Need help with DC Blocking capacitor value.

[davepusey]

I'm currently building a Caller ID interface for my PC, so that I can log/announce incomming calls on the network.

To provide isolation from the telephone line I am using a 600ohm 1:1 transformer. The datasheet provides a recommended circuit...

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I have built this part of the circuit and it works well, but the problem is the capacitor (highlighted in green) is such a large value that I cannot obtain from anywhere. The only thing I could make it were three old 2.2uF capacitors in parallel, but these are quite bulky.

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I'm hoping someone can advise me if I can use a much smaller value capacitor so that I can reduce the bulk and also make it easier for others if I decide to publish my circuit or make kits of it in the future.

I'm also sure i read somewhere that the capacitor will also act as a high-pass filter, with smaller value=higher frequency and vice versa. Is this correct?

 

[canadaelk]

If your network is connected to phoneline all the time tha AC load could give you grief. I have used a 10k:600 Ohm transformer with a 0.1u cap to block DC. E

 

[davepusey]

I'm sorry, but what AC load? An idle line only has 48V DC, no AC.

How did you arrive at the value of 0.1uF?

 

[MikeMl]

Dont forget the 100V 20Hz AC ringing signal. The reactance of the coupling capacitor should be quite high at 20Hz, low at 300 Hz (audio). The voltage rating of the capacitor should be at least 200V.

 

[davepusey]

So how do I calculate the reactance then?

 

[canadaelk]

Why do you need reactance? To calculate that, you need a fair bit of info on the transformer. As Mike pointed out: the impedance (or AC resistance) of the circuit at 20 to 30Hz ringing signal should be high enough to not load it because than your phone may not ring. It should be much less than 1REN (ringer equivalent number (I did not make that up!)). The 90V ringing signal is superimposed on the line voltage, so a 200VAC cap is needed.
If you are interested in reactance: google smith or bell reactance chart.
E

 

[davepusey]

All i want is a capacitor value that will block the dc voltage from reaching the transformer (prevent saturation of the windings) but still pass the voice band (300Hz to 3300Hz).

I know i'll need at least a rating of 200V.

The secondary side of the transformer will have a reversed pair of diodes to limit the max voltage swing, and will then be output as line-level audio.