Ampmeters

From what I gather, a "shunt" ammeter allows a known percentage af actual current through a shunt - which can take the total current without vaporising. If we say a shunt meter is designed to let 90% of the current through the shunt, we assume that 10%(remainder) is going though the meter. We apply the multiplier percentages and display 100% on the meter (which was logically what it actually carried - times ten).

Just a guess... but can't be far from the truth I assume..............

Although the answers already given are correct, a vital piece of information is missing.
An ammeter must be inserted in series in a circuit to measure current. It will not measure across (parallel) in a circuit as the current will take the path of least resistance.
Ammeters are sensative instruments and therfore generally don't like huge currents. Typically for greatest accuracy, you wouldn't use a non shunt ammeter to measure currents over say, 2A. A shunt ammeter uses a shunt, material with a known resistance that the majority of the current flows through so as to not inhibit the circuit. The ammeter is itself in parallel with the shunt, measuring the now reduced current through the ammeter itself.
Shunts and ammeters are matched to each other.
Larger current circuits, you would now use a CT, (Current Transformer) which transforms the current in a circuit to allow instrumentation to measure it, particularly useful for digital display current meters, or for displaying on computer pages.

Cheers,

Dion.

Yeh, Dion is absolutely correct in that.
And further, to make it a bit less technical.... ie: quick and simple....
ANY ammeter you see in a car will use either an internal OR external shunt. It's that simple really.

Ray Bates,
Just to make it complicated, ALL ampmeters are shunt meters. An ampmeter is simply a moving coil or digital voltmeter that measures the voltage drop accross a 1 ohm shunt. If you remember ohm's law, then I=V/R. R = Resistance, V = Voltage and I = Current. So, if the resistance of the shunt is 1 ohm, then the Current reading is read as Voltage. The only other thing is that meters have series resisters to calibrate the scale that the meter operates at, and that the current rating of the shunt will determine the maximum current that the meter can handle.
Hope that helps.

Chaz

Ray,

Chaz is quite correct. All Ammeters are shunt meters. Its just the location of the shunt that varies. I tend to think of a shunt driven Ammeter as a Voltmeter. Most basic Shunts present about 1mOhm of resistance. So 1Amp of current will provide 1mV of voltage difference (potential) across the shunt (V=IR). The Voltmeter simply measure the voltage difference but presents it as Amps. You can build a very simple ammeter by using a shunt and digital voltmeter.

Shunts are very handy because they allow you to place your guage along way from your measurement point. ie You can measure the current into your battery without routing the heavy cables into the cabin to the back of the guage which is also alot safer.

Just recently I embarked on a project to build a combined volt and ammeter. Basically I wanted to measure battery volts to determine battery state, plus also be able to measure current into & outof my 2nd battery. Problem is, most ammeters are symmetrical (ie -1000A to +100A) but I wanted to be able to measure input current (approx 80A) but also output current (approx 4A) and a symmetrical meter was too inaccurate at low ranges. So I built a cheap & cheerful Volt & Amp meter. I discussed this a few days ago on a different thread and had abit of interest so I drew it up & posted a link.

You can find my cheap & cheerful Volt & Amp meter herehere. If you want to learn abit about shunts & meters, it's not a bad project to begin with.

Hope my diatribe helps.

Andrew

I'm not certain what you mean by an "ordinary ammeters" but you may be referring to a "moving iron meter" which will measure both AC and DC currents but does not use a current shunt instead it uses magnetics.

Shunt meters may loose quite a bit of power in the shunt and it will certainly rob the load of some voltage - which at low volts is usually bad. They are normally used with moving coil or electronic (digital) meters.

Moving iron meters don't take very much power from the circuit under test but are relatively insensitive and not suited to the measurement of very low currents. For 12V camping / 4WD applications they would be ideal - better than a shunt meter.
(Have I missed anything? 30 years since I did moving iron meters :).

Mike Harding

mike_harding@fastmail.fm