Path: utzoo!attcan!uunet!lll-winken!netsys!vector!nobody From: kitty!larry@cs.buffalo.edu Newsgroups: comp.dcom.telecom Subject: Submission for comp.dcom.telecom Message-ID: Date: 9 Oct 88 15:49:02 GMT Sender: chip@vector.UUCP Lines: 46 Approved: telecom-request@vector.uucp (USENET Telecom Moderator) X-TELECOM-Digest: volume 8, issue 157, message 6 X-Submissions-To: telecom@bu-cs.bu.edu (TELECOM Digest Coordinator) X-Administrivia-To: telecom-request@vector.uucp (USENET Telecom Moderator) In article tedk@ihuxv.ATT.COM (Kekatos) writes: > | I'm looking for a relay to place in series with a telephone which will > | open (or close) when the set is offhook. The closest I've come is with > | a 200 ohm 4.5 mA relay. It didn't affect the phone operation too much, > | but wouldn't quite pull in. If I pressed the armature in, it would > | latch. Close, but no cigar. > | So, what type of relay should I be looking for (impedance/current). Is > | such a thing commonly available? Will this scheme work? Does ringing > | current cause any problems? > > Sorry, This can not be done with a ONLY a relay. You just can not > draw enough current from the phone line. This is incorrect. Since the beginning of time :-), relays have been available which operate on series telephone loop current. Typical off-hook loop current ranges from a low of 0.020 A (long CO loop) to 0.1 A (sitting next to a PABX). In the "traditional" key telephone system world, the 6-type KTU (Key Telephone Unit) has been around for at least 50 years. This device consists of a single WECO B-type relay wired to screw terminals on a small mounting bracket. I don't have the exact relay specs handy, but the relay picks up at somewhat less than 0.010 A and has an internal resistance of around 20 ohms. The relay will withstand at least 0.2 A of overcurrent with no damage. There are various types of non-polar relays which exhibit this degree of sensitivity. In the past 25 years, reed relays have been particularly popular for loop current detection, since a reed relay is sealed and requires no adjustment. With respect to the article by the original poster, a relay with a series resistance of 200 ohms introduces too much series resistance for a "good design". A series resistance of less than 75 ohms should be used. A common circuit design practice is to place a non-polarized capacitor (0.5 to 2.0 uF) across the relay winding to minimize voice frequency attenuation. While the 200 ohm relay used by the original poster has an undesirable high resistance, it SHOULD have still worked in a normal telephone loop. If it did not work as described, that it was either maladjusted, damaged or did not in fact have a sensitivity of 4.5 mA. Bear in mind that I am not advocating the use of relays in place of solid-state device (like the opto-isolator). I am merely pointing out that relays will in fact work, and for some applications, a scrounged up 6C KTU is still the simplest solution. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {att|hplabs|mtune|utzoo|uunet}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"