Two blade equipments that require a shunt EMI filter capacitor to chassis also benefit from the polarization. By connecting the filter capacitor to the neutral terminal, it avoids inducing capacitor leakage current on the chassis and reduces the risk of electric shock in case of capacitor failure. Even below the 3.5mArms safety limit, leakage current can be very annoying to touch. If your adapter is a switchmode converter, which is becoming more and more common, it may have an EMI capacitor between neutral and one of the DC output terminals. Furthermore, for audio equipment, the leakage current induces high voltage on audio inputs when connecting and disconnecting audio cables which for the most part do not have a first make last brake feature on the ground terminal of their connectors. This is also true for many other type of signal interfaces of analog and digital equipment. So the polarized plug reduces the risk of annoyance and damage by limiting leakage current to chassis ground. Best Regards, Eric
_____ De : [email protected] [mailto:[email protected]] De la part de Price, Ed Envoyé : Thursday, August 10, 2006 9:04 AM À : [email protected] Objet : Power Plug Blade Configuration I recently bought a little Radio Shack wallwart power supply, equipped with two parallel-blade AC power terminals (with no ground blade) and a 6 VDC output wire pair. As you would expect, one of the blades is slightly wider than the other (implying that this is the neutral or return line), conforming to the convention of the typical NEMA 15 Amp female power receptacle. Since the AC power blades are located near one edge of the wallwart, you can plug the wallwart into a receptacle and the bulk of the wallwart body hangs to one side of the receptacle. Unfortunately, when you try to plug in a second wallwart, the bulk of the wallwart body prevents you from plugging it in, and the larger AC power blade prevents you from facing this second wallwart opposite the first one. My question is, why does this appliance have a polarized blade power connector? I am assuming that the input power pins go directly to the primary of a transformer, so why should we worry about which side is "high" or "return"? Could wallwarts be designed using identical power blades, allowing the option of a 180-degree rotation installation? Ed Price [email protected] WB6WSN NARTE Certified EMC Engineer & Technician Electromagnetic Compatibility Lab Cubic Defense Applications San Diego, CA USA 858-505-2780 (Voice) 858-505-1583 (Fax) Military & Avionics EMC Is Our Specialty - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] 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 [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc ______________________________________________________________________ This email has been scanned by the MessageLabs Email Security System. For more information please visit http://www.messagelabs.com/email ______________________________________________________________________ - ---------------------------------------------------------------- This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. Website: http://www.ieee-pses.org/ To post a message to the list, send your e-mail to [email protected] 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 [email protected] Mike Cantwell [email protected] For policy questions, send mail to: Jim Bacher: [email protected] David Heald: [email protected] All emc-pstc postings are archived and searchable on the web at: http://www.ieeecommunities.org/emc-pstc
