How about using a hall effect type of system? I've never tried it but I think it should work. TTYL > -----Original Message----- > From: pic microcontroller discussion list > [mailto:PICLIST@MITVMA.MIT.EDU]On Behalf Of Jim > Sent: Thursday, May 09, 2002 20:07 > To: PICLIST@MITVMA.MIT.EDU > Subject: Re: [EE]: Measuring automotive current.... > > > For "engineering" purposes while testing charging > systems on autos I have simply used a digital > millivoltmeter and two points tapped in either > the negative or positive battery leads - > > - choosing points with enough separated to provide > a suitable 'scale factor' while drawing a known > 'calibration current' (like 10 Amps) ... > > Jim > > ----- Original Message ----- > From: "Pic Dude" > To: > Sent: Thursday, May 09, 2002 6:34 PM > Subject: [EE]: Measuring automotive current.... > > > > Hi all, > > > > Back from a "break" which has me running on 9 fingers and 9 toes for > > a while. :-( But fortunately, I'll be back to 10 and 10 in some weeks. > > > > Trying to figure out the best way to measure current in a car > (12V neg gnd > > system). The current path to be measured is that going into/out of the > > battery. For now, I'll use a "safe" figure of 100A, although > the current > > drain/alternator charge current should not be anywhere near that. > > > > First thought is to use a very small resistance with high current > capacity. > > I believe that is what a shunt is, so a quick web search found me some > > shunts rated at a certain millivolt value and max current. For example, > > 100mV and 100A. My guess is that this means that the voltage drop at > > 100A will be 100mV, so the resistance is 0.001 ohms. That's small ! > > So I'll use these figures for now. > > > > However, with this small of a resistance, max power = 100A x 0.1V = 10W. > > Not bad. And 100mV of voltage drop will not be a problem. Now > all I have > > to do is measure the voltage drop across the 2 resistor terminals. > > > > The questions are: > > - I have an ammeter that is "shuntless". Does this mean that it has an > > internal resistor for voltage drop? Or some other method of current > > measurement? If internal resistance, how does it deal with resistance > > of the (long) connecting wires? > > - How do I get a PIC to measure -100mV to 100mV accurately? A small > > voltage reference would be necessary, but would the PIC be accurate > > given noise, etc? Or does the 100mV now need to be amplified? > > - How about getting an accurate resistor -- if I use an actual resistor > > instead of a shunt, can I get a resistor that small and that is accurate > > to within a couple percent? > > > > Or am I going about this all wrong? > > > > Cheers, > > -Neil. > > > > -- > > http://www.piclist.com hint: The PICList is archived three different > > ways. See http://www.piclist.com/#archives for details. > > > > > > -- > http://www.piclist.com hint: The PICList is archived three different > ways. See http://www.piclist.com/#archives for details. > > > -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email listserv@mitvma.mit.edu with SET PICList DIGEST in the body