Hi John, Even theory has to comply with practice, so i took my soldering iron...
I have to admit that modern capacitors do keep their charge too long. I did some test with newer Y and X caps of 0.1 uF and they keep their charge way too long - over 1 minute - without notable loss of voltage (< 10%) (real life voltages choosen :310V) Probably the quality of the dielectricum has been improved, or the use of paper has been abandoned in favor of modern equivalent plastics. (any one knows ?). Of course the increased leakage of the older caps was not meant to be part of the specs, and this is again a good example of how relying on hidden specifications may in time lead to undesired results: standards ignoring the effect of increased leakage resistance. I also tried the discharge between two fingers, and found the result to be unpleasant at least. Time to change standards....... Gert Gremmen ce-test, qualified testing -----Original Message----- From: [email protected] [mailto:[email protected]]On Behalf Of John Allen Sent: Thursday, September 19, 2002 9:24 AM To: 'Grasso, Charles'; 'Warren Birmingham' Cc: [email protected] Subject: RE: Question: Discharge capacitance 0.1 uF Hi Charles, Warren Seems that a few of us know what DOES happen and the longterm results, but quite a few others don't believe that it does - and that even it does then it is not very "important". The difference between reality and theory! I suggest that the "non-believers" try it for themselves - by unplugging a suitable piece of equipment and picking it up - AND then touching the pins of the plug! (the unplugging process may need to be repeated a few times until the capacitor is disconnected when the mains is high at the time of disconnection and so gets a "decent" charge!) However before they do try it, I suggest they wear safety boots and also use a piece of equipment which can then be discarded due to the damage it received when it fell on the foot and/or the ground! Regards John Allen -----Original Message----- From: Grasso, Charles [mailto:[email protected]] Sent: 18 September 2002 19:07 To: 'John Allen'; [email protected] Subject: RE: Question: Discharge capacitance 0.1 uF Hi All, From personal experience I can tell you that the involuntary reaction to a shock can have serious consequences to the sales of a company. In a former life - a previous employer OEM'd a PC from a Korean Company. The PC had all the relevant marks but somehow the resistor that was supposed to bleed off the caps didn't make it into production. A customer , moving said model from one location to another, touched the mains terminals and felt a shock. The customer fell over, the PC landed on the customer, the customer sued and the story ended up in the papers. The sales of PCs essentially died after that. - All for the sake of one resistor. Best Regards Charles Grasso Senior Compliance Engineer Echostar Communications Corp. Tel: 303-706-5467 Fax: 303-799-6222 Cell: 303-204-2974 Email: [email protected]; Email Alternate: [email protected] -----Original Message----- From: John Allen [mailto:[email protected]] Sent: Wednesday, September 18, 2002 11:11 AM To: [email protected] Subject: Question: Discharge capacitance 0.1 uF Hello Folks Tomonori Sato commented "However, I think discharge from 0.1uF capacitor charged to the mains peak voltage can be quite uncomfortable." I believe that to be true from personal experience and from having to investigate the results of a number of such incidents, and so would remind member of a point that I made several years ago on this forum: The primary shock almost certainly will NOT hurt a person, but the involuntary reaction TO the shock may well have much more seriousconsequences. This type of shock is often encountered by people who pick up equipment which they have just unplugged from the AC mains in order to carry it elsewhere. If they then touch the pins of the plug there are numerous reported incidences of them involuntarily dropping the unit - and that can possibly be on their own feet - and from a height of about 3ft/1m! If the unit is more than a couple of pounds (about one kilo) then the injury to t! he feet can be substantial. Worse situations could occur in industrial equipment when a service engineer opens a cabinet to perform a service operation - the reaction from the "shock" could cause him to strike touch other hazardous electrical or mechanical parts (which probably should also not be there, I do agree!) which then cause him serious actual injury. These types of incident do not make the equipment supplier very "popular" to say the least, and could result in product liability claims. The main basis for the claims would be that the supplier had not adequately assessed the hazards and taken the appropriate simple precautions which are easily and cheaply available - fit a bleeder resistor across the capacitor, or use a filter with a resistor already built in (or with transformer/inductor windings directly across the capacitor - which achieve the same result) ! Again from personal experience I can say that it is a very "embarassing" and un! comfortable experience to have to write to an injured or anno! yed person, or to his employer, to say "sorry, but that is what the safety standard allows". It is just not good "business sense". Therefore, regardless of the requirements of the various standards and this argument over capacitor value and/or charging voltage, I firmly believe that the use of bleeder resistors should be considered effectively mandatory, and have always recommended it to engineers I have advised on product safety. Regards John Allen Technical Consultant Electromagnetics, Safety and Reliability Group ERA Technology Ltd Cleeve Rd Leatherhead Surrey KT22 7SA Tel: +44 (0) 1372-367025 (Direct) +44 (0) 1372-367000 (Switchboard) Fax: +44 (0) 1372-367102 (Fax) ---------------------------------------------------------------------- Replies to this message may be posted in the following public forum: Question: Discharge capacitance 0.1 uF _____________________________________________________________________ This e-mail has been scanned for viruses by the WorldCom Internet Managed Scanning Service - powered by MessageLabs. For further information visit http://www.worldcom.com ************************************************************************* Copyright ERA Technology Ltd. 2002. (www.era.co.uk). All rights reserved. The information supplied in this email should be treated in confidence. 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