🚀 go-pugleaf

In article <636@serene.UUCP>, gbell@pnet12.cts.com (Greg Bell) writes:
> This is in response to the person who was talking about lighting an LED from
> the 50 VDC phone line voltage as an indicator for hook status:
> 
> That's the easy project!  But, how 'bout making a circuit that will light an
> LED when the line voltage DROPS (ie. the phone goes off hook).  I tried it
> once, and got it to work moderately well.   I used a combination of voltage
> dividers to keep a transistor off if the voltage is 50 or so.  Worked alright
> until the phone rang... the circuit answered the phone!  Got a little
> annoying.

	No kidding! :-)  That's what I'm been trying to tell sci.electronics
readers for quite some time.

> 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!).  
> 
> 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?

	Again, here are some assorted comments about telephone loop voltages
and currents:

1.	The on-hook (i.e., idle) voltage of the vast majority of central
	office telephone loops ranges between 48 to 52 volts, with 50 volts
	being a pretty typical value.  However, this is NOT ALWAYS the
	case, since on-hook voltages of lines supplied from various
	subscriber line concentrators, subscriber line carrier apparatus
	and subscriber lines on long loops with loop extenders can vary
	from a low of 6 volts (that's right, 6 volts using Continental
	FDM AML subscriber line carrier) to a high of 96 volts (using
	Cook Electric or equivalent loop extenders).  A few electronic
	PABX's may have 24 volts instead of 48.

	There are two morals to the above:

	(a) Only a 500-type telephone set with passive network will work
	    on all telephone loops at all times; anything else will fail
	    under some circumstances.

	(b) If in doubt, measure the on-hook voltage of the telephone loop.

2.	In general, good telephone circuit design practice dictates that
	no less than 100,000 ohms should be bridged across a telephone
	line in an on-hook state.  That means you can draw a maximum of
	0.5 milliampere of current with _assurance_ of not causing trouble
	with your telephone and not causing telephone company automatic
	line insulation test (ALIT) apparatus to print false trouble
	reports.

	0.5 mA is, by the way, sufficient current to keep a well-designed
	reperatory dialer memory or clock circuit alive.

3.	In general, good telephone circuit design practice dictates that
	no current path to ground should ever exist between tip and/or ring
	of the telephone loop and ground.  From a practical standpoint,
	a value of at least 100,000 ohms leakage resistance can be used.

	The only exception to the above is equipment for use on ground-start
	PBX loops and for test purposes.

	An additional exception to the above involves party line circuits,
	but most telephone company tariffs prohibit the use of ANY apparatus
	on a party line circuit that is not directly obtained from the
	operating telephone company.
	
4.	From a practical standpoint, bridging less than 20,000 ohms across
	a telephone line (i.e., a current flow of more than 2.5 mA) in an
	on-hook state is an immediate invitation to trouble, especially with
	respect to false ring-tripping as described in the referenced
	article. 

5.	In the off-hook state power to operate amplifiers and other circuit
	elements may be obtained from the voltage drop across a series
	resistor (i.e., in series with the talk circuit).  From a practical
	standpoint, any such series resistance must be limited such that
	no less than 30 mA of loop current flows in an off-hook state.
	Taking an example value of a 250 ohm resistor on a 30 mA loop, we
	have 7.5 volts available with a DC power of 225 mW.  225 mW is
	actually a decent amount of energy to play with (in this day and
	age of CMOS circuits) - but you will ONLY have it available when
	the telephone is off-hook.

	If you attempt to obtain a larger voltage drop through a higher
	series resistor, such that the off-hook loop current drops below
	30 mA you run the risk of: (a) having the central office apparatus
	fail to detect an off-hook condition; (b) being unable to trip
	ringing; (c) being unable to send rotary dial pulsing signals to
	the central office; and (d) having insufficent current available
	to operate telephone set DTMF dialing and talk circuits.

6.	The "series resistor" described in (5) above may be effectively
	replaced with solid-state circuit elements, such as a constant
	current regulator set for a minimum of 30 mA.

7.	Any series element for the purpose of obtaining DC power as described
	above should be bridged with a non-polarized capacitor to minimize
	the voice-frequency insertion loss of such element.

	It should be obvious from the above guidelines that any BRIDGED
connection of an LED will draw excessive on-hook current, but that a series
connection (i.e., "in use" indicator) is okay.

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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