Shunts in Ammeters

Name: Austin
Status: student
Grade: 9-12
Location: WV

Question: In electrical measurement, can somebody please explain to me why an 
ammeter requires an internal shunt, but a voltmeter does not?
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Austin -

A volt meter is rather high in resistance and only a small current travels 
through the meter itself.  It is used in parallel ... meaning that it is not 
the only path for the current.

An ammeter is used in series to measure the current flow in a particular 
circuit.  It must provide a path for the entire current.  It would be difficult 
to develop a meter that would handle significant amounts of current.  With that 
in mind, the ammeter uses a shunt to allow some of the current to travel 
through a course parallel to the meter.  If the shunt malfunctions, it is 
likely the meter will not be able to handle the current.

Larry Krengel
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Voltmeters and ammeters typically have a similar problem: the sensing unit
has a smaller sensing range than is desired. To extend the range, you add
circuits that adjust the voltage or current to be within the sensor's range.
To understand why a shunt is used in an ammeter but not a voltmeter, you
need to understand Ohm's law, and how it differs between serial and parallel
circuits.

In the case of voltmeters, you are trying to reduce the voltage, but not the
current, so you can add a resistor in series. Voltmeters typically have
several circuits (different resistors) to allow reading voltages across a
very wide range.

For an ammeter, to adjust current instead of voltage, you do not use an
in-line resistor, but a parallel resistor. With a resistor in parallel, the
current is split, but the voltage is the same. This parallel resistor is the
'shunt'.

For more information, and a more detailed explanation, this site is very
good:

Ammeter: http://www.allaboutcircuits.com/vol_1/chpt_8/4.html

Voltmeter: http://www.allaboutcircuits.com/vol_1/chpt_8/2.html

Hope this helps,
Burr Zimmerman
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Hi Austin

An ideal voltmeter measures the potential difference (voltage)
between two points and it has an infinitely high internal resistance
so there would be no current flow through the voltmeter. In that way,
the voltmeter can be used to probe a circuit without it affecting the
operation of the circuit.

An ideal ammeter has zero resistance so current can flow directly
through the meter, unimpeded. Furthermore, from Ohm's law, V=IR (V is
voltage, R is resistance and I is current) if resistance is zero, the
voltage across the ammeter will also be zero. This way the ammeter
can be inserted into a current path, current will flow through it
like it was just a piece of wire, and it will not affect the voltage
distribution in the circuit.

Okay, those are ideal voltmeters and ammeters, but nothing is really
ideal. So let us make a real ammeter out of a voltmeter.

Because of Ohm's law, we can use a voltmeter to indicate current just
by measuring the voltage present across a resistor (If we know V and
we know R, we can calculate I). In a setup like this, the resistor we
measure across must be very small - that is, current flows through it
very easily - and that resistor is called a shunt.  Since V=IR, if R
is very small, for any value of I, V will be very small. In order to
read that voltage, you will need a very sensitive voltmeter. Put the
sensitive voltmeter together with a shunt and you have an ammeter.
All you need to do now is to change the scale on the voltmeter from
volts to amps following Ohm's law solved for current, I=V/R.

Hope this helps!
Bob Froehlich
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A voltmeter is in series (usually) with the power source. Inside the 
voltmeter is a resistor of known number of ohms and this is the "voltage" 
you read. If the voltage is high the voltmeter may contain an internal 
shunt in parallel, to "siphon off" the large share of the electrical 
current.

In the case of an ammeter, the shunt in parallel with the ammeter (which 
also may be measuring the voltage drop across a resistor) is necessary 
because in an ammeter the current is typically too large, and several 
amps may "blow" the internal circuitry. Therefore it is more important 
to have a shunt resistor(s) in parallel with the meter to "siphon off" 
the large share of the electric current.

Vince Calder
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Austin,

An ammeter requires an internal shunt and a voltmeter requires an
internal resistor.  Quite often this resistor is already part of the
voltmeter.  Actually, a shunt is just a VERY SMALL resistor.

Every ammeter and voltmeter has a coil or semiconductor device that has
its own internal resistance.  This internal device cannot handle a large
current.  To keep the current down, the voltage across this internal
device must be kept fairly small.

An ammeter, once structured, needs a VERY SMALL resistance, with almost
no current flowing through the internal device.  The easiest way to
accomplish this is to put the internal device in parallel with a VERY
SMALL resistor, a shunt.  A large resistor in parallel with a small
resistor has a very small resistance.  Almost all current will flow
through the shunt.  When in series with a circuit object, such as a
light bulb, the ammeter has full current (through the shunt) and very
small voltage.  It does not affect the rest of the circuit.  Changing
the shunt resistance can change the scale of the ammeter.

A voltmeter, once structured, needs a VERY LARGE resistance, with almost
no current flowing through the internal device.  The easiest way to
accomplish this is to put the internal device in series with a VERY
LARGE resistor.  A large resistor in series with a small resistor has a
large resistance.  When in parallel with a circuit object, such as a
light bulb, the ammeter has full voltage (most of it across the large
resistor) and very small current.  IT does not affect the rest of the
circuit.  Change the large resistor can change the scale of the
voltmeter.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College
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Both ammeters and voltmeters use coils that respond to a voltage.  They are 
typically capable of withstanding only millivolts of voltage difference 
between the terminals, and can tolerate only tiny currents within the 
coils of the meter movement.  You could make a meter movement that would 
allow large current to flow through it, but it would be enormous.

