John is right. You want to feed LEDs a constant current, and let them drop whatever voltage they need. If I am not mistaken, LEDs have a negative voltage temperature coefficient. With this being the case, feeding a constant voltage will cause the current to increase as the diodes warm up. The additional current will cause more heating. This can lead to a "runaway" situation.
Note that feeding a constant current to a lamp is not unusual. All gas discharge lamps (neon, fluorescent, mercury vapor, etc) are run this way. Incandescent lamps are an exception since they are fairly nice resistors. However, to maintain more constant illumination levels as the lamp ages, constant current works better than constant voltage. The tungsten filament in the lamp gets thinner as the lamp ages, and its metal gets deposited on the inside surface of the glass. This blocks some of the light. The loss of material from the filament causes the resistance to go up. Using a constant current causes the filament to run a bit hotter (and brighter) due to the higher resistance. This compensates for the blocking of the light by the deposited metal. Donald Borowski EMC Compliance Engineer Schweitzer Engineering Labs Pullman, WA, USA From: John Woodgate <j...@jmwa.demon.co.uk> To: EMC-PSTC@LISTSERV.IEEE.ORG Date: 03/01/2013 07:12 AM Subject: Re: LED Chamber Lighting - drivers outside the chamber Sent by: emc-p...@ieee.org In message <FCA549BE3ECF9D4CB8CB8576837EA489140D6F@ZEUS.cetest.local>, dated Fri, 1 Mar 2013, "ce-test, qualified testing bv - Gert Gremmen" <g.grem...@cetest.nl> writes: >I will try using a voltage stabilizer first. >If the terminal voltage varies much with temperature a current driver >approach may be necessary. I strongly advise you to use a current source. -- OOO - Own Opinions Only. See www.jmwa.demon.co.uk SHOCK HORROR! Dinosaur-like DNA found in chicken and turkey meals John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com> - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org> All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc. Website: http://www.ieee-pses.org/ Instructions: http://listserv.ieee.org/request/user-guide.html List rules: http://www.ieee-pses.org/listrules.html For help, send mail to the list administrators: Scott Douglas <emcp...@radiusnorth.net> Mike Cantwell <mcantw...@ieee.org> For policy questions, send mail to: Jim Bacher: <j.bac...@ieee.org> David Heald: <dhe...@gmail.com>
