Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!csd4.milw.wisc.edu!lll-winken!uunet!snjsn1!bilbo!greg From: greg@bilbo (Greg Wageman) Newsgroups: sci.electronics Subject: Re: Using "Free" telephone power Message-ID: <920@snjsn1.SJ.ATE.SLB.COM> Date: 1 May 89 19:26:47 GMT References: <636@serene.UUCP> Sender: news@SJ.ATE.SLB.COM Reply-To: greg@sj.ate.slb.com (Greg Wageman) Organization: Schlumberger ATE, San Jose, CA Lines: 47 In article <636@serene.UUCP> gbell@pnet12.cts.com (Greg Bell) writes: > >I'm still amazed at how phones run entirely off the phone line power. They >have lit dials, and audio amplifiers... and I had trouble lighting an LED! >(trouble means the phone company equipment gets loaded down enough that it >shuts off your line for a while!). An off-hook telephone is *required* to draw a minimum amount of current. I believe that it is permitted to draw about 20 milliamps from the line in an *off-hook* condition. The Central Office's switching equipment takes current draw above a certain limit as an indication of an off-hook condition. If you were trying to light that LED with the phone on-hook, you were actually taking the line off-hook. After 15 seconds or so, the dial tone generator would time out and you would have an open line. Please note that the (Western Electric) telephones (e. g. "Princess") with lighted dials are supplied 6 VAC by a small line transformer which is typically connected to the black and yellow "unused" wires in the phone jack. Multi-line phones are a whole 'nother ballgame, with a closet full of electronics and power supplies to support them. >Can anybody offer insite on the secrets of running audio amps and chips off >the meagre phone line power? How 'bout the ammount of current the phone line >can source? I've used line-powered modems. They use all CMOS parts to keep current consumption down, and as I said, since an off-hook device is required to draw a certain amount of current, why not use it to power the modem? Don't expect to be able to use phone line current and keep the circuit on-hook, however. Even if you could, you would have to cope with the possibility of an incoming ring signal, which is typically 60-90 VAC! (Yes, Virginia, you can ring a phone with house current.) Longish .signature follows. Skip now, or don't complain. Greg Wageman DOMAIN: greg@sj.ate.slb.com Schlumberger Technologies UUCP: ...!uunet!sjsca4!greg 1601 Technology Drive BIX: gwage San Jose, CA 95110-1397 CIS: 74016,352 (408) 437-5198 GEnie: G.WAGEMAN ------------------ "Live Free; Die Anyway." ------------------ Opinions expressed herein are solely the responsibility of the author.