There are also some nice current sense amplifiers from Maxim and Linear Technology. Some of these have an internal sense resistor, some rely on an external one. Most of the ones I've seen are high side current sensors. They are nice in that they already have a fixed gain, so you don't need any precision resistors around an op amp. Also, they have very good CMRR, which requires even better resistor matching around an op amp. I used a Maxim high side current sensor to monitor the current into a 1.5A LED (really bright!). The output of the Maxim chip drove the feedback input of a Linear Tech boost converter driving the LED. The boost converter then had a regulated current output instead of its usual regulated voltage output. To prevent infinite voltage should the LED open, I put a zener between the output of the Maxim chip and the output of the boost converter. If the boost converter voltage went above 12V, it started pulling up the output of the current sense amplifier so the boost converter would then regulate at about 13.25V (instead of infinity) when driving an open circuit. So, the Maxim current sense amplifier worked out real well in that application. Originally we had a problem with noise in the circuit (due to the boost converter) that would sometimes drive one of the Maxim pins negative. It did NOT like that, and blew the chip. We fixed that by adding another layer to the circuit board as a ground plane. Another problem that was difficult to find was when the user slid the plastic probe into the product (the LED light was sent down a plastic light guide), sometimes static would be generated. This would arc to one of the LED leads, which would then blow out the Maxim chip. We fixed that by bending the LED lead away from the light guide and adding more insulation. There's always something! Anyway, I like current sense amplifiers! Harold > All good reasons to buffer the dropping resistor before putting the signal > into your PIC. > > You really need to be using an op-amp buffer to take a small signal, say > from a 0.05 ohm "resistor" (I.E. a wire) and amplify it, low pass filter > it, and clamp it so it won't blow up your PIC. Depending on your power > supply arrangement it may need to be a rail-to-rail op amp. > > > Or you could go high tech and use another current measurement technique > like a hall effect or a current transformer. > > > -- Lawrence Lile, P.E. > Electrical and Electronic Solutions > Project Solutions Companies > www.projsolco.com > >> -----Original Message----- >> From: Alan B. Pearce [mailto:A.B.Pearce@rl.ac.uk] >> Sent: Monday, September 13, 2004 8:45 AM >> To: Microcontroller discussion list - Public. >> Subject: Re: [PIC] Current measurement using PIC >> >> >> So if I use a 1 ohm, ground line sense resistor, I can get a 0-5v >> (for >> >> 0-5A) across the resistor and PIC16D876 ADC can measure that range! >> >> Right? >> > >> >Yes, but that will dissipate 25W at full current. It will also use up >> 5V >> of >> >the available voltage, but I don't know if that's an issue in your >> circuit >> >or not. >> >> And what happens with a short circuit current surge? over voltage on the >> PIC >> input? >> >> _______________________________________________ >> http://www.piclist.com >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> >> --- >> Incoming mail is certified Virus Free. >> Checked by AVG anti-virus system (http://www.grisoft.com). >> Version: 6.0.744 / Virus Database: 496 - Release Date: 8/24/2004 >> > > --- > Outgoing mail is certified Virus Free. > Checked by AVG anti-virus system (http://www.grisoft.com). > Version: 6.0.744 / Virus Database: 496 - Release Date: 8/24/2004 > > > _______________________________________________ > http://www.piclist.com > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- FCC Rules Online at http://www.hallikainen.com _______________________________________________ http://www.piclist.com View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist