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#### ablyss

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##### Pool Shark
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I was reading up on old phone tech where the ph co. runs there service generally around 50 Volts DC. Then, they will superimpose 90 volts AC @ 20 Hertz to make the phone ring.

What is stopping the two voltages from conflicting with each other?
Aren't the two voltages additive in some way, shape or form?

This is got me all worked up.

#### stuiec

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##### cog
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I was reading up on old phone tech where the ph co. runs there service generally around 50 Volts DC. Then, they will superimpose 90 volts AC @ 20 Hertz to make the phone ring.

What is stopping the two voltages from conflicting with each other?
Aren't the two voltages additive in some way, shape or form?

This is got me all worked up.
Draw it out.

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##### Registered
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2,176 Posts
Because the rest of the system never see's it.

As the diagram shows your phone is normal open till one lifts the hand set.

ablyss

#### ablyss

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##### Pool Shark
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Discussion Starter · ·
Because the rest of the system never see's it.
I think I understand. When the phone is picked up and a dial tone is present that is when the 50 Volts DC is on the line. When the phone is ringing, that is when the 90 Volts AC @ 20Hertz is on the line, and anytime else it has no voltage because the circuit is open?

The article I read used the term superimpose to refer to both AC and DC. I think the term superimposed would be incorrect, since both voltages are not ran together. What do you think?

#### drsparky

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##### Senior Moment
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AC can ride on a DC reference. Instead of AC swinging from 0 volt reference it can swing for any DC voltage and still work fine.

#### wildleg

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##### animal lover /rat bastard
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put your tongue on tip, and connect your right hand to ring.

you will likely feel nothing.

have someone within sight ring the phone. you will taste the AC.

disclaimer: I will not be held responsible for anyone trying the above. Especially pansies.

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I never read up on things of the telephone!
I was only going off what the old physic's teacher said;
"A telephone is NO and is closed when the handset is lifted"

I've frankly never heard of AC on this DC system. Guess it makes sense
per the diagram. DC couldn't pass through a Capacitor can it/could it?
Thus my statement about not seeing the AC power, IE is drained off
with the Cap. and also helps the bell ring.

I'm not ever sure of the use of a triac in the handset but both
AC or sparingly(singulary) for DC can be used by a design.

By no means am I an electronic geek.

#### micromind

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##### Bilge Rat
motors and controls.........
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Because the rest of the system never see's it.

As the diagram shows your phone is normal open till one lifts the hand set.

Note the 2 uf capacitor in series with the bell. This is important when dealing with AC superimposed on DC. A capacitor will block DC current but will allow AC to pass.

In this case, the capacitor will cause the bell to ring when AC is applied, but it won't allow the DC to burn up the coil.

#### ablyss

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##### Pool Shark
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Discussion Starter · ·
Note the 2 uf capacitor in series with the bell. This is important when dealing with AC superimposed on DC. A capacitor will block DC current but will allow AC to pass.

In this case, the capacitor will cause the bell to ring when AC is applied, but it won't allow the DC to burn up the coil.
I think I understand. When AC and DC are applied together on the same wire, the trick is to make sure they don't actually energize at the same time, hence the use of the capacitor.

What do you think would happen if they both energized at the same time? I'm not about to go try this in my garage and have a battery blow up in my face. Been there done that already.

#### wildleg

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##### animal lover /rat bastard
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the phone has dc on it. an ac signal is sent to ring it. the ac and dc are on the same 2 wires. think of waves on the ocean. waves on top of the tide. some times it adds, sometimes it subtracts. in this case it is additive. put a voltmeter on your phone and watch when you ring it.

ablyss

#### LARMGUY

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##### Conservitum Americum
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the phone has dc on it. an ac signal is sent to ring it. the ac and dc are on the same 2 wires. think of waves on the ocean. waves on top of the tide. some times it adds, sometimes it subtracts. in this case it is additive. put a voltmeter on your phone and watch when you ring it.
Better yet, wet your fingers and put them on the 66 block when someone rings in. Pow!

wildleg

#### RFguy

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##### Registered
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Here's a visual of the ring voltage (90 volts at 20 Hz) superimposed on the -48 VDC.

#### ohmontherange

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##### Registered
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Ever wonder why an o-scope has AC, DC, & AC+ DC coupling?

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