So how do you use a small meter movement to measure, say, 500 Amps of 
current?  the answer in found in Ohm's law, which states that the voltage 
across an element is equal to the current flowing through it multiplied by 
the resistance (V=I*R).  If we place a very small resistance in the circuit, 
with a value we know fairly accurately, we can measure the current flowing 
in the circuit by placing the leads of the meter on either side of the 
resistance we added.  The resistor is in parallel with the meter.  The 
resistors used for such purposes are called shunts.

A typical shunt value is 500 Amps and 50 milliVolts.  This means that there 
will be 50 milliVolts across the shunt when 500 amps are flowing through it.  
So a small 50 milliVolt meter movement can be used to measure a very large 
current.

Shunts such as this are pretty big, so the ones used in small hand held meters 
are typically smaller, about 10 Amps.

To use the same meter movement to measure a voltage, we connect a high resistance 
in series with the movement, so that even though we connect a large voltage to 
it, the current through the meter movement will be limited.  Hand held meters 
use the dial to select a variety of resistors to allow the meter to measure 
different ranges of voltages.

David Brandt
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Hi Austin.
In that last several decades, we tend to make just one kind of meter
element, or meter-core,
around which we place voltage-dividers or current-shunt resistors to get it
to read the right thing.
After all, everybody wants a different meter.
Or thirty different meters in one little box, for multi-meters.

Suppose all meter-cores are purely voltage-sensitive.
Then we need a "dropping resistor" (the shunt resistor)
so our current will cause a certain amount of voltage-drop
that our meter will read properly.

It is not really true; a mechanical meter-core is often around 20-60 ohms
and needs maybe 1mA at 20-60mV to deflect to the top of its scale.
It requires both voltage and current.
In practice their manufactured sensitivity varies a little,
and is usually adjusted to some exact value by putting a largish resistor
in series,
which makes them a bit more voltage-like,
maybe 1mA at 0.100v , which is 100 ohms.
Or even 1mA at 1.00v, which is 1000 ohms.

Digital electronic meter-cores are definitely purely voltage-sensitive;
the FET transistors inside draw less than 1 millionth of an amp from the
wires being measured.
So meter-circuitry using them is easier to figure out.

Either way, a current-meter needs to be a much lower resistance.
So we get a low-value resistor and put one of our voltage-sensitive
meter-cores across it.
If we have a meter-core that needs 0.100v to go to full-scale,
and we want 1.00 Amp to push it to full-scale,
we need an 0.1-ohm shunt resistor.
V = I x R
0.1v = 1Amp x 0.1 ohm

With algebra you can turn it around any way that makes sense, such as:
we know we have an 0.1v meter and want to measure 1 Amp,
but we need a formula to tell use what shunt resistor to use.
V = I x R
can be changed to
V/I => R
to tell us directly.
0.10v / 1.0 A  =>  0.1 ohm
I do this all the time in my lab.

If your current-meter does not have a low-enough resistance,
two problems can happen:

  1) the voltage it takes up is a significant part of
        the voltage in the circuit you are measuring,
         so your ammeter actually starts choking down the current a little.
     This is considered a yucky inaccuracy even if it is only 10% current
reduction.
     (Possibly you would not be so picky.)
     So we would not use a 1-volt meter core to make an ammeter to use on
12v cars.
     It would have to be an 0.1v meter core. Or less, if we like.

  2) the resistor can get too hot and burn up.
           P = I x V
           10 amps x 1 volt = 10 watts
          This means If you try to measure headlight current draw
               using a 1-volt meter core and the right 0.1-ohm resistor,
           that resistor has to also be pretty big, a "power resistor" we
call it.

       But if you use an 0.10volt meter core, the right resistor is
smaller, only 0.01 ohms
          and P = I x V:  10 amps x 0.10 volt  =  1 Watt.
          A fairly small resistor can do it, and will not get hot enough to
burn itself out or hurt when touched.

So we have two good reasons to use a low-voltage meter core (0.1v or less).
And being a voltmeter inside, it needs a current-shunt resistor to make it
a current-meter.

Meter-cores that are primarily current-sensitive can exist, sure.
A wire carrying your current, passing a tiny bar-magnet on spring-bearings,
with needle sticking out to a scale,
would be a current-sensitive meter-core, and maybe it was made many years
ago.
A magnetic compass kind of does this today, if you arrange it right with
respect to the wire and the north pole.
Digikey sells newer transistor-like devices that measure the magnetic field
around a wire
(which is proportional to current), which  are gradually becoming popular
too.
Guess what, they output 0-5 volts that goes to your favorite, uh,
VOLTmeter.
DC amp-clamps are part of this trend.  Slowly they are getting cheap
(<$50).
They do not involve current-shunt resistors,
and you will not have to figure out what is inside if you do not want to.
You do not even have to break open the measured circuit to put it through your
ammeter; that is great...
But so far, not having one on hand, most people still use shunt resistors
and voltmeters.

By the way, almost all voltmeters use voltage-dividers inside, made of two
or more resistors,
to get them to read the right full-scale range.
They are not really simpler, just more likely pre-packaged in the box.
And they are harder to burn out, so they need less education of their user.

It is a pretty broad philosophical essay to explain why we have liked voltage
more than current.
I am sure some of our favorite electronic devices have something to do with
it.

Jim Swenson
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