DC VOLTMETER Done By: Ala’ Al-Hiary Nidal Al-Hamad

The Volt Volt: is the SI derived unit for electric potential electric potential difference electromotive force.

History of the Volt The volt is named in honor of the Italian physicist Alessandro Volta (1745–1827), who invented the voltaic pile, possibly the first chemical battery

voltaic pile is a set of individual Galvanic cells placed in series. the first electric battery invent ed by Alessandro Volta in 1800

What is voltage??? Volta characterized the emf of a pair of metals in terms of the difference in their voltages, which he could measure

VOLTMETER A voltmeter is an instrument used for measuring electrical potential difference (voltage) between two points in an electric circuit.

How does is work??? When an electric current is applied, the galvanometer's indicator rotates and compresses a small spring. The angular rotation is proportional to the current through the coil.

The connection The voltmeter must be connected in parallel with the element we want to measure the voltage across.

Types of Voltmeters: Analog Voltmeter Digital Voltmeter

Analog Voltmeters Analog voltmeters move a pointer across a scale in proportion to the voltage of the circuit.

Analog Voltmeters

Digital Voltmeters Digital voltmeters give a numerical display of voltage by use of an analog to digital converter. ADC

Digital Voltmeters

DC Voltmeter Equivalent Circuit RsRs RmRm ImIm V + -

RsRs RmRm ImIm V + - >> A moving coil galvanometer can be used as a voltmeter by inserting a resistor in series with it. >> R s limits the current through the coil not to exceed the full scale deflection current.

DC Voltmeter Equivalent Circuit RsRs RmRm ImIm V + - >> R s is relatively large so that the voltmeter does not affect the current to be measured.

DC Voltmeter Equivalent Circuit RsRs RmRm ImIm V + - Im : deflection current of the movement. Rm : internal resistance of the movement. V : the full range voltage of the voltmeter.

DC Voltmeter Equivalent Circuit RsRs RmRm ImIm V + - V = RI V = (Rs + Rm)Im Rs = (V – ImRm)/Im Rs = V/Im – Rm

Multi-Range Voltmeter >> Here, we use a number of resistors in order to control the voltage range. RmRm ImIm

Multi-Range Voltmeter

R1R1 RmRm ImIm V1V1 + - V2V2 V3V3 V4V4 R2R2 R3R3 R4R4

0 to V1 Range R1 RmRm ImIm + - R2 R3 R4 R1 Rm Im + -

0 to V2 Range RmRm ImIm + - R2R2 R1R1 R3R3 R4R4

R2R2 RmRm ImIm + -

0 to V3 Range RmRm ImIm + - R3R3 R1R1 R2R2 R4R4

R3R3 RmRm ImIm + -

Multi-Range Voltmeter R1R1 RmRm + - R2R2 R3R3 R4R4 V1V1 V2V2 V3V3 V4V4

0 to V4 Range RmRm + - R4R4 R2R2 R3R3 R1R1 V1V1 V2V2 V3V3 V4V4

0 to V3 Range RmRm + - R3R3 R4R4 V3V3 R1R1 R2R2 V1V1 V2V2 V4V4

0 to V2 Range RmRm + - R2R2 R3R3 R4R4 V2V2 V3V3 R1R1 V1V1 V4V4

0 to V1 Range R1R1 RmRm + - R2R2 R3R3 R4R4 V1V1 V2V2 V3V3 V4V4

Sensitivity of Voltmeters >> The sensitivity of a voltmeter is defined as the inverse of the full scale deflection of the element. S = 1 / (I full scale)

Sensitivity of Voltmeters RsRs RmRm ImIm + - >> Using “S” we can determine the value of Rs, as follows : R total = S*V R total = Rs + Rm Rs = S*V - Rm

Example A galvanometer with internal resistance of (200 ohm) and maximum current (500 uA) to be converted to a multi-range voltmeter with the following voltage ranges : 1)0 to 100 mV 2)0 to 1 V 3)0 to 10 V 4)0 to 100 V

Solution R1R1 RmRm + - R2R2 R3R3 R4R4 S = 1/(I full scale) = 1/500 uA = 2000 ohm/V >>Range (0 to 100 mV) : R4 = Rs = S*V – Rm = 2000*0.1 – 200 = 0 R4 is S.C.

Solution R1R1 RmRm + - R2R2 R3R3 R4R4 >> Range (0 to 1 V) : Rs = R3 + R4 R3 + R4 = S*V – Rm = 2000*1 – 200 = 1800 ohm R4 = 0, so : R3 = 1800 ohm = 1.8k

Solution R1R1 RmRm + - R2R2 R3R3 R4R4 >> Range (0 to 10 V) : Rs = R2 + R3 + R4 So: R2 + R3 + R4 = S*V – Rm = 2000 * 10 – 200 = ohm R2 = – 1800 R2 = ohm = 18k

Solution R1R1 RmRm + - R2R2 R3R3 R4R4 >> Range (0 to 100 V) : Rs = R1 + R2 + R3 + R4 So: R1+ R2 + R3 + R4 = S*V – Rm = 2000 * 100 – 200 = ohm R1 = – R1 = ohm = 180k

Solution 1.8 k RmRm k 180 k R1 = 180 k R2 = 18 k R3 = 1.8 k R4 = 0 (i.e. S.C.)

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