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.

An ammeter that is extended by a number of shunts selected by a “range switch” 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.

Universal-Ayrton Shunt : This type eliminates the possibility of having the ammeter without a shunt.

......... (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