On Thu, 10 Sep 1998 20:27:09 -0500 David VanHorn writes: [Paul Webster wrote] >> Aw come on, do a *decent* job! For phone circuits, use a >regulator, >>e.g. 78L08 (as well as a couple of electrolytics). Not a good idea, phones work on current rather than voltage. Some won't like having 8V forced into them. Others won't work on only 8V. The simulated "phone company" you supply should look like a 50 V source with about 1.2K ohm DC resistance, and 600 ohm AC impedance. It can be difficult to supply the full 50 V, so another workable model is a constant current source of 25 mA or so with a compliance to 20 V. >> You've got me there! The only diodes a modem would normally have >are >>those in the ring detector, and those are DC isolated by a capacitor >>anyway. You are talking of line-*powered* (PCMCIA) modems? > > >We did line voltage sensing, so we had a bridge on the input. Some >also use >optoisolated interfaces (no transformer) and they also require loop >current >to run. You gotta light the LEDs with something. Especially at higher speeds, it's a good idea to not run the loop current through the transformer, since the DC bias will cause the iron core to saturate early and increase distortion. Many that I have seen use a transistor active load to pass the line current, then couple to the transformer through a capacitor. Since the active load needs a proper positive DC bias to work, there is a diode bridge in line to the whole affair. I suppose the bridge could just be in line to the active load, but then a non-polarized capacitor would be needed to couple to the transformer. Some designs switch the load on and off to control on/off hook, but relays are still popular. In external modems, I suppose it is possible to use a larger transformer that would be more immune from saturation. The early external modems just had a transformer and a relay at the input, so they could be connected directly without needing to supply DC bias. _____________________________________________________________________ You don't need to buy Internet access to use free Internet e-mail. Get completely free e-mail from Juno at http://www.juno.com Or call Juno at (800) 654-JUNO [654-5866]