----- Original Message ----- From: Simon Nield To: Sent: Friday, July 28, 2000 6:38 PM Subject: [EE]: Re: Capturing power line waveform > >Please take note that operational amplifiers do not amplify voltage > >difference! > > four resistors and they do... however the term I used was a more general "differential amplifier > stage". I suspect I would still use an opamp for this. Same thing. They all have huge differential amplification and operate on same principle. Feedback network matters, not opamp. > > If you want such circuit to work, voltage form each side of resistor to > > ground plane of measuring circuit should not be greater than OPAMP supply > > voltage, otherwise OPAMP will not be able to do compensation across feedback > > network and circuit will not work at all (you will just get common-mode > > amplification). > > good point. Typical resistor values would be < 1 ohm though, otherwise you perturb the measured > current. I would recommend using series resistors with the sense wires and then clamping diodes to > the rails of the opamp. If ground floats on mains voltage, this is not problem. Shunt resistance is small enough so you cannot get huge voltage here. Jerko Golubovic 9A6JGJ -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.