-bit
DACs are down on 0v, probably powered from 12-0-12 supplies,
and developing 0 to 5v under programme control. The two
diodes are just protection diodes against what can happen
when in use.
The V-controlled Current-Source is fairly straightforward;
An opamp+MOSFET powered off -12v relative to the raw dc,
with the control from the DAC being sent up to it as a
The V-controlled Shunt-reg isn't sorted yet though. It has
to be not less than (say) 1 megohm shunt resistance when
not regulating and has to be programmable to as low a
voltage as practical.... less than 1v would be nice.
I also have to detect and indicate when in CV or CI mode.
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