campbell@.ai.mit.edu (02/10/91)
I am working on a modem circuit and I'd like to power part of it off the phone line itself. Because of the nature of the phone line, I figured that the most efficient, reliable +/- 5 volt power supply would be using a charge pumped device (DC-DC charge pumped devices, there are lots of these listed at +/-48 volts in the catalogs I have). 1. Will this work? I'm not entirely sure so the only way to tell is to experiment. 2. Is this really the best method to go about it? Is there some other way? 3. (The part I really wonder about) Will the CPD introduce noise on the line if I run it in parallel with everything else, so that my original intention of powering the tone generators/readers by the phone line will fail due to noise?
jon_sree@world.std.com (Jon Sreekanth) (02/12/91)
In article <13284@life.ai.mit.edu> campbell@.ai.mit.edu writes:
charge pumped device (DC-DC charge pumped devices, there are lots of these
listed at +/-48 volts in the catalogs I have).
I'm wondering if you intend to draw power from the telephone line when
it's on-hook (meaning, not being used). The on-hook DC resistance of
any device you connect to the telephone line should be more than
5 Meg ohms. 5M gives you a ringer equivalence number of 5, which is the
maximum permissible, meaning you cannot have any other device on the
line. (REN measures how much DC and AC load you impose upon the telephone
line. Common phones have a REN of 1.0. FCC limits you to a maximum of
5.0 when you add up all the devices connected to that line. )
1. Will this work? I'm not entirely sure so the only way to tell is to
experiment.
I've seen ads for laptop modems that are line powered, so it's doable.
After you've picked up the line, you can draw a fair amount of DC
power from the line : usually there's 20 - 25 mA at about 8V.
Regards,
/ Jon Sreekanth
Assabet Valley Microsystems Fax and PC products
346 Lincoln St #722, Marlboro, MA 01752 508-562-0722
jon_sree@world.std.com