Adapting Meters

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Adapting Meters

Digital meters, although versatile, are sometimes inferior in practice to analogue meters (i.e. meters with a sweeping arm on a graduated dial). Nowadays the analogue meters are often made using "Analogue to Digital Convertes" or ADCs. These too have some limitations and occaisionally only a "good old-fashioned" meter will do.
Using analogue meters you can see changes that are not so easily seen with a digital display. For example: if there is no steady reading (where a meter tries to "hunt" the average), in the case the digital display, the value dispalyed is often meaningless.
Similarly there are circumstanceswhere it is useful to change the display so that the whole of a range on the display is used. Or it may also be necessary to have ammeter to cover a range that you do not have a dedicated meter for. For example a microwave detector can be more easily understood when used to illustrate the principle of diffraction if the current is over the full scale. This can be done by adding resistances to the meter.
In order to create range to suit a few bits of information are required:
These meters are either moving coil (where the magnetic effect of the current passing in the coil of the meter tries to move the pointer) or moving iron (where the magnetic effect is to pull the iron load of the pointer).
Need to Know:

  1. Resistance of meter
  2. Current (or sometimes Voltage) at FSD

Explanation:
Understanding how one meter can be made to cover different ranges relies on two relatively simple bits of theory:
1 The sum of the currents entering a node(junction)is equal to sum of the currents leaving that node. This is Kirchoff's first law. If you think in terms of the water analogy what you pour into a pipe will come out the other end.
2 link to potential divider theory.


Shunts & multipliers

A common example of a moving coil meter is a 100 microamp 1500 Ohm meter (which can be found in most electronic catalogues). This can be used to create an ammeter and a voltmeter.
Two cases are now considered to illustrate how calculations may be made
e.g. For 100muA FSD meter 1500 Ohm
Set up as an Ammeter to give 0.5A at Full scale deflection (FSD)
As a Voltmeter set to give 20V at FSD.

Shunt.jpg
In the above diagram the current I at point B is split so that it flows through the meter as I' and through the "shunt" resistor "Resistor 2" as I".
Note only Resistor 1 or Resistor 2 would normally be in circuit at any one time.

From Kirchoff's First law I =(I')+(I" )


Worked Examples

We know that the maximum currrent through the meter is when it is at FSD (full scale deflection)
In our real life example this is 100 micro amps (100muA).

Creating a 0.5A meter

We want a total current entering B of 0.5A. The meter has a FSD of 100muA. This means that the current I" is 0.5-100muA or:
I"=500000-100uA
=499900uA
=0.4999A
As the voltage across the meter must equal the voltage across the “Shunt”,
V = I’R’=I"R"
R"=I’R’/I"
R"=100x 10-6 x1500/0.4999
=0.300 Ohm
This goes between B and C in the diagram.
This can be obtained from a length of constantan wire.
Note: A shunt is always of low resistance.


Creating a 20V meter

We know that the current through the meter at FSD is 100muA.
And that the meter has a resistance of 1500 ohm.
This means that the voltage dropped across the resistor is IR=0.15V.
The "Multiplier" resistor "Resistor 2" or R2 must then have a voltage dropped across it of:
V2=20-0.15
= 19.85V

We already know the current (100muA), so from Ohm’s law
R2=V2/I =19.85/100 x10-6

=198500 Ohm
In practice a 220K Ohm resistor would be used.
Note A "Multiplier" has always a high vlalue
This goes between A and B in the diagram.

--D.B.Ferguson 20:42, 10 October 2007 (BST)


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