Download Analog Meters 1

Analog Meters
1- DC Ammeters :
Shunt resistor ammeter :
Here, the basic movement is a PMMC
“permanent magnet movement coil ”
→ Galvanometer
The coil of movement can carry only very small
current.
In this case when large currents are to be measured it
is necessary to by base the major part of the current
through a resistance called “shunt resistance”
Multirange Ammeter:
Several values of shunt resistors and a rotary switch
are used to select the desired range of current to
measure. A make-before break rotary switch insures
that there is a shunt resistor connected at all times.
a- Aytron Shunt :
Is an ammeter that is extended by a number of shunts
selected by a “range switch” .
(This is called a multi range ammeter)
As can be seen, the resistors are connected in a way
that they look in parallel. Here, the switch can be
moved to any of the four shunts.
b- Universal Shunt :
This type eliminates the possibility of having the
ammeter without a shunt.
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......... (1)
.......... (2)
........ (3)
Notes:
- The larger the current, the value of the shunt
resistance may become very small.
- When using an ammeter you have to be aware of
the following:
1. Observe the correct polarity.
reversed polarity causes the movement to deflect
against the mechanical stop (which cause some
damage to the movement).
2. Never connect the ammeter across the element to
be measured.
Here, due to the low resistance of the ammeter, this
would draw a damaging high current and destroy
the movement.
Therefore, an ammeter must always be connected
in series with the element to be measured.
3. When use a multi range ammeter first use the high
current range, then decrease as required.
B.
Voltmeter sensitivity :
This sensitivity is known as “ohms-per-volt” rating the voltmeter .
When using a voltmeter :
1. observe the correct polarity
2. place the voltmeter across the element that its voltage
to be measured
3. when using a multi range voltmeter always use the
highest voltage range first
4. always be aware of the loading effect.
b. Shunt type Ohmmeter:
This type ohmmeter is good for low resistance measurement
Voltmeters, Ammeters and Ohmmeters :
All of these devices use the d’Arsonval movement the
difference between them is the setting of the circuit they use
Hence, the multimeter is the combination of an ammeter
voltmeter and an ohmmeter
Ohmmeter calibration :
A rough calibration of an ohmmeter may be performed by
measuring a standard resistance and noting the reading
Do this similar standard procedure with different resistance
values , This will allow one to obtain an indication of the
correct measurements