Why have a capacitor on a telephone input?

[abdullahseba]

Hi
I am in the middle of designing a telephone interface for a pc. I noticed that most examples have two 100nF capacitors at the input of the audio transformer. This makes sense for a ring detection circuit but would it not block the audio from coming out?
Example:

Also what's the purpose of the zener diodes?
Thanks in advance.

[Seekonk]

The telephone line is at 50V DC, phone will never unhook with a resistive load.  The ringer is at 150V AC, hence you need zeners to limit voltage that will damage external devices.

[madires]

And for the VMWI there could 90V DC.

[abdullahseba]

The telephone line is at 50V DC, phone will never unhook with a resistive load.  The ringer is at 150V AC, hence you need zeners to limit voltage that will damage external devices.

Thanks.
Do you know you would accept a call? For example do you apply a resistor between the A and B wires or a tone?

[SeanB]

Normally to accept a call you would provide a 600R load, made up from the transformer primary winding resistance and impedance, but the transformer must be able to handle the 50mA or so of DC current that will flow through it.  Commonly these days you use a transistor constant current circuit ( and a bridge rectifier so it only has to handle DC, and does not care if the line polarity is reversed for some reason) to pass around 40mA of current, which does the off hook emulation, and the constant current circuit gives a high AC impedance so the audio signals are not going to be attenuated by it, and the capacitors on the transformer primary still provide DC isolation. The constant current circuit however has to handle up to 200VDC on it, as it has to provide the current sink even during a ring cycle, as the system provides from 60-160VAC 25Hz ( for the UK) to ring the bells, riding on top of the -48VDC on hook line voltage.

[Benta]

The capacitors are for DC blocking. Without them, the transformers would saturate.
What your schematic shows is only the audio part.
Ring detection and off-hook signalling are two additional circuits.

[abdullahseba]

Normally to accept a call you would provide a 600R load, made up from the transformer primary winding resistance and impedance, but the transformer must be able to handle the 50mA or so of DC current that will flow through it.  Commonly these days you use a transistor constant current circuit ( and a bridge rectifier so it only has to handle DC, and does not care if the line polarity is reversed for some reason) to pass around 40mA of current, which does the off hook emulation, and the constant current circuit gives a high AC impedance so the audio signals are not going to be attenuated by it, and the capacitors on the transformer primary still provide DC isolation. The constant current circuit however has to handle up to 200VDC on it, as it has to provide the current sink even during a ring cycle, as the system provides from 60-160VAC 25Hz ( for the UK) to ring the bells, riding on top of the -48VDC on hook line voltage.

Would something like this work?

[SeanB]

Not with the triac, but will work with a high voltage optotransistor. There are optoisolators designed for telephone line off hook work, or simply do what has been done before, and use a relay with a 5V coil instead.

[abdullahseba]

Not with the triac, but will work with a high voltage optotransistor. There are optoisolators designed for telephone line off hook work, or simply do what has been done before, and use a relay with a 5V coil instead.

Just out of curiosity, why can't I use a triac? It's used for 240v ac.

[SeanB]

Triac turns on on DC, but will not turn off till the current drops below holding current. Thus on AC it turns off every half cycle once triggered, but on DC it just stays on till you disconnect the phone line. You can run them on DC but then the circuit gets a lot more complex, and a relay will work out both cheaper, more reliable and smaller.

[abdullahseba]

Triac turns on on DC, but will not turn off till the current drops below holding current. Thus on AC it turns off every half cycle once triggered, but on DC it just stays on till you disconnect the phone line. You can run them on DC but then the circuit gets a lot more complex, and a relay will work out both cheaper, more reliable and smaller.

Thanks
So does it act as a hook switch? Example if I wanted to place a call I'd turn on the relay than send the DTMF tones and when I release the relay the call would end?


I've included the full schematic below.
Sorry about the mess, but I'm not fond of diagrams in sections .
https://1drv.ms/b/s!AqGREZOPB-wppHqhXGhMM4ibbB-m