Landline AC and DC Voltages

Thread Starter

maudbenoit

Joined Aug 31, 2022
4
I did not know that the landline carries AC superimposed on the DC V. The only reason I put a multimeter on it al all is that my landline works sporadically. Usually when it is not working the ringer shorts out so quickly it can't be answered. I measured ~ 47V DC and 103.4 V AC. Reading this forum tells me that 90V AC is delivered when the phone rings. I can measure 103V 24/7. Why does this happening and is it the cause of my service problem?
 

Audioguru again

Joined Oct 21, 2019
5,148
We do not know the specs or country of your phone system.
Here in North America the DC when on-hook is about 51VDC. Off-hook draws some current which is sensed and produces a dial tone or it answers ringing.
Ringing is 90VAC at 20Hz on the 51VDC.

If your phone is continuously ringing then your telephone service provider should fix it.
 

Thread Starter

maudbenoit

Joined Aug 31, 2022
4
Edit: I called the number with the meter attached. The VAC ranged from 60 to 106 VAC and averaged about 80-90 as the phone rang. When the answering machine picked up the VAC was 15V.
 

Audioguru again

Joined Oct 21, 2019
5,148
Your meter probably cannot accurately measure the level of 20Hz ringing.
When your answering machine "picks up" Then the telephone line should have no audio 300Hz to 3kHz AC on it. It might be picking up 60Hz electricity interference which might be from your meter unbalancing the telephone line.
 

DickCappels

Joined Aug 21, 2008
9,289
In North America the DC level on the line drops from a nominal 48 VDC to around 15 to 20VDC because of the IR drop across the telephone office equipment.

Your voltmeter might be measuring a function of DC + AC. Try measuring with a capacitor in series with the meter. When the line is idle there should be practically no AC at or below audio frequencies. In some systems phantom lines are /were made using VLF sub-carriers (very old information) so your meter might be seeing that through they are too high a frequency to affect off-hook status, ring the bell, or be heard.

(some text omitted for clarity) Usually when it is not working the ringer shorts out so quickly it can't be answered.
It is possible that your device (answering machine or telephone for example) is malfunctioning.

I made a transistor circuit that did that in the 1960's. One day I came home to find that a telephone company employee had cut the wires I had connected to the phone line off at the wall. End of experiment.
 

Thread Starter

maudbenoit

Joined Aug 31, 2022
4
I figured it out. Although the ringer shorting out is an ongoing issue it has nothing to do with what I have observed. I've suspected the multimeter itself is involved. This is a cheap non rms meter. The AC measuring circuit must be a half wave diode rectifier and an averaging filter. I found a post which questioned a 12V DC output giving a 26VAC reading. This is the same ratio observed with a 47 DC and 103 VAC reading.
 

sghioto

Joined Dec 31, 2017
3,397
The AC measuring circuit must be a half wave diode rectifier and an averaging filter.
If that's the case why would the meter read almost double on AC? Not saying the meter isn't at fault but shouldn't the AC reading be the same as the DC minus the diode drop.
Can you provide more detail about that post you are referring to?
 
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