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Old 26th Feb 2007, 7:50 pm   #6
AndiiT
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Join Date: Aug 2005
Location: Saltburn-East, Cleveland, UK.
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Default Re: Pulse (Loop Disconnect) to Tone (DTMF) Converter?

Quote:
Originally Posted by beery View Post
I cheated by not including a bridge rectifier which has made the phone polarity sensitive, but that is ok as line reversal is not used these days.
Andy
Hi,
Whilst line reversal isn't used for metering purposes (like it used to be) I have seen line reversal on some switches (exchanges) on an incoming call, also what about protection of the semiconductors (unless I have missed something in the circuit) during the AC ringing cycle?
In some cases ringing current is generated by chopping (and reversing polarity at 25Hz) the DC line voltage.

Additionally although line polarity is supposed to be maintained some faults/linesmen aren't too fussy about this so you cannot guarantee that the 'B' Leg will always be the -ve one.

Personally I would always fit a bridge rectifier in any device which is to be powered from a POTS or (analogue) PBX line

Regards
Andrew
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