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From: "Fritz Schlunder"
Subject: Re: Help, design behind a variable current power supply, w/o pots.
Date: Mon, 21 Oct 2002 21:52:23 -0700
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"Fritz Schlunder" wrote in message
> "Sam Nichols" wrote in message
> > In article ,
> > "Sir Charles W. Shults III" wrote:
> > > Another solution is to use something like a transmission gate or
> > > multiplexor to select a control resistor in a current amplifier. Even
> > > transistors could switch a control voltage or a resistor in a current
> > > regulator.
> > > There are a lot of simple ways to do this.
> > >
> > > Cheers!
> > >
> > > Chip Shults
> > > My robotics, space and CGI web page - http://home.cfl.rr.com/aichip
> > >
> > >
> > >
> > Sorry, most of that went over my head, I'll look this stuff up on the
> > web but could you be more specific? How could a transistor switch a
> > resistor? Also, I looked around at digital pots, I want to be able to
> > go from 10 mA to 5000 mA, granted that might not all happen from one pot
> > but I can't find any pots that handle even close to 1000 mA.
> 10mA to 5A eh'? That is a rather large dynamic range although certainly
> impossible to achieve, but you definitely don't crank all that up to 5A of
> current through the digital pot directly. I'm not sure what kind of
> limitied powersupply you have in mind, but perhaps something like the
> titled "High Compliance Current Sink" on page 15 of this op-amp datasheet
> (http://www.national.com/ds/LM/LM158.pdf) could be modified for your use.
> Choose an appropriate sense resistor value and large enough gain
> (or perhaps consider replacing it with a MOSFET), and then feed the
> non-inverting input pin with a voltage divider from some fixed reference
> voltage using an adjustable resistive divider using your digital pot as
> of the resistors in the divider.
Oh yeah, one more thing. If you chose a sense resistor of 1 ohm, that only
produces a rather meager 10mV at 10mA, which is rather small. But then at
5A your 1 ohm resistor is dissipating a rather massive 25Watts. Though I've
never tried doing it, it seems to me you could increase the practical
dynamic range of a single high compliance current sink by paralleling it
with other high compliance current sinks that are contructed identically.
Perhaps a practical implementation of such a concept might be something like
this: Use a quad op-amp with a common mode input voltage range that reaches
GND (LM324 for instance). Use four transistors with sufficient gain that
can handle 1.25Amps each with a base drive of 10-20mA or less (the
approximate output capability of the LM324). Perhaps choose sense resistor
values of around 1 ohm. Then hook the non-inverting inputs of the op-amps
up to separate resistive dividers using separate digital pots. At low
currents you would only turn on one of the current sinks, but for the full
5Amps you would turn them all on.
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