Posted by David Sherman on Friday, August 10, 2007 at 18:51:22 :
In Reply to: Re: Amp meter is pegged? posted by Arthur P. Bloom on Friday, August 10, 2007 at 16:23:41 :
In theory, that would work fine. In practice, home-made high current shunts are difficult to parallel with any semblance of accuracy the trouble is that you're dealing with milli-ohms of resistance, and things like thermal expansion and contraction, oxidation, the tightness of the mounting screws, and so on are quite capable of making a big different in the way the current splits between the two shunts. If the meter is a type that uses an external shunt, then the thing to do is change the shunt to a single shunt of a lower resistance. Lots of (most?) automotive ammeters are of the "moving magnet" design where a 1-turn "coil" of heavy wire creates enough of a magnetic field to move the pointer. In this case there is no shunt, but if you try to add one externally, you'll still get into all the problems of it drifting all over the place with time, temperature, vibration, etc. I know how to make proper shunts that work right, but there are all kinds of second-order effects involved, and it's not just a matter of soldering a couple resistors together.
It's one thing to plug some numbers into a calculator and find that a .01 ohm shunt will turn your 30 amp meter into a 60 amp meter. It's quite another to actually make a .01 ohm shunt and connect it to the meter in a way that causes it not to fluctuate by more than .001 ohms (10%) with time, heat, and vibration. You don't just go down to Radio Shack, ask for a .01 ohm resistor, and stick it between the terminals on the back of the meter. Any time you're dealing with milli-ohms, things get tricky. Probably the only practical way to do it would be to put the ammeter in question in series with another meter of higher capacity and known accuracy, get a foot or so of #14 wire, screw it tightly between the ammeter terminals, and see if it give you the full-scale reading you want. If the meter's still pegged, shorten the wire until it reads right. If it reads too low, try some #16 wire. Keep experimenting with the wire length and gage until you get the right reading, then coil the wire up neatly, and solder the ends of it to the meter terminals. Since all that really matters with a truck ammeter is whether the alternator is charging hard, charging a little, or not charging at all, a jerry-rigged shunt like this might be good enough.