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can some one tell what is the voltage (ac and dc both )In a phone line? Answered

i have a DMM but it is of low quality so i assume may be my readings were wrong ...

i found out to be 50V DC (when knob was at DC) and a Vrms of 110V when i turned my DMM's knob to AC ...
even when the phone was not ringing

is my method of finding the voltage right?

can someone check what is the actual voltage in a phone line

part 2
if the voltage is 50v Dc then when i connect a resistor it is lowered to 16V and if the resistor is 240 ohm then it is more lowered to 8V
 what possibly the reason could be ?
and when an led is attached directly to 50V it glows BUT theoretically it should Blow up. any idea whats going on ?



Best Answer 6 years ago

If you measure the voltage using a DMM, then the DMM only draws MICROamps from the phone line. So it does not trip the phoneline to go "OFF-HOOK". If you place a load across the phone line that draws more than a few milliamperes... then a relay or other device at the phone office detects you "lifted the receiver" and it goes "off hook" now you will get a dialtone and voltage goes down as you noticed. When you placed your meter to "AC" and saw 110 volts this could occur because the meter only draws MICROAMPS and presents NO LOAD to the stray ac that may be present on the miles long wire from your house to the phone office. This stray ac will give you a reading on a voltmeter, but it is just a "ghost" of a voltage... that may cause slight hum in your line, but it is not full powered AC 110 volts that would run anything. (has no AMPS behind it). PS.. i worked 10 years in a phone office as a technician troubleshooting these kinds of things.

sure only you got my point right.

now i wanted to know that only DC of the phone can be used
why not the ac we can step up the AC and use it then after rectification etc.


can this GHOST voltage means the stray voltage cause high current flow in spite of having passed through a bridge rectifier ? because i used transistors for interfacing a DTMF output to the telephone line which is connected via a bridge rectifier and 110 v MOV, but the transistor blows up after 2 months of installation giving continuous proper service . WHY?

Hi Framistan... 10 years aye, am a bit of a PHreAk and always got telecommunications questions heepz... can i add you too facebook (if have) on standby for pHrEakiN just askin ???

Stephen Berkeley look for popeye (am a skipper when im not burning my fingers)

Do not try to tap the phone-line for power: it screws the phone system-up and then the Phone company get cross with you.


thanks for the precaution
BUT what actually happens? the line gets damaged or the phone?

The line can behave in a faulty-way* and the phone company send some one out to find out what the problem is.


*because you're connecting something to it that isn't a phone.

dear brother wanted the measured value as framistan said that the ac is ghost voltage and has no link so i would never find that on google

"When the telephone is NOT in use (on hook) the voltage across the two wires (tip and ring) is about 48 volts D.C.

When the telephone IS in use (off hook) the voltage across the tip and ring wires drops to about 6 volts D.C.

When a ringing signal is being sent there is an A.C. voltage "superimposed" on top of the normal D.C. voltage. This "ringing voltage" is nominally about 90 volts at 20 Hertz (cycles) but could be as high as 130 volts and at different frequencies."


6 years ago

Theres sems to be a whole lot of people who refuse to do even basic search with a searchengine before posting questions - i find this a very sad direction :(

But to answer the question you could have found quite easilly: http://wiki.answers.com/Q/What_is_the_voltage_of_telephone_line

if it is only 50v dc then what is the 110 thing my DMM is reading?

That I can not answer. i saw a some info on the ringing signal being AC but if that is related i do not know. it could also "just" be a bad DMM - I am not really an expert on electricity so anything from here on would be uneducaetd guessing. Perhaps someone with more knowledge on the subject can answer