Jim's Not-To-Be-Patented or Certified or Type-Accepted 'Type I' Modem Condom

I will not over-justify the technical limitations or argue techniques over this circuit. There are slightly more advanced variations applicable to those who can or want to figure out what's really going on here. Another $3-5 worth of parts and an earth ground connection will make it better - but who can find or want to run a GROUND wire on the 15th floor of some hotel?

Here goes - bear with the ASCII representations here - I'll do a properly drawn GIF or JPEG someday - but this is not bad for something I pulled out of my head one day to help a friend who was having more than his share of on-line connection and noise problems. (Yes, it is good for 14.4, 28.8, 33.6, etc. - slightly modified it may also be OK for ISDN.)

R in the drawing represents a 47 to 68 ohm 1/2 watt resistor. C in the drawing represents a .01 ufd, 400-1000 volt capacitor (Mylar or ceramic) M in the drawing represents a 130 volt large or small Metal Oxide Varistor

The unit is symetric, such that it does not matter which end/side of the Red/Green wire pairing goes to the wall jack or to the modem.

For ISDN circuits I reduce the C value to .001 ufd. For the added protection (a Type 2 unit) to provide enhanced relief, add a capacitor and an MOV (2 more each) from each of the junctions of the C,M or Rs and tie the other ends to a common point that will wire to Ground. For a Type 3 unit I use little RF chokes instead of resistors.)

This is all neatly built into a surface mount RJ-11 jack with 6-wire screw terminals inside, removing the Blue, White, Yellow and Black wires, and adding a short jumper wire with an RJ-11 plug at the end for the wall-socket connection.

This little gem is not certifiable or technically acceptable for application to the public telephone network - if only because it exceeds certain technical ratings established by the phone companies LONG ago. Still, it is a reasonable device to add to any office or portable modem setup to protect you against the mysteries of those unknown phone jacks.

The basic premise is that NO phone line should ever impart more than 130 volts (maximum phone ringing energy) to the device attached to the line, thus anything over 130 volts must be considered waste or harmful. The MOV's limit the voltage on the line to 130 volts. The resistors help soften the limiting affect, and. working with the capacitor(s) reduce the amount of non-essential static and line noise (above the frequency range the modem needs to 'hear' and send) to reduce noise related data errors. An added benefit, especially with the added caps to ground, is that annoying "CB radio" garble interference is often removed with this unit as well.

That's all folks!