Download Voltmeter-Rev1 - Electro Tech Online

Multi Voltmeter

A volt meter simply measures voltage

The meter is actually a milli-ammeter meter on its own. That’s why you have to convert it to
a voltmeter via the above circuit. The meter is actually called a movement and will have a
characteristic called Full scale Deflection, such as 30uA FSD or 50uA FSD. This means the
needle will swing fully across the scale when 50 microamps flows in the coil.

When the movement is connected to a series resistor, it is converted to a voltmeter. This
means you can connect a battery to the circuit and get an accurate reading.

A
scale
will
be
printed
on
the
movement,
such
as
0
to
10.
The circuit above will change the range of the movement, so it can be selectable to read 0 to
10v or 0 to 100v etc.

The volt meter works by using two probes which touch the positive and negative on a power
source, such as a battery

The common probe is connected to negative or ground.

The red probe is connected to positive

This circuit above has no exact values shown. This is because there are many different
movements available, each with different sensitivity.

The movement has a low resistance of its own. It may be something like 50 - 200Ω. It is not
important to know this value

Figure 1 shows a simple voltmeter circuit. A voltage is applied to the circuit which has a fixed
resistance

Since there is a voltage (the battery being measured) and a resistance (of the circuit), Ohm’s
law says a current will flow

This current determines how far the needle moves across the scale

If the range of the meter is 10V, and you connect a 1.5V battery, the needle (pointer) will
move to the 1.5v mark.

In our circuit, there is a switch which selects one of three paths for current to flow

One path has no resistor. There
is no path with no resistor
 Change the circuit diagram

This uses the normal range of the meter, and is essentially the circuit shown in figure 1

The other two resistors increase the meter’s range

Putting a resistor in series with the meter has the effect of reducing the current through the
circuit and thus it takes a higher voltage to make the pointer travel full scale.


Because the resistance is higher, more voltage is required to get the same meter movement

If the resistor (R2) is the correct value to make the meter’s range 100V, a 15V battery would
be required to move the meter’s needle up to the 15v mark

R1 would be higher again to give a higher range, such as 1000V

Variable resistors can be used to determine the required value of R1 and R2
The values of resistance are so high that you don’t need to
have a pot.

Using just one variable resistor would be inaccurate. Instead of using just a variable resistor,
a resistor network could be used. See figure 2
o
In this network, a resistor is placed in parallel with the variable resistor
o
Having the parallel resistor makes the range of the variable resistor decrease. This
means that it is less sensitive, which means it is more fine tuneable
o
The total range of the parallel resistors can be found with the
NO
formula
NO
o
If the variable resistor is turned down to zero, which is the lowest range, the total is
also zero
NO
o
If it is turned up to maximum, which is 10KΩ in this schematic, the maximum range
will be 5KΩ
o
The resistor R1 is placed in series with VR1 and R2. This is so that if VR1 is turned
right down, the total resistance of the network does not drop to zero

o
This means the resistors network’s range in this schematic is 10KΩ - 15KΩ
o
The values or VR1 and R2 could be changed to give different ranges
o
If VR1 and R2 were both 100KΩ, the range would now be 10KΩ - 60KΩ
Replacing R1 and R2 in the volt meter with a resistor network will allow for adjustments to
be made

This is especially useful, as resistors normally have tolerance values, meaning that their
values may be a little out. Using the resistor network will allow for more accuracy
You have got the concept of a voltmeter entirely wrong
Re-write is and send it back
NO
Fig. 1 – Simple Voltmeter
NO
Fig. 2 – Resistor Network
NO