Telephone Question

It's probably powered by the phone line, which is 6-8 volts off-hook. Is that a problem?

Joe Gwinn

That is correct.

Technically, it's powered using the loop current that flows when the 'phone is "off-hook".

When the 'phone is "on-hook" it draws no power, though I've seen hacks that drew hundreds of micro-amps when "on-hook", and they got away with it because of the TELCO allowed-leakage spec. ...Jim Thompson

Could be much higher, upto 48V DC. Line distance (resistance) could drop this somewhat.

Yep. I found that out as a kid when a telephone man turned up to find the cause of a leak - I had a high-impedance relay (26 kohms) across the phone line so I could tell if the phone was in use while I was in the basement. On the basement wall, on a piece of plywood, was a working telephone consisting of the innards of a 1920s or 1930s telephone, the kind with a hybrid transformer whose core is a bundle of parallel black iron wire strands. The phone man took one look, understood, explained the leakage issue to me, and drove away. The leak never came back.

Joe Gwinn

The 48 volts is the on-hook voltage. In addition to line resistance there is a current-limiting resistor, line relay, and so on, so the voltage is either 48 volts, or less than 12 volts or so.

Joe Gwinn

I've seen 56V here in Arizona.

Though, as of two days ago, I'm rid of the high cost land-line... Ooma for the two lines in the house, plus cellphones.

I haven't measured what Ooma puts out for on-hook... but their ring generator is easily handling 6 phones. ...Jim Thompson