Since this is a single supply with no reference common to anything else,
put the current sense resistor in the ground lead.  No problem!

dwayne

At 04:28 PM 10/29/02 -0800, William Chops Westfield wrote:
>I have a hypothetical "hobbyist power supply" that I'm thinking about
>(partially inspired by the recently mentioned dead bench power supply I'd
>mentioned elsewhere, and also by accidentally buying too many LM317s.)  It
>would provide CCCV output, 0-20V or so at about 1A max.  I'd use a 24V or so
>wall wart input, a bunch of LM317 style regulators, and a PIC with LED/LCD
>to display voltage and current.  This is, in theory, the sort of thing a
>hobbyist could build easilly and cheaply from surplus parts, but would cost
>a pretty bundle if you tried to make it from NEW parts.
>
>There are a couple of complications that have already turned up...
>The obvious way to measure the not-so-large currents I'm thinking of
>is to put something like a 0.1 ohm power resistor in the output, and
>measure the maximum voltage of I*R (.15V) to get current (at 1.5A max,
>you only need to dissipate about 1/4W...)  HOWEVER, unless your PIC
>has a completely separate and isolated power supply, you're not measuring,
>0.15V, you're measuring the difference between two voltages that are...
>quite high compared to what you can connect to a PIC input, or even your
>average opamp.  Isolated power supplies might be tempting (add another
>$0.99 wall wart), but lack elegance :-)  Batteries might be OK for an
>LCD display...
>
>LM317 info is sketchier than I'd have thought as well.  The data sheets all
>have a basic switching regulator, and a circuit that uses a separate LM317
>as a pre-regulator, but little in the way of explanation, and no obvious
>extension of "switching pre-regulator."  There's circuits that parallel 317s
>for increased output capability, but again there is little explanation, and
>those circuits aren't combined with the current limitter circuits...  If
>anyone knows of any more complete ... tutorials on using the LM317, please
>let me know!
>
>BillW
>
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Dwayne Reid   <dwayner@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
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