How Measure Telephone line voltage with mcu

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Hello

 

i want to measure telephone line voltage without affecting line

with a mcu

 

mcu will have independent power supply (obviously) therefore isolation is required

Ii was thinking about use of optocoupler's i tried a moc4020 with a 47k in series but drops voltage of line about 3 volts (from 48 to 45)

therefore i guess its not "best idea" !

 

does anoyne have an idea and/or link to some manual on this topic?

 

thanx

 

 

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It's more complicated than most people would think. Some details can be found here: http://www.epanorama.net/circuit...

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Hello,

 

I've done this years ago without isolation (for a private line) but with very high impedance, so the line was not affected by the measure.

If isolation is mandatory, you may use a DC/DC for powering the measure circuit, including ADC, and optocouplers to send digital signals.

Schematic

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Remember that there may be legal and/or regulatory prohibitions on connecting "unapproved" equipment to the public telephone system.

 

Remember also that there is a direct, dedicated piece of wire from the exchange to your house - so, if you do cause any problems, they can quickly trace exactly who did it ...

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Also be aware that there are many national variations - so be very careful to check whether anything you find on the internet is actually applicable to your local systems ...

 

It's all summed-up by BORSCHThttps://en.wikipedia.org/wiki/BO...

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Note, in the US it's -48 volts dc, but the ring voltages are AC and IIRC about 90volts or so.

 

 

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RMS. And the frequencies are 16.6 hertz to 66 hertz. (Most common is 20 Hz)

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thanks all for your reply

 

i will most probably go with proton88 suggestion, since seems everything else to more complicated and its just a simple project :

 

@anweil & ki0bk , us and Greece (Europe) has same regulation and voltages only difference is the free line tone (we use german type , tout touuut, tout touutttt), instead of single tone)

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Agreed, a high impedance "tap" is a good approach.

The impedance loading on the line is often measured in REN, Ringer Equivalence Numbers.

An old, black, rotary, dial phone with a big transformer had an REN of 1.

Most lines were allowed up to 3.

Now days most phones have a REN of 0.1 or 0, as they use a Hi Z approach, also.

 

You ought to keep the term DAA in mind when doing your Google searching, that is the data interface for a POTS (Plain Old Telephone, i.e. physical Tip & Ring line).

Lots of circuits out there.

 

Another option would be to look at Caller ID Chips.

Some of these will incorporate the DAA, or at least much of it, on chip, and give you on/off hook status, and CID info.

 

A typical ringer voltage is 90 V, but IIRC one typically designs for 300 V for a phone front end.

You need to protect against lightning strikes near by the long lines, and perhaps to the lines, as well as power lines shorting out to the phone line during a storm, or if a car crash takes out a telephone pole.

The transients can be short duration, but large.

 

In the USA I think the outside of the house typically has two fusable links for the phone lines, but your circuitry will be exposed to a large transient long before those blow.

 

Lastly, a reminder:

DON'T connect your O'scope to the phone line unless you really, really, really know what you are doing.

The POTS float, and your scope doesn't.

That said, a small battery powered scope can be used, then you can see the on/off hook voltage, the ringer signal, the answer pulse, the CID info, etc.

 

And finally, (really, this time), many circuits use the fact that the phone line sits at about -48 V or whatever when off hook, and this might drop to -12 V on hook, (highly variable), but you have to be careful with designing for the variation in voltage, unless it is a one-off project and you can tweak the thresholds for your specific situation.  The POTS is, by design, a current based system, not a voltage based system.  That works better for variable length lines from the local office to all of the customer's sites.

 

Good luck with your project!

 

JC

 

Edit: Typos

Last Edited: Tue. May 3, 2016 - 04:49 PM
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It's an old project, is was about detecting line failures or short circuits (0V), and defective modems that kept connected (from 5 to 15V for a long time). The ring voltage (75Vrms, 50Hz in France) is filtered by the RC input network.

 

As I said, it was for a private telephone network. As JC said, it should be protected, and I think the best is to protect the differential amplifier input with diodes, and add bidirectional suppressor diodes from C1 pins to earth. For a modern design (mine is from 2004 :-) ), I'd use an AVR with amplified differential ADC input directly connected to the divider network, and AVcc connected to VCC/2.

 

Stephane

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DocJC wrote:

Agreed, a high impedance "tap" is a good approach.

The impedance loading on the line is often measured in REN, Ringer Equivalence Numbers.

An old, black, rotary, dial phone with a big transformer had an REN of 1.

Most lines were allowed up to 3.

Now days most phones have a REN of 0.1 or 0, as they use a Hi Z approach, also.

 

You ought to keep the term DAA in mind when doing your Google searching, that is the data interface for a POTS (Plain Old Telephone, i.e. physical Tip & Ring line).

Lots of circuits out there.

 

Another option would be to look at Caller ID Chips.

Some of these will incorporate the DAA, or at least much of it, on chip, and give you on/off hook status, and CID info.

 

A typical ringer voltage is 90 V, but IIRC one typically designs for 300 V for a phone front end.

You need to protect against lightning strikes near by the long lines, and perhaps to the lines, as well as power lines shorting out to the phone line during a storm, or if a car crash takes out a telephone pole.

The transients can be short duration, but large.

 

In the USA I think the outside of the house typically has two fusable links for the phone lines, but your circuitry will be exposed to a large transient long before those blow.

 

Lastly, a reminder:

DON'T connect your O'scope to the phone line unless you really, really, really know what you are doing.

The POTS float, and your scope doesn't.

That said, a small battery powered scope can be used, then you can see the on/off hook voltage, the ringer signal, the answer pulse, the CID info, etc.

 

And finally, (really, this time), many circuits use the fact that the phone line sits at about -48 V or whatever when off hook, and this might drop to -12 V on hook, (highly variable), but you have to be careful with designing for the variation in voltage, unless it is a one-off project and you can tweak the thresholds for your specific situation.  The POTS is, by design, a current based system, not a voltage based system.  That works better for variable length lines from the local office to all of the customer's sites.

 

Good luck with your project!

 

JC

 

Edit: Typos

 

The 90 Vrms is also known as 300 V peak to peak. (I know that they are not equal but I believe we rounded a tad on both numbers.) It has been a while but my only patent was a ringing generator. JC, looks like you might have done a bit of telephony too! :) Ah the good old days, when dirt was still rock!

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It has been a while but my only patent was a ringing generator

Congrats on the patent!

 

  Ah the good old days  (Regarding telephony projects...)

Ah, yes.  When one either got a job offer, or went to jail...

 

JC 

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DocJC wrote:

Ah, yes.  When one either got a job offer, or went to jail...

 

JC 

 

Jay, getting a job here was much, much easier. cheeky Maybe not so today though.

Ross McKenzie ValuSoft Melbourne Australia