Download 07-Voltmeters

VOLTMETERS
Done by:
Naser M.Jamal
Ibrahim Shahbaz
WHAT IS A VOLTMETER?
A voltmeter is an instrument
used for measuring electrical
potential difference between
two points in an electric
circuit.
WHAT IS A VOLTMETER?
Some voltmeters are intended
for use in direct current (DC)
circuits; others are designed
for alternating current (AC)
circuits.
HOW DOES IT WORK?
The main principle
of voltmeter is that
it must be
connected in
parallel with the
element we want
to measure the
voltage across.
HOW DOES IT WORK?
The parallel connection is used
because a voltmeter is
constructed in such a way that it
has a very high value of internal
resistance.
HOW DOES IT WORK?
 So if that high resistance is connected
in series than the current flow will be
almost zero which means the circuit has
become open. If it is connected in
parallel, than the load impedance comes
parallel with the high resistance of the
voltmeter and hence the combination
will give almost the same the impedance
that the load had.
T YPES OF VOLTMETERS
Analog Voltmeter
Digital Voltmeter
 The readings are shown
on a scale.
 The readings are
displayed on an LCD or
LED display panel.
T YPES OF VOLTMETERS
1. Analog voltmeter
• Permanent Magnet Moving coil Voltmeter.
• Moving Iron Voltmeter.
• Electro Dynamometer Voltmeter.
• Rectifier Voltmeter.
• Induction Voltmeter.
• Electrostatic Voltmeter.
2. Digital Voltmeter.
PERMANENT MAGNET MOVING COIL
VOLTMETER
1.With the probes
unconnected, the meter
is like an open circuit, no
current can flow into the
meter or the coil inside
it.
2.With no current flowing,
the coil generates no
magnetic field and the
pointer stays at zero.
PERMANENT MAGNET MOVING COIL
VOLTMETER
3. Connect the meter
probes to something
you're testing (such
as a circuit board),
and current will
immediately start to
flow through the
meter and the coil
inside it.
PERMANENT MAGNET MOVING COIL
VOLTMETER
4. The moving current creates
a temporary magnetic field
around the coil that repels
the magnetic field created
by the permanent magnet
causing the movement of
the pointer. The strength of
the magnetic field is
directly related to the size
of the current that flows
through the coil.
PERMANENT MAGNET MOVING COIL
VOLTMETER
5. The greater the
voltage we are
measuring, the
greater the
magnetic field
produced by the
coil, and the
higher up the dial
the pointer moves.
MULTI-RANGE VOLTMETER
The range of the basic voltmeter can be
extended by using number of multipliers
and a selector switch. Such a meter is
called multi-range voltmeter.
NOTES
When using a voltmeter we
have three points to take into
consideration:
1.Polarity
2.Sensitivity
3.Loading effect
POLARIT Y
The analog voltmeter indicates polarity by the
moving direction of the needle (CW or CCW).
while the digital voltmeter indicates polarity
by the sign of the numerical value displayed
on the LED.
SENSITIVIT Y
The sensitivity is known as “ohms-per-volt”
rating of the voltmeter and is given by:
S=1/(full scale current)
 Voltmeters with low sensitivity may give
correct readings when measuring voltages in
low resistive circuits, but unreliable readings
in high resistive circuits.
LOADING EFFECT
The voltmeter will have an internal resistance
(R M).
when measuring voltage we connect a resistance
in parallel with the component concerned which
changes the resistance in the circuit and
therefore changes the voltage we are trying to
measure.
this effect is known as loading.
 How to minimize the loading effect?