telephone related question

[walid1]

any modern telephone has 4 diodes at its input i knowthat these diodes are for polarity protection but voice signals into or out of telephone are pass through these diodes how these signals are affected  are they rectified, i know that they are not rectified  are they carried over a DC level that enable them to fully pass, please i need an explenation about this point, thanks.

 

[audioguru2]

Hi Walid,
Two of the diodes are forward-biased all the time that the phone is active, so rectification of the audio doesn't happen. The audio passes through these diodes like they are a low-value resistor.

 

[walid1]

Hi Mr. audioguru, thank you very much for your response, I agree with you, there are two of the diodes are forward-biased all the time and I'm sure that rectification of the audio doesn't happen, but how??? Let us examine what I think (it may be wrong!). Look at fig.1 if we put a sinusoidal sig of 1V peak between T & R (Caller audio), and if we assume that there is no 8.3 VDC between T & R, then I expect to receive a rectified sig of 0.4V peak between P1 & P2. BUT since there is 8.3 VDC D1 & D4 are already conduct so a full sinusoidal 1V peak appear between P1 & P2 "I think this is true, please comment".

View attachment 37188

 

[audioguru2]

Hi Walid,
For a phone to be detected as being off-hook, and to remain off-hook, it must draw a certain amount of current DC continuously. When a phone receives or transmits audio, the audio modulates the off-hook current only a little, so that the minimum amount of current flows and the diodes are forward biased continuously. With the diodes forward biased all the time, rectification of the audio doesn't happen. ;D

 

[walid1]

Hi Mr. audioguru, I'm coming back, I know I annoying you with my foolish
questions but i expect patient, so lets go back to our discussion:
Believe me i agree with all your saying, but your answers are not pour
directly at what i ask, please tell me directly if true or false and tell
me where i'm true and where I'm false, ==>
please look at Fig.2, if a certain amount of DC current continuously pass
through the diodes in the direction from line to telephone (not vice versa)
you will agree with me that only D1 & D4 are forward biased continuously, OK.
Now if we put a signal like that in Fig.2 in the location shown, tell me
exactly through any diodes it will pass and how.
sorry for my rambling and hope you can understand this with my bad grammar
Thanx a million for the information....
NOTE: I want to tell you that it takes more than an hour to prepare this
small letter for sending.

View attachment 37199

 

[audioguru2]

Hi Walid,
Your sketch clearly shows that the phone's current continues, forward-biasing D1 and D4 continuously, but the current is modulated a little by the audio. The other two diodes don't do anything.

The full-wave bridge rectifier is used only when electronics in the phone needs a certain polarity, even if the phone or its line is connected backwards then the other two diodes would be forward-biased and pass the phone's current with the same polarity. ;D

 

[walid1]

From all of this discussion with Mr. audioguru about telephone diodes I can tell others that in general any diode in conduction state(forward bias) acts as a very small resistance and it can pass a small signal in two way direction, in other word look you can look at this diode as a small segment copper wire. isn't true Mr. audioguru.
thank you very much for reaching this big result....

 

[audioguru2]

Hi Walid,
Thanks, I'm glad to help you understand. ;D