Olin Lathrop wrote: > > Personally I would start with a piece of copper 1/2" water pipe, work out > > from theoretical resistance of copper how far apart the tap points should > > be, and solder a couple of wires on and give it a try. > > A piece of #12 wire will be easier to deal with, and the much smaller > outer diameter will make it easier to compute the expected resistance. This is true. > Computing the resistance of a hollow tube between two point connections to > the outside of the tube is not a trivial matter. No, but that shouldn't even be an issue. Assuming he's building a proper 4-wire shunt, the potential around any circumference along the tube should be the same. The load current should be applied at the centers of caps attached to the ends of the tube. The sense wires can be tacked on at points on the outside of the tube. BTW, you can buy SMT milliohm shunts commercially, and they're not all that expensive. You do need to be careful in your layout so that the sense traces come directly off the shunt's pads and not from elsewhere along the load current path. Also, I have reverse-engineered some APC UPS units, and they just use a PCB trace as the shunt. Of course, they aren't going for incredible accuracy; they're more interested in just detecting overloads. -- Dave Tweed -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body