Question About Safety Of Voltage Supply

The phone line current is limited to something like 20-80 mA, which may or may not be below the level of serious danger. See

for devices that claim to use the telco power. The maximum power you can draw is probably at the point where the line voltage drops to half (about 24 VDC), at which line current would probably be about half (about 40 mA maximum), so power would be just about

1 watt. There will be a point at which the telco senses an off-hook condition, and will send voice and tone warnings on the line.

The ring voltage is about 20 Hz, which IIRC is not as likely to cause fibrillation, and it is applied and removed every couple of seconds. It is also current limited to about 50 mA. It is based on REN (Ringer Equivalence Number), which is 8.75 mA, and the phone company generally supplies 5 REN, or 44 mA. This works out to 2.6 watts. But it is duty cycle limited to an average of about 1 watt.

See

for some interesting information. It shows 84 VRMS ring voltage at 1.4 REN. The threshold for ringing is about 60 VAC, but can be much lower for modern phones. These numbers were based on the old Bell type 2500 with electromechanical bells.

Any voltage source should be treated with some respect and knowledge of possible hazards, and one should take any reasonable precautions.

Paul

lco.htmlfor devices that claim to use the telco

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off topic, but what with april fools day coming up: when you're on the phone with somebody, suddenly say 'hey, what are these two wires coming out of the phone? AAAAAAHHHHHHHHHHHH!!!!!"