POTS - Telephone Ring Voltage

[synthetic]

Hi,

I want to use a circuit like this as a hook-switch for a phone system (not connected to actual phone lines, just in the building)



Wikipedia says that the ring voltage can be up to 90V ac superimposed on the -48V dc.

My question is if that 90V ac is centered or based on the -48V dc?

If the voltage is alternating between peaks of  +42V (-48V + 90V) and troughs of -48V then I can use regular 2n2222s. If its between -93V (-48V - 45V) and -3V (-48V + 45V) Then I cannot use 2n2222s as they have a collector emitter maximum voltage of 60V.

[bktemp]

The 90Vac gets added to the 48Vdc, so you get over 100V peak. So you should use transistors rated for at least 200Vdc.

[synthetic]

Assuming that I get high enough rated components will this circuit work?

[danadak]

These guys in US have a large knowledge base, ap notes.

http://www.cermetek.com/Catalog/Telephone-Line-Interface/

Regards, Dana.

[synthetic]

Thanks for the resource, but I don't intend to use a DAA, I am just wondering if the circuit i suggested will be able to switch an arbitrary AC signal on and off.

[IanB]

Thanks for the resource, but I don't intend to use a DAA, I am just wondering if the circuit i suggested will be able to switch an arbitrary AC signal on and off.

You have connected R1 to the bases of the transistors. Now if you follow the current flow, where is the closed current loop between the transistor base terminals and back to the transistors? In other words, at which other terminals of the transistors is the base drive current coupled?

[Audioguru]

The polarity of the diodes is backwards so the transistors will never conduct normally. Instead the transistors will have emitter-base avalanche breakdown when they become reverse-biased at about 6V to 8V.

[danadak]

The idea of the DAA manufacturer was not to design in DAAs, rather to use their
knowledge in their ap notes of the POTs environment.


Regards, Dana.

[synthetic]

You have connected R1 to the bases of the transistors. Now if you follow the current flow, where is the closed current loop between the transistor base terminals and back to the transistors? In other words, at which other terminals of the transistors is the base drive current coupled?

If I used MOSFETs instead would that problem be fixed? If I understand correctly mosfets don't have a current path from the base to the emitter.

The polarity of the diodes is backwards so the transistors will never conduct normally. Instead the transistors will have emitter-base avalanche breakdown when they become reverse-biased at about 6V to 8V.

Oops!

[danadak]

MOSFETS have a Drain and Source, principal current carrying / switch connections,
and  a Gate to turn on/off the MOSFET.

http://www.irf.com/technical-info/appnotes/an-1084.pdf

Regards, Dana.

[IanB]

If I used MOSFETs instead would that problem be fixed? If I understand correctly mosfets don't have a current path from the base to the emitter.

Electrical circuits always consist of one or more closed loops (what is why they are called circuits). If you don't have a closed loop you don't have a circuit and then it will not function. A MOSFET does not relieve you of this burden, even if the gate is nominally insulated from the drain and source.