Hello,

As C3 gets charged over R4, R3 and R5, the voltage over R4 will get less than 0.7 Volts
and the base of transistor T1 (it is an PNP transistor) will get no current anymore.
The relays will fall off at that moment.

Greetings,
Bertus

 

Hello,

C2 is a decoupling capacitor (normaly done afther a "polarity safety" diode).
R4 pulls the base of the transistor T1 high when no signal is present.

Greetings,
Bertus

Any idea how do we determine the value of C2 in this case? wat is the formula to use?

 

Hello,

There is no real formula for C2.
It is there to catch the dip when the relays attracts.

Greetings,
Bertus

 

Hello,

C2 is a decoupling capacitor (normaly done afther a "polarity safety" diode).
R4 pulls the base of the transistor T1 high when no signal is present.

Greetings,
Bertus

Can you tell me what is the reason for choosing such a large resistance value of 100K? Since most of the current from 12V supply will flow thru emitter of
T1 instead.

 

Hello,

You are right that current is flowing through the emitter.
This happens until the voltage over the resistor falls below 0.7 volts, then the transistor is cut the current.
This happens when the capacitor gets full.

Greetings,
Bertus

 

Hello,

You are right that current is flowing through the emitter.
This happens until the voltage over the resistor falls below 0.7 volts, then the transistor is cut the current.
This happens when the capacitor gets full.

Greetings,
Bertus

So i guess the large resistance value of R4 is have a longer time constant so as to for the relay to flap back at a later time.

 

Hello,

Yes, you are right.
As the capacitor gets over 11.2 Volts, the relays will fall off.
This 11.2 volts is calculates this way:
R4 takes about 0.7 Volts. The R2, R3 and R4 are in series,
so the voltage until the capacitor is ((R2 + R3 + R4) / R4) * 0.7 = (116 / 100) * 0.7 = 0.812. 12 - 0.812 = 11.2 (rounded).

Greetings,
Bertus

 

Hello,

This is what happens at a ringpulse :
The ringpulse will let the LED of the 4n27 blink.
The transistor if the 4n27 will conduct.
When R5 and C3 are connected, C3 will be discharged over R3 and the transistor of the 4n27.
When there is no ringpulse the C3 will be charged by R3,R4 and R5 (the timeconstant is much higher).
At the next ringpulse C3 is discharged again and so on.
When no ringpulses are coming anymore, C3 will be fully charged and the relays will fall off afther some time.

Greetings,
Bertus

Can I confirm again C3 is discharge through R3 and R5 and transistor of 4N27?Reason I ask is becos you did not mention R5 in yur previous reply.

 

Hello,

Here is the circuit again.

C3 is charged by R4,R3 and R5.
C3 is discharged by R5 and the transistor of the 4N27.
R3 protects the transistor in the 4N27 and T1 against a high current.

Greetings,
Bertus

 

If the resistor were any smaller, the phone company would detect the leakage on the line and come investigate it. Happened to me once -the phone guy came out and clipped the wire that ran from the wall to my "invention."

 

Hello,

The capacitor is a resistor for AC without the dissipation.
The resistor will dissipate heat if the value is much lower.
The resistor used across the capacitor is called "bleeder resistor" and is placed there for the safety.

Here is the complete schematic :

Greetings,
Bertus

Hi,
Does anyone knows how much is the AC ringing current in this case from the telephone line when the signal comes in????

 

No - the spec is for voltage, 90VAC, 20 Hz - http://en.wikipedia.org/wiki/Ringtone.

 

No - the spec is for voltage, 90VAC, 20 Hz - http://en.wikipedia.org/wiki/Ringtone.

If i do not have a telephone line in the lab and would like to measure the actual voltage and freq and reflect using the osciloscope what should i use to subsitute in this case?? Function gen to for freq how about the 90Vac? I don't think i can use the stnadard DC power supply?