[TheOutcaste]

Originally Posted by JohnWill

I can't imagine anything that sips power from the USB connection can generate enough ringing current to ring an old style mechanical ringer! The total power available through the USB jack if 2.5W, 5V @ 0.5A maximum.

That was my first thought. Having to come up with 40-90 VAC and enough current to ring, than a USB port can provide. The DC loop doesn't take much, 48 V at a minimum 23 mA. But most phones will work just fine on 9-12 volts, it's the current that matters for the DC loop.
The numbers show it's possible:
1 REN is roughly equivalent to a 7000 ohm load, so you get:

Voltage Current    Power
Ring
40 V    5.714 mA   229 mW
50 V    7.142 mA   357 mW
60 V    8.571 mA   514 mW
90 V   12.857 mA  1157 mW
Loop
12 v       23 mA   276 mW
24 V       23 mA   552 mW
48 V       23 mA  1104 mW
USB Supplies:
 5 V      500 mA  2500 mW

Assuming an 80% conversion efficiency, the USB port can provide 2000 mW of power.

So it depends on how they regulate the Ring voltage/current, and what voltage/current your phones actually need to ring. If they will ring at 50 volts, you could get 5 old style phones to work (The full loop current isn't needed until the phone is off hook).

No real reason to use an old phone on this though -- only reason to have one is because they work when the power is out. Magicjack won't, unless you have a UPS for the PC, Modem, and router. In that case, you can plug the phone's power into the UPS as well, or get a cheap line powered electronic phone.

I don't think it will power my TELCO POWERED™ CAR CHARGER though, so I'm going to stick with the regular phone for now

Jerry

[vicks]

You can receive calls even when the computer is off. Just set the call forwarding to your cell phone... or land line.
Works great.
Vicks

[JohnWill]

What is ring signal ?

The telephone company sends a ringing signal which is an AC waveform. Although the common frequency used in the United States is 20 HZ and in Europe is typically 25 Hz, it can be any frequency between 15 and 68 Hz. Most of the world uses frequencies between 20 and 40 Hz. The voltage at the subscribers end depends upon loop length and number of ringers attached to the line; it could be between 40 and 150 Volts. The ringing cadence - the timing of ringing to pause - varies from telephone company to company.

The usual arrangement is to feed the 75 V a.c. ringing current (backed by earth) down one wire of the phone line. On the other wire is placed a slugged relay (or equivalent) which is backed by -48V d.c. When you pick up the phone, the relay operates to the loop d.c. current and trips the ringing current. It also triggers a further device to put the transmission bridge in circuit to enable speech to take place, together with supervision of the calling and called loops. The ralay needs to be a slugged relay to prevent premature ring trip by the a.c. ringing current.

In USA minimum ring voltage supplied is 40Vrms (delivered into a 5 REN load). This is the must detect limit. There is also a minimum must ignore value of 10Vrms. Milage on individual PBX's will vary greatly. But most guarantee to deliver 40Vrms into a 3 to 5 REN load.

When the telephone ring signal is sen to the telephone, the ring voltage is not applied constanly to the line. Typically ring timing is 2 seconds on and 4 seconds off in the US. In the UK ring timing goes .4 sec on, .2 sec off, .4 sec on, 2 sec off then repeats. In toher countries the ring timign cna vary from country to country (even from operator to operator) and you should check the local regulations if you want to get to know the actual ring signal timing in use.

For more information, check Understanding Telephones article by Julian Macassey at http://www.egyed.com/phonework.html and appropriate BellCore documents.

What is REN ?

REN stand for Ringer Equivalen Number. It is a measurment of how mugh ringing power certain telephone equipment takes. REN numbers are used in USA to determine how many telephoen equipments you can connect to same telephone line and still get them ringing properly (typical line can drive about 3-5 REN load).

The definition of 1 REN is the ringer power required by one ringer of an AT&T standard 500 series telephone set in single-party configuration (ringer placed ACROSS the line). One place to find the exact info: get a copy of 47CFR Part 68 - this is the FCC technical specs (and other info) regarding the PSTN (public switched telephone network). This info also may be available from the FCC's web site.