Re: Question About Safety Of Voltage Supply


"z" <gzuckier@xxxxxxxxxxxxxx> wrote in message
news:f3356871-04ac-4e0b-a419-67044fe9b9bc@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

46 volts is about what's on the phone line, IIRC, and nobody really
worries about that. (until you're holding the wire and the phone
rings, ring votage being like 90 volts. whoo hee.)

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
http://www.sandman.com/telco.html 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 http://www.sandman.com/ringvoltbul.html 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


.

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