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/iframe></center><br><br> <br><br> </center> </td></tr> </table> <center> <table> <tr> <td width="600"> <center><hr width="400"><font size="2"><b>The <a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a> Usenet Archive Feeds Directly<br>From The Open And Publicly Available Newsgroup<br>Sci.Electronics.Design<br><br>This Group And Thousands Of Others Are Available<br>On Most IS NNTP News Servers On Port 119.<br><br><a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a> Is NOT Responsible For Any Topic,<br>Opinions Or Content Posted To This Or Any Other<br>Newsgroup. This Web Archive Of The Newsgroup And<br>Posts Are For Informational Purposes Only.<br><br></b></font><hr width="400"></center> <center><font size="2"><pre><b> From: "Fritz Schlunder" <srentacow@nospam.yahoo.com> Newsgroups: sci.electronics.design References: <sizam-3AA5BB.14100721102002@news.stanford.edu> <mS%s9.10716$r7.382095@twister.tampabay.rr.com> <sizam-6B287E.16294221102002@news.stanford.edu> <3db4ce71_2@corp.newsgroups.com> Subject: Re: Help, design behind a variable current power supply, w/o pots. Date: Mon, 21 Oct 2002 21:52:23 -0700 X-Newsreader: Microsoft Outlook Express 5.50.4522.1200 NNTP-Posting-Host: 66.218.239.188 Message-ID: <3db4d4a0$1_2@corp.newsgroups.com> X-Comments: This message was posted through Newsfeeds.com X-Comments2: IMPORTANT: Newsfeeds.com does not condone, nor support, spam or any illegal or copyrighted postings. X-Comments3: IMPORTANT: Under NO circumstances will postings containing illegal or copyrighted material through this service be tolerated!! X-Report: Please report illegal or inappropriate use to <abuse@newsfeeds.com> X-Abuse-Info: Please be sure to forward a copy of ALL headers, INCLUDING the body (DO NOT SEND ATTACHMENTS) Organization: Newsfeeds.com http://www.newsfeeds.com 80,000+ UNCENSORED Newsgroups. "Fritz Schlunder" <srentacow@nospam.yahoo.com> wrote in message news:3db4ce71_2@corp.newsgroups.com... > > "Sam Nichols" <sizam@inerd.net> wrote in message > news:sizam-6B287E.16294221102002@news.stanford.edu... > > In article <mS%s9.10716$r7.382095@twister.tampabay.rr.com>, > > "Sir Charles W. Shults III" <aichip@cfl.rr.com> wrote: > > > > > Another solution is to use something like a transmission gate or > analog > > > 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. 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. -----= Posted via Newsfeeds.Com, Uncensored Usenet News =----- http://www.newsfeeds.com - The #1 Newsgroup Service in the World! -----== Over 80,000 Newsgroups - 16 Different Servers! =----- </b></pre></font><hr width="650"><a href="http://www.cyber-spy.com/electo-index.html"><font size="3">Go Back To The <a href="http://www.cyber-spy.com/">Cyber-Spy.Com</a><br>Usenet Web Archive Index Of<br>The sci.electronics.design Newsgroup</font></a><br><br></center></td><img src="http://cyber-spy.com/cgi-bin/livecntr.pl?0+1+unetsci" width="1" height="1" alt="The Cyber-Spy Toolkit - powerful internet spy software - live police scanners - usenet fbi secrets archive - newsgroup spy electronics archive - search engine spy tools - download TCST today!"> </tr> </table> </center> <center> <br><center><hr size="3"></center> <table width="100%" cellpadding="0" border="0" cellspacing="0"> <tr> <td align="left" valign="top"><small><b><i><a href="http://www.cyber-spy.com/index.html">Cyber-Spy.Com Live Resources</a></i></b></small></td> <td align="right" valign="top"><small> Copyright Alpha Omega Delta,<br> All rights reserved. </small></td> </tr> </table> <center><hr size="3"></center> <br><br> <br><br></center> </body></html>