How Measure Telephone line voltage with mcu

[plouf]

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


thanx

 

[betwixt]

Connect a bridge rectifier across the line (because you can never be sure of it's polarity), connect it's negative side to VSS and its positive side to a resistive divider. Measure the voltage at the output of the divider then multiply the result by the divider ratio to discover what went in to it. Finally, because some voltage is lost in the rectifier, add 2 x Vf of the bridge diodes to the result.

Brian.

 

[crutschow]

If your circuit has a separate power supply that is not connected to earth ground then you don't need isolation to measure the phone voltage. But, if your circuit can't measure bipolar voltages, then you may have to put an op-amp full-wave ideal rectifier in from of it, unless you determine the line polarity before you connect the circuit.

You also may need to add a voltage clamp so that the high ring voltage (whatever that is in your country) doesn't damage your circuit.

 

[chuckey]

Your circuit would do, you could increase the 47K. What you are finding is that the telephone volts come from behind a relay coil. In your case it drops 3V at 1mA, so it must be about 3K.
Frank

 

[c_mitra]

You also may need to add a voltage clamp so that the high ring voltage (whatever that is in your country) doesn't damage your circuit.

It is useful to remember that ring voltages can be 48V or even higher in some cases. Better be ready for a 100V as a conservative value.

 

[betwixt]

In the UK, the ringing voltage is superimposed on the line voltage so the peak can be >125V (~75 on top of ~48), better to rate everything at 200V or more. There is also a line voltage reversal just before the first ringing burst if the consumer has caller ID enabled so if a rectifier is used, it should be capable of resisting twice the peak voltage. Most UK phones I've seen use 1N4006 rectifiers.


Brian.

 

[andre_luis]

In the UK, the ringing voltage is superimposed on the line voltage so the peak can be >125V (~75 on top of ~48), better to rate everything at 200V or more

I’m unaware about other countries, but here where I live the telephony lines are always affected by electromagnetic discharges of atmosphere, particularly during summer storms. In matter of protection circuitry I would even consider additional components to protect against higher voltages.

 

[plouf]

thanx all

will most probably go with a high impedance i.e 6MOhm and opamp gain, since seems to be simplest (grrr)

ring voltage is as high as 100-120VAC correct
and storm may affect circuit but not so important (eill replace it )

 

[KlausST]

Hi,

in my region the telephon lines mostely are underground.

***
What exactely do you want to measure.
* The idle DC voltage
* The active DC voltage
* the ringing AC voltage
* the transmitted voice signal.

What do you want to achieve?


***
Please try to answer in values rather than textual description:

What measurement speed/time are you thinking of?
What precision/resolution do you want?

Klaus

 

[plouf]

HI

minimun requirements

wanna know if line active or not (i.e. not been cutted) i.e 6 to 120volts
plus if its active i.e. about 6-10 volts

speed can as slow as a few seconts !

 

[KlausST]

Hi,

seems to be possible.

**
some calculations:
120V AC is +/-170V. means a range of 340V.
With some headroom ... maybe 400V.

If you do it with an 10 bit ADC, then
* you get a resolution of about 0.4V.
* Easy to detect DC 6..120V
* and ... possible to measure 120V AC.
* it will be preicse, flexible and fast.
* but it will need relatively high effort in software.

If you are experienced in using ADC and interrupts, then I´d go for it.

Klaus

 

[plouf]

hi

adc and mcu software is not my problem-question

question is how to interface, secure, telephone interface to my circuit without affect telephone line and/or other devices f.e modem etc

 

[c_mitra]

Why don't you try a transformer coupling? I do not know much about how to secure...

 

[plouf]

Why don't you try a transformer coupling? I do not know much about how to secure...

transformer proides 0 output at DC current's so will only provide output when talking (voice) at offline state it will 0 out

 

[betwixt]

A telephone does not care which way around it is connected, the line going in to it connects to a ring voltage detector (capacitor --> back to back Zeners --> ringer) and a bridge rectifier to provide supply and a current to modulate. If you wire a bridge rectifier in the phone line it will still work normally but you can be sure of it's polarity.

