Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!husc6!cs.utexas.edu!uunet!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Ringing telephones with line voltage
Summary: I suppose anything will ring if you hit it hard enough... :-)
Keywords: bandpass filters: are they there?
Message-ID: <3144@kitty.UUCP>
Date: 5 May 89 03:10:46 GMT
References: <636@serene.UUCP> <920@snjsn1.SJ.ATE.SLB.COM> <3139@kitty.UUCP> <5012@charon.unm.edu>
Organization: Recognition Research Corp., Clarence, NY
Lines: 47

In article <5012@charon.unm.edu>, ee5391aa@hydra.unm.edu (Duke McMullan n5gax) writes:
> >	Sorry, but you canNOT ring a phone with "house current".  Virtually
> >all conventional telephone ringers are frequency selective, and operate only
> >between 16 and 32 Hz; 60 Hz won't cut it.  Some ringers used on older
> >party line systems (like the "harmonic", "decimonic" and "synchronic") were
> >_very_ freqeuncy selective, with a "bandpass" of 5 Hz or less.
> 
> Wellll -- I do it on occasion. I generally pull the voltage down to about
> 90 VAC with a variable autotransformer for safety, but the 'phone rings loudly
> and continuously.

	Well, I suppose anything is possible if you drive the ringer hard
enough.  Ringing voltage as provided from central office apparatus is well
limited as to current.  For many years the method of current limiting was
a resistance lamp; this method was also used in earlier ESS systems.  Newer
apparatus uses solid-state current limiting circuits, although resistance
lamps are still used in some apparatus because they are so _simple_ and
reliable. 

	A WECO 13G resistance lamp was typically used for current limiting
on ringing circuits and has a current limit of around 400 mA.  At a typical
20 Hz ringing voltage of 100 VAC, a 500 ohm subscriber loop would of itself
limit ringing current to 200 mA.  In addition, the central apparatus trunk
circuit introduces further resistance from the ring-trip relay or series
resistor used for electronic current sensing.

	Ringing current is further limited by a series capacitor in the
telephone set itself to 50 mA or less.

	A typical mechanical ringer is well intentionally designed for
frequency selectivity though the use of a careful core design, a permanent
magnet for magnetic bias, and an adjustable spring for mechanical bias.
I don't doubt that it will ring to 60 Hz if you drive it hard enough to
overcome the magnetic and mechanical bias elements.

	Under the circumstances, I bet you were either: (1) driving the
ringer at current levels >> greater than 50 mA, and/or (2) driving the
ringer without the usual series capacitor.
 
> If you close the hookswitch, you get a loud buzz. ;^)

	See any smoke? :-)

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