Like very low-drift analog circuits, high impedance circuits require attention to detail normally dismissed as negligible in 'everyday' circuit design. _______________________________________________________________________________
Ralph McDiarmid | Schneider Electric | Solar Business | CANADA | Regulatory Compliance Engineering From: Ted Eckert <[email protected]> To: [email protected], Date: 06/29/2015 12:55 PM Subject: Re: [PSES] Optos for PELV isolation I’ve designed systems where the optocouplers were required to have an unusually high insulation resistance. I worked for a company that designed industrial water treatment equipment. The pH probes we used act like a battery. The more current you draw out of them, the sooner they need to be replaced. If you wanted to get at least 6 months operation out of a pH probe, the current draw had to be limited to no more than a few picoamperes. DC-DC converters with a high level of insulation (and lousy efficiencies) were used to get power to the pH probe’s analog amplifier circuitry and optocouplers were used to get the data back to the digital electronics. The high insulation values were not for safety; they were just to limit current from the pH probe finding a lower impedance leakage path. I remember a case where a mishandled board failed because a fingerprint was left on the solder side of the board under the optocoupler. It got encapsulated by the conformal coating and the oil from the fingerprint provided enough of a current path that the system was depleting pH probes about once every two weeks. Ted Eckert Compliance Engineer Microsoft Corporation [email protected] The opinions expressed are my own and do not necessarily reflect those of my employer. From: Nyffenegger, Dave [mailto:[email protected]] Sent: Monday, June 29, 2015 12:07 PM To: [email protected] Subject: Re: [PSES] Optos for PELV isolation HI Rich, The standard also says 5 second interval between discharges. And the test must be run on 20 samples. We know UL likes to test. First edition of the standard is 1984, the test has probably been in there since the beginning. -Dave From: Richard Nute [mailto:[email protected]] Sent: Monday, June 29, 2015 1:51 PM To: Nyffenegger, Dave; [email protected] Subject: RE: [PSES] Optos for PELV isolation Hi Dave: “subjected to 50 discharges from a 0.0005 microfarad capacitor that has been charged to a potential of 20 kV between the short-circuited input and short circuited output terminals.” Okay, 500 pF charged to 20 kV then applied to the opto isolation. The input-to-output capacitance is on the order of 1 pF. That means nearly the full 20 kV is applied across the isolation barrier, and exponentially decays by the insulation resistance. One opto specs the insulation resistance at 100 Gohms minimum! Time constant is 500e-9 x 100e+9 or 50000 seconds or 833 minutes or 13 hours! Repeated 50 times! 50 days to do the test, at 5 days per week, 10 weeks, 2-1/2 months! I must be missing something. Test does not make much sense. Best regards, Rich - ---------------------------------------------------------------- - ---------------------------------------------------------------- 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 <[email protected]> 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://www.ieee-pses.org/list.html (including how to unsubscribe) 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]>
