John, To follow on from Stuart's question, the circuit you describe, relies on the grounding for EMC being provided by another device (usually the PC). How effective is the suppression of the Y cap when it has the inductance on the USB cable etc. in series with it?
Regards, Dave C. From: Tyra, John [mailto:[email protected]] Sent: 18 April 2006 15:44 To: Coleman, David; [email protected] Cc: [email protected] Subject: RE: Portable hard disk power supply Hello Dave, I don't think your drive supply is defective as I have seen this problem in class 2 (non earthed )switching power supplies. In the case I have seen the higher voltage on the secondary to earth was caused by Agency approved Y type coupling caps inserted between primary and secondary used for EMC suppression purposes since there was no earth ground to work with. While the leakage current from the caps was limited by the devices inherent impedance, and below the IEC60065/IEC60950 Standards touch current limits, there was enough current to be physically felt if you happened to touch exposed metal and earth at the same time... I have also seen this problem with a class 2 linear supply which had no direct coupling between primary and secondary but used a toroidal transformer. I am no transformer expert but was told that the leakage current was a result of parasitic capacitance between primary and secondary of the transformer...Sounded like as good a theory as any but I cannot confirm this was the case... So it pays to limit any capacitance primary to secondary in any class 2 (nonearthed) mains connected power supply design Not a design defect but certainly could be perceived as such by a user!! Best regards, John Tyra Manager Product Safety Bose Corporation The Mountain, MS-450 Framingham, MA 01701-9168 Phone: 508-766-1502 Fax: 508-766-1145 From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: Tuesday, April 18, 2006 8:53 AM To: [email protected] Cc: [email protected] Subject: RE: Portable hard disk power supply Stuart, I have also found a large number of devices (usually mains powered USB, via a wall-wart) with sufficient potential difference between the USB shield and PC chassis, to cause a spark when connecting to the PC. Whilst the leakage (touch) current is low enough to pass the standard's requirements, it can give un-educated users a surprise. Dave C. From: [email protected] [mailto:[email protected]] On Behalf Of Stuart Miller - Ablewisp Sent: 18 April 2006 12:02 To: Emc-Pstc Subject: Portable hard disk power supply Hi group I hope I can get some advice on an issue with a power supply. It was supplied, as a separate unit, with a Maxtor portable hard disk drive that we use for backups in the office (UK based). The drive sits on a pc case. The supply is manufactured by DVE, model number 0421S. It provides 12V dc at 3A. Rated input is 100-240V, 50/60Hz, 1.2A. Its CE marked, UL listed and has a TuV mark stating BSEN60950. It's marked as double insulated. I guess its around 18 months old and has had an "easy" life in the office, with no visible case damage. After connecting the data cable (Firewire, already connected to the pc) to the drive, I touched its metal case while touching the outer of the dc connector and got an unpleasant tingle. I measured 230 V from the metal case to the outer of the dc connector (not connected to the portable hard disk). To isolate the drive from ac ground, I removed the data cable from the drive and measured 230V from the metal case to ac power ground (the drive is insulated from the pc case by plastic feet). Current was around 0.2mA. When the dc connector is connected, the drive performs normally. The drive was removed for secure storage when we left the office over the Easter holiday. In normal use it remains connected to one pc for weeks at a time. It would be exciting to move the drive from pc to pc! I couldn't find a recall notice for the DVE part number on Google. I'm interested in possible failure modes (I'm not a switch mode power supply expert) that could result in this shock hazard. I assume its due to degrading components and will get worse! Many thanks in advance Stuart Miller Ablewisp Limited www.ablewisp.com ph: 01803 843502 -- - This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. 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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
