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.