To measure the voltage, use the MCUs ADC and scale the line voltage with a potential divider so ~200V gives FSD, this will give you best voltage accuracy. To see if the line is in use, wire an LED in series with it and check if it is lit up. A normal POTS phone can draw up to about 100mA so use a suitably rated LED or an opto-coupler with a shunt resistor across it. Most phones appear as a constant current load across the line so adding a fixed LED voltage drop in the line will make little difference to its performance.

I made a portable line tester for a telecoms company a few years ago, it had a three digit LED display which showed the line voltage, line current and how many mS it took for line current to settle when the phone was taken off hook. It just plugged in series between the phone and it's wall socket. It used a series resistor in one leg of the line and monitored the voltage across it to read the line current. The voltage was measured using a potential divider as I described. The timing was a software measurement, starting when the line current exceeded about 1mA and stopping when it reached 50mA. It could also read pulse dialed digits but it had no decoder for checking DTMF tones. I used a PIC16C71 but there are better alternatives available now.

Brian.

 

[Audioguru]

You want to measure the voltage on a telephone line to see if it has been cut? If it is not cut then it has a DC voltage and the capacitance of the long wires holds the voltage if it is cut. Its capacitance is very high so you do not want to measure its voltage with a 6M ohms circuit that will take a long time to discharge the capacitance of the wires. You also do not want to "load down" the voltage which might cause the line to go off hook or trigger a fault alarm. It is illegal to "steal current" from a telephone line which your voltmeter circuit will be doing.

 

[plouf]

You want to measure the voltage on a telephone line to see if it has been cut? If it is not cut then it has a DC voltage and the capacitance of the long wires holds the voltage if it is cut. Its capacitance is very high so you do not want to measure its voltage with a 6M ohms circuit that will take a long time to discharge the capacitance of the wires. You also do not want to "load down" the voltage which might cause the line to go off hook or trigger a fault alarm. It is illegal to "steal current" from a telephone line which your voltmeter circuit will be doing.

at 6mohm it will draw around 8uAmp's i thought it would be low enough to be safe

if not what is your suggestion to measure voltage ?

 

[KlausST]

Hi,

In my eyes measuring voltage gives good information.
Current measurement also gives good information.
A combination of both voltage and current measurement maybe is the best.

I'd say a voltage measurement with high ohmic voltage divider is a good point to start.
Maybe one needs to optimize on resistance.

You want galvanic isolation for the measuremen tool. I don't see that this is necessary.

***
You don't want to talk about ADC.
On one side there are the two telephone lines, on the other side any device that can detect the voltage (optocoupler, bjt, fet, ADC)
But inbetween I see only two resistors for each telephone line.

I'm not sure... is your question answered?

Klaus

 

[plouf]

the idea of galvanic isolation only said since i have doubt of how much it will affect telephone circuit, my circuit that it may (or may not) powered from a device with constant ground as minus
as i understand most "olders" say it is not a problem , therefore i will leave it as is !

i dont speak about ADC because its simple, i will insert a a divider betwer i.e. 4Moh-100kohm-100kohm-4MOhm, amplify out with a opamp (maybe an instrumentation opamp) and measure it with some MCU with internal ADC,high degree of accurancy it not needed so


i guess its answered, unless someone has an idea of true isolation (opto isolation , transformer or whatever)

 

[KlausST]

Hi,

Galvanic isolation or not - you need some device to detect voltage.
Galvanic isolation or not does notnecessarily influence resistor values.

You say your mcu has an independent power supply. Because it is "independent" there is no need for galvanic isolation.

Why amplifier, opamp, instrumentation opamp? You make it difficult instead of simple.
Now you talk about mcu inside ADC. Just to clarify: I didn't mean an external ADC. You may use the internal ADC....but it doesn't change anything (wether external or internal).

I'd say a transformer won't work, because you want to measure DC voltages.

Klaus