<html><head></head><body><div style="color:#000; background-color:#fff; font-family:Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:13px"><div id="yui_3_16_0_ym19_1_1498181992358_8965" dir="ltr"><span id="yui_3_16_0_ym19_1_1498181992358_8964">I will add to your discussion that I have used a 50 ohm 25 watt unit for 12+ years in all positions with no problems.</span></div><div id="yui_3_16_0_ym19_1_1498181992358_8965" dir="ltr"><span>Rob Brockway</span></div><div class="qtdSeparateBR" id="yui_3_16_0_ym19_1_1498181992358_8963"><br><br></div><div class="yahoo_quoted" id="yui_3_16_0_ym19_1_1498181992358_8944" style="display: block;"> <div style="font-family: Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 13px;" id="yui_3_16_0_ym19_1_1498181992358_8943"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;" id="yui_3_16_0_ym19_1_1498181992358_8942"> <div dir="ltr" id="yui_3_16_0_ym19_1_1498181992358_8941"> <font size="2" face="Arial" id="yui_3_16_0_ym19_1_1498181992358_8979"> <hr size="1" id="yui_3_16_0_ym19_1_1498181992358_8978"> <b><span style="font-weight:bold;">From:</span></b> David Roden <etpost@drmm.net><br> <b><span style="font-weight: bold;">To:</span></b> elec-trak@cosmos.phy.tufts.edu <br> <b><span style="font-weight: bold;">Sent:</span></b> Thursday, June 22, 2017 8:34 PM<br> <b><span style="font-weight: bold;">Subject:</span></b> Re: (ET) field weakening potentiometer with a Curtis 1204<br> </font> </div> <div class="y_msg_container" id="yui_3_16_0_ym19_1_1498181992358_8996"><br><div dir="ltr" id="yui_3_16_0_ym19_1_1498181992358_8997">On 22 Jun 2017 at 7:30, Robert Laird wrote:<br clear="none"><br clear="none">> You are only using a small portion of the total area available to<br clear="none">> dissipate the heat. <br clear="none"><br clear="none">Ah, thanks for the clarification, NOW I see what you're getting at, a heat <br clear="none">dissipation problem. You don't want your rheostat acting like a fuse. I'll <br clear="none">agree that you need to check the rheostat manufacturer's datasheet to make <br clear="none">sure its current rating is >= the load's full current.<br clear="none"><br clear="none">That's one big advantage to using fixed field resistors as GE did -- you can <br clear="none">calculate the power rating required for each fixed step, and set the steps <br clear="none">so that it's low enough to keep the component costs down.<br clear="none"><br clear="none">Or you can take the modern route (paved) and use a low power PWM controller <br clear="none">to drive the field. I see several 36v / ~350W DC brush motor controllers <br clear="none">offered on Ebay in the $20-30 range. Disclaimers: I haven't presonally <br clear="none">tried this, and I know nothing about the quality or reliability of these <br clear="none">little silver no-doubt-Chinese boxes.<div class="yqt4217756478" id="yqtfd29868"><br clear="none"><br clear="none"><br clear="none">David Roden - Akron, Ohio, USA<br clear="none"><br clear="none">= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = <br clear="none">Note: mail sent to the "etpost" address will not reach me. To send <br clear="none">me a private message, please use the address shown at the bottom<br clear="none">of this page : <a shape="rect" href="http://www.evdl.org/help/" target="_blank">http://www.evdl.org/help/</a><br clear="none">= = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = = =<br clear="none"><br clear="none"><br clear="none"><br clear="none"><br clear="none">_______________________________________________<br clear="none">Elec-trak mailing list<br clear="none"><a shape="rect" ymailto="mailto:Elec-trak@cosmos.phy.tufts.edu" href="mailto:Elec-trak@cosmos.phy.tufts.edu">Elec-trak@cosmos.phy.tufts.edu</a><br clear="none"><a shape="rect" href="https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak" target="_blank">https://cosmos.phy.tufts.edu/mailman/listinfo/elec-trak</a><br clear="none"></div></div><br><br></div> </div> </div> </div></div></body></html>