POTS - Telephone Ring Voltage - Page 1
Forum section: electronics / beginners

author: synthetic
author_rank: Contributor
timestamp: March 02, 2016, 09:12:29 pm
content: 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 acsuperimposedon 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.

author: bktemp
author_rank: Super Contributor
timestamp: March 02, 2016, 09:16:39 pm
content: The 90Vac gets added to the 48Vdc, so you get over 100V peak. So you should use transistors rated for at least 200Vdc.

author: synthetic
author_rank: Contributor
timestamp: March 02, 2016, 09:52:51 pm
content: Assuming that I get high enough rated components will this circuit work?

author: danadak
author_rank: Super Contributor
timestamp: March 02, 2016, 09:56:32 pm
content: These guys in US have a large knowledge base, ap notes.http://www.cermetek.com/Catalog/Telephone-Line-Interface/Regards, Dana.

author: synthetic
author_rank: Contributor
timestamp: March 02, 2016, 10:26:36 pm
content: 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.

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2016, 10:33:40 pm
content: Quote from: synthetic on March 02, 2016, 10:26:36 pmYou 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?

author: Audioguru
author_rank: Super Contributor
timestamp: March 02, 2016, 10:41:22 pm
content: 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.

author: danadak
author_rank: Super Contributor
timestamp: March 02, 2016, 10:47:32 pm
content: The idea of the DAA manufacturer was not to design in DAAs, rather to use theirknowledge in their ap notes of the POTs environment.Regards, Dana.

author: synthetic
author_rank: Contributor
timestamp: March 02, 2016, 11:03:00 pm
content: Quote from: IanB on March 02, 2016, 10:33:40 pmIf 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.Quote from: Audioguru on March 02, 2016, 10:41:22 pmOops!

author: danadak
author_rank: Super Contributor
timestamp: March 02, 2016, 11:35:15 pm
content: 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.pdfRegards, Dana.

author: IanB
author_rank: Super Contributor
timestamp: March 02, 2016, 11:55:06 pm
content: Quote from: synthetic on March 02, 2016, 11:03:00 pmElectrical circuits always consist of one or more closed loops (what is why they are calledcircuits). 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.