Amp Meter Shunt

1 Posts
Mar 23, 2010 07:33 am
Amp Meter Shunt

Whats the purpose of a shunt on the meter when measuring a DC current? Thanks Jon
97 Posts
Mar 23, 2010 03:03 pm
Re: Amp Meter Shunt

Jon, the purpose of the shunt on an ammeter is to pass whatever current that cannot be passed by the meter itself.  The meter has a rather high internal resistance and will reach full deflection with a matter of microamps or milliamps of current.  The shunt on the other hand has a very low resistance, and when placed in parallel with the meter, will pass the majority of the current flow that the meter is calibrated to indicate.  Jon C. (in the case of a voltmeter, a Multiplier resistor is used in series with the meter to limit current flow to the calibrated value on the meter)
Mar 26, 2010 03:04 am
Re: Amp Meter Shunt

If Alt-E will allow it Jon S., and while not meaning to "out do" anybody, here is a link to a more, illustrative description of what Jon C. wrote.
If Alt-E will not, then...
Its funny sometimes what can clear up an understanding of how something works. Take batteries for example. Once I had read from a high school chemistry text book that, a battery doesn't store electricity, that it converts electricity into a chemical energy and stores that, battery types and their ratings were no longer a mystery to me.
Think about the starter motor on an automobile for a moment and how, most of them can "pull" around 80 amps at 12 volts from the cranking battery. Imagine what size of a switch would be on the steering column (picture Frankenstein's laboratory) if there wasn't an electromagnetic contactor or "solenoid switch" on that circuit.
Instead, a small amount of amps runs along a small wire to a small switch on the steering column and then to the solenoid which charges an electromagnet which in turn pulls in the large contact that carries the higher amperage through larger wire to the starter from the battery.
If you think about how an analog ampmeter, with the moving needle, works then you can see that its a tiny little motor of sorts that drives the needle. Now, imagine how big an ampmeter would have to be to read 180 amps of current on 0000 wire.
Picture, David and Goliath.
The shunt (David) knocks down the high amperages (Goliath) and cuts off his head so that the meter (Israeli army) can... Well, maybe thats not such great analogy. But, do you get the picture?
« Last Edit: Mar 26, 2010 03:17 am by Thomas Allen Schmidt »
184 Posts
Mar 28, 2010 04:05 pm
Re: Amp Meter Shunt


While voltage is measured “across” a device, current flows “through” a device.  If resistance is low, current flow will be heavy.  If resistance is high, current flow will be light.  Current is the same in all parts of a series circuit.  A meter is a sensitive device, only able to pass a small amount of current.  But a light bulb (a typical load), requires a relatively high current.  Putting a light bulb in series with a current meter would quickly destroy the meter.  The solution is to create a parallel circuit, diverting most of the current around the meter, and to the load.  That is what a shunt does.  A shunt is a precision resistor, of low resistance.  As a hypothetical example:  If the meter represents 999 ohms of resistance, and the shunt represents 1 ohm, and the total current flow is 1 amp, then 0.999 amps flow through the shunt, while 0.001 amp flows through the meter.  The meter is calibrated in such a way to indicate 1 amp, while the actual current flow through the meter is only 0.001 amp. 

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