From: "Fritz Schlunder"
Subject: Re: Help, design behind a variable current power supply, w/o pots.
Date: Mon, 21 Oct 2002 21:25:58 -0700
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"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 not
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 current
limitied powersupply you have in mind, but perhaps something like the figure
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 transistor
(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 one
of the resistors in the divider.
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