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'[EE]:Adding a DC bias to and AC signal'
2002\02\04@222255 by Steve Kasm

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Hello All,



A prelude to my last post about Ac signals and a problem with detecting
an off hook if I can't get this condition detected without the dc
component of the signal. How can I add the dc component, even if it's
only 12vdc instead of the regular 48vdc the Telco uses so that I can use
it to detect an off hook condition. I'm detecting this condition in a
closed loop off a relay that is not connected to the telephone company
it is a ringing signal that I have created on my circuit that does not
have the dc component, unfortunately this does not give me a way to
detect an off hook. A bridge rectification of my ringing signal gives me
41dc during ringing and 14vd off-hook but the ring cadence would be a
problem as it would provide a false reset signal during ringing silence
between ringing pulses and the dc voltages are too high. The bridge
rectifier also seems to load the ringer circuit down a little too much.
So my next thought is to add enough dc to the loop to detect the off
hook and get a good logic high or low that is independent of the
ringing, I am not sure how to add dc to the ac without blowing the thing
up.



Any ideas?



Steve

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2002\02\04@230555 by David Duffy

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part 1 1760 bytes content-type:text/plain; charset="us-ascii"; format=flowedAt 10:20 PM 04/02/2002 -0500, you wrote:
>Hello All,
>
>A prelude to my last post about Ac signals and a problem with detecting
>an off hook if I can't get this condition detected without the dc
>component of the signal. How can I add the dc component, even if it's
>only 12vdc instead of the regular 48vdc the Telco uses so that I can use
>it to detect an off hook condition. I'm detecting this condition in a
>closed loop off a relay that is not connected to the telephone company
>it is a ringing signal that I have created on my circuit that does not
>have the dc component, unfortunately this does not give me a way to
>detect an off hook. A bridge rectification of my ringing signal gives me
>41dc during ringing and 14vd off-hook but the ring cadence would be a
>problem as it would provide a false reset signal during ringing silence
>between ringing pulses and the dc voltages are too high. The bridge
>rectifier also seems to load the ringer circuit down a little too much.
>So my next thought is to add enough dc to the loop to detect the off
>hook and get a good logic high or low that is independent of the
>ringing, I am not sure how to add dc to the ac without blowing the thing
>up.

Are you using a transformer to step up the ring voltage? If so, connect
the secondary winding "on top" of the 12V rail so that you have 12Vdc
with the ring voltage riding on top of it. The phone hangs between the
free end of the tx sec and a 1K resistor that goes back to 0V. The
junction of the phone/1K goes via a 10K to a 10u bipolar capacitor
which feeds the base of an NPN transistor via a signal diode...
Oh crud, this is too hard to explain... Here's a quick drawing !  :-)
Regards...


part 2 3535 bytes content-type:image/gif; name="ringer.gif"; (decode)


part 3 136 bytes
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