I agree with Rich's suggestion re treating this as a Limited Current Circuit, and carrying out the test in 2.4 with a 2000 ohm resistor. We have done this for several LCD displays we use, to satisfy UL, and they seem happy to accept this. We also short circuit the transformer output to prove that no hazard results, and if I recall correctly, this also serves to satisfy 5.4.4 c) if the operational creepage and clearances at the transformer output are insufficient (which is quite likely).
Note that the wiring from the backlight circuit to the lamp needs to be rated for the voltage being produced - normal 600V wiring won't be good enough. Our vendors typically supply the backlight circuit board with flying leads using silicn rubber insulation. Regards, John Crabb, Development Excellence (Product Safety) , NCR Financial Solutions Group Ltd., Kingsway West, Dundee, Scotland. DD2 3XX E-Mail :john.cr...@scotland.ncr.com Tel: +44 (0)1382-592289 (direct ). Fax +44 (0)1382-622243. VoicePlus 6-341-2289. > -----Original Message----- > From: pe...@itl.co.il [SMTP:pe...@itl.co.il] > Sent: 18 May 1999 20:30 > To: 'emc-pstc'; treg@world.std.com > Cc: pmerguer...@itl.co.il; al...@itl.co.il; ico...@itl.co.il; > plei...@itl.co.il; zoh...@itl.co.il > Subject: Back-Light Circuit > > > Dear All, > > I am investigating an ITE product with a backlight-circuit (used to > provide the background lighting for an LCD display located in > SELV). However, manufacturer of product cannot get hold of the > backlight circuit schematics. I intend to use Clause 2.3.5 of the > UL1950/EN 60 950 standard. This allows the SELV to be > connected to hazardous voltages (approx 1500 V) in the backlight > circuit as long as the SELV limits are maintained even after single > faults. > > Missing the schematics, I wonder if if anyone can explain how a > backlight circuit works and what components in such a circuit are > candidates for shorting and/or open-circuiting? > > Thanks and Best Regards, > PETER S. MERGUERIAN > MANAGING DIRECTOR > PRODUCT TESTING DIVISION > I.T.L. (PRODUCT TESTING) LTD. > HACHAROSHET 26, P.O.B. 211 > OR YEHUDA 60251, ISRAEL > > TEL: 972-3-5339022 > FAX: 972-3-5339019 > E-MAIL: pe...@itl.co.il > Visit our Website: http://www.itl.co.il > --------- This message is coming from the emc-pstc discussion list. To cancel your subscription, send mail to majord...@ieee.org with the single line: "unsubscribe emc-pstc" (without the quotes). For help, send mail to ed.pr...@cubic.com, jim_bac...@monarch.com, ri...@sdd.hp.com, or roger.volgst...@compaq.com (the list administrators).
