Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!decvax!ucbvax!larry@seismo.CSS.GOV@kitty.UUCP From: larry@seismo.CSS.GOV@kitty.UUCP Newsgroups: mod.telecom Subject: Re: Hold Circuit Message-ID: <8701210626.AA18289@ucbvax.Berkeley.EDU> Date: Thu, 25-Dec-86 23:20:16 EST Article-I.D.: ucbvax.8701210626.AA18289 Posted: Thu Dec 25 23:20:16 1986 Date-Received: Wed, 21-Jan-87 18:43:40 EST Sender: daemon@ucbvax.BERKELEY.EDU Organization: The ARPA Internet Lines: 60 Approved: telecom@xx.lcs.mit.edu >> From: tj >> >> put a resistor, and LED, a Zener, and an SCR in series. The Gate of the scr >> goes through a resistor and a pushbutton to + phone line. Lets draw a >> schematic online... [schematic delected for brevity] >> The SCR is off until pushbutton PB is pressed making contact thus >> turning it on. Upper Zener is some voltage higher than the off hook >> voltage of the line. (say 10 volts) Until the line goes higher than >> 10 volts it is off, even though the SCR is on. Hold the push button >> as you hang up. This turns on the SCR and as the line voltage rises >> the zener conducts and the current through the SCR will hold it on >> (get a low hold current scr, say 3 ma) The resistor in series with >> the push button supplies the small turn on current. 5 k should do. >> The lower zener limits the voltage across the LED. Make say 5 >> volts. This would put the r in series with the LEd at about 150 >> ohms. > I doubt this circuit would work, and it could be dangerous. There is > no impedience between a 28 volt source and ground; that voltage will > be pulled down to 15 volts. What would limit the current through the > SCR and two zener diodes? The circuit should work under most circumstances, and is by no means "dangerous". The current on a telephone subscriber loop is limited by the central office apparatus. In most central offices there is effectively 200 ohms between the tip of the line and ground, and 200 ohms between the ring of the line and -48 volt office battery. This results in a total of 400 ohms resistance so that from Ohm's law a short circuit across the loop sitting right at the CO would draw 0.120 ampere. There are two discreet resistances as described above so that the subscriber line will be longitudinally balanced with respect to ground. Since it is necessary for the CO apparatus to detect current flow in the subcriber loop to indicate an off-hook condition, the above resistances may be incorporated as two windings in a "line relay", ferrod sensor, repeating coil, etc. While a line relay and ferrod sensor sense current flow directly, the voltage drop across a resistor or repeating coil is used for current detection in other types of CO's. So the point is: 0.120 ampere _worst_ case isn't going to hurt anything or destroy a properly selected SCR (I would use 200 V @ 1 ampere to be conservative and handle surge voltages). A more typical short circuit current value for a subscriber loop (since most subscribers don't live next door to the CO) is .030 to .060 ampere. One minor improvement which could be made to the circuit is to make it polarity-independent and protect against possible false ring-tripping by placing a full-wave bridge rectifier (use 200 PIV @ 1 ampere to be conservative) between the circuit and the telephone line (the AC leads go to the telephone line). Also, a small resistance may be required in series with the SCR cathode to make certain that the circuit releases properly under some subscriber loop conditions. You could add say, a 500 ohm pot and know that you'll be in range. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|bbncca|decvax|nike|rocksanne|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"