Electrical question: how does a shunt work? Instead of a 12 or 10 gauge wire like the old AD's have to their amp gauge the newer cars have a 16 or 18g wire to them. The wiring schematic shows a "shunt". Here what I envision; the shunt is like the little line to my ice maker, it feeds the ice maker but doesn't really affect the water going through the big main line. I guess pressure would be the same but just a fraction of the volume. Way off base or a little bit on track? Thanks; Evan
Old ammeters had the full load run through them. A cheap, but not terribly safe, way to conduct business. The primary circuit in old trucks was not typically protected with a fuse before it got to the meter. If the insulation on the wire gets rubbed off, the terminal on the ammeter comes loose, etc. there is a good chance for fire under the dash. Later model ammeters used an external shunt. This is actually a very low resistance (0.01 ohms or less) resistor. The ammeter measures the direction and amplitude of the voltage drop across the shunt. Doesn't need big wires to the meter and could be fuse protected. Even if no fuse, shorting the wire to ground would probably not cause shuch a big fire before the wire burned in two. I'm no EE, but I have caused lots of sparks. Hope this helps Evan.
Ohm's Law! Voltage=Current times Resistance. V=IxR. Very simply put, a shunt is merely a resistor installed in series with the ammeter to lower the amount of current going through the ammeter. If the voltage remains constant, increasing the resistance lowers the current.
Amps or Volts? Both are basically correct---but---maybe not? Think of filling a pool with two hoses at the same time. One half inch hose and one one inch hose. Both with equal pressure (volts), which one has more water flow (amps). That's the way the shunt works. It takes the bulk of the flow. Like Bill said very small resistance for high amps directly around the meter so the meter takes very little to show a "scaled" reading. The older shunts looked like a metal plate bolted on the back of the meter. Again, small resistance and large enough to handle the wattage (heat). This way, if the meter burns out, the circuit still works through the shunt. Now with that said, newer meters might actually be volt meters, not amp meters. They work off the voltage being dropped across a shunt and read that voltage which is scaled to look like amps. Cheeper to make a volt meter. When we went to 12 volts from 6, the amps got cut in half for the same resistance, hence the smaller gauge wires. Disclaimer:This is not all totally accurate but simplified for easier understanding. Rick
Close Ken, you got this in while I was typing. You got it all right except the "series" part. It's parallel (across the meter). Current in a series circuit is always the same anywhere in that circuit. Rick
Shunt Typ Gauges Evan ; You're correct but the thing I remember from the 1960's GM's was : the damned ammeter NEVER MOVED ! not once , not even when the cars (SS Chevies mostly) were brandy new . I think a volt gauge is far more useful . Safer , too .
