Dc Meter Instrumen-2

DIRECT CURRENT METER

ByM. Rahmad

Physics EducationFKIP UR

2013

Meter DC, by M. Rahmad 1

DC Voltmeter

Fig 1 Simple DC voltmeter

+

-

Rs Im

Rm

description:Im = Current max d’arsonval movementRs = Multiplied resistanceRm = internalresistanceV = Voltage .

V

Voltage V = Im (Rs+Rm)

and the multiple resistance mmm

mms R

I

V

I

RIVR


Multi Range VoltmeterWe can be designed with the addition of a multiplier resistance with the switch range approprite. Example 1: A d’Arsonval movements with Rm = 100 Ohm, Im = 1 mA. Determine multipler resistance to make multi range Voltmeters according to 0-10 V, 0-50 V, 0-250 V, 0-500 V.


Fig.2 Multi range Voltmeter

+

-

V4

Im

Rm

V1

V3V2

R1 R2 R3R4


Solution:


K10mA1

V104

mI

VR K9,91,0104 mRRR

K50mA1

V503

mI

VR K401050)( 43 RRRR m

K250mA1

V2502

mI

VR K20050250)( 432 RRRRR m

K500mA1

V5001

mI

VR K250250500)( 4321 RRRRRR m

#for range voltage0-10 V di V4

# for range voltage 0-50V di V3

# for range voltage n 0-250V di V2

# for range voltage n 0-500V di V1

and

ad

and

and

The alternative to determine voltmeter multiplier resistance for multi range, we can use voltmeter sensitivity methode : follow the equation

Where Rs = multiplier resistance

S = Sensitivity or 1/Idp

V = Range of voltage Rm = internal resistance


ms RSR )V(

Fig 3. Two range VoltmeterMeter DC, by M. Rahmad 7

Voltmeter loading effectThe sensitivity of a DC voltmeter is an important factor when the selecting a meter for a certain voltage measurement. A low sensitivity may give correct reading when measuring voltage in low resistance circuits, but it is certain to produce very unrealible readings in high resistance circuits. So measuring voltage influence the voltmeter sensitivity. For low sesntivity caused loading effect. Example 2:The circuit from beside figure measured with two voltmeter and have sensitivity first 1000 Ohm/V and second 20 Kohm/V. Compute the each voltmeter reading and error reading

V

+

-

50K150V

100 K

R1

Rm

R2


Solution:Through the proper voltage on the calculation resistance R = 50 KΩ is

If using voltmeter wiht S = 1000/V, for 50 V range, voltmeter sensitivity is

Sv= 50 V1000/V = 50 KΩ. So that

and

V50V150K150

K50

K25K50K50

K50K502

R

30VV150K125

K25V


If we using voltmeter with sensitivity s = 20K/V, so if the rang 50 V, the resistance = 50 V20/V = 1 M.

so , and we find

What the error percent?

K6,47K50M1

K50M12

R

V36,48V150K6,147

K6,47V


1. Use the correct polarity2. connect the parallel to the circuit to be measured3. use range from high to appropriate,4. Be careful the effects of the loading

How to use voltmeter?


OHMMETER

Fig 4. Ohmmeter type


The d’Arsonval meter movement may also be used in conjunction with a battery and resistor to construct a simple ohmmeter circuit such as fig 5.If point x and y are connected, we have a simple series circuit with current through the meter movement coused by voltage source V.

The resistor Rz consists of a fixed portion and a variable portion. Cocnnecting point x and y is equivalent to shorting the test probes together on an ohmmeter to zero the instrument before using it. After points x and y are connected the variable part of resistor Rz is adjusted to obtain exactly full scale deflection on the meter movement.


Figure 5. Basic Ohmmeter circuitMeter DC, by M. Rahmad 14

Rm

Im

0,9Rz 0,1Rz

Rz

x y

Rx

The amplitude of the current thruogh the meter movement can be determined by applying Ohm’s law as

between points x and y. If we connect the unknown resistorbetween points x znd y, the circuit current is now expressed as


The current I less than the current Im, because ot the additional resistance Rx. The ratio of I to Im is equal

If we let P represent the ratio of I to Im, we can say that


ExampleA 1 mA full scale deflection current meter movement is to be used in an ohmmeter circuit. Internal resistance 100 Ohm and a 3 volt battery will be used in the circuit. Markof the meter face


SolutionThe value of Rz which will limit current to full scale deflection current is

=2,9k

The value of Rx with 20% full scale deflection is

We will find for the other Rx are:For 40% = 4,5k, 50% = 3k, 75%= 1k


We can draw the ohmmeter scale, shown in fig.6

Fig. 6 Ohmmeter scaleMeter DC, by M. Rahmad 19

0

3k

100%

75%50%40%

20%

0%

1k4,5k

12k

We find two interesting and important facts may be seen from scale ohmmeter. First, the ohmmeter scale is non linier. This is due to the high internal resistance of an ohmmeter. Second, at half scale deflection, the value of Rx is equal to the value of the internal resistance and Rz of an ohmmeter. Figure 7. Multiple range ohmmeter


Rm =2K

Im

0,9Rz 0,1Rz

Rz =28K

x y

Rx

10 Ohm

100 Ohm

1 K

Example 3a) In fig. 7 if we want to use measure Rx= 20 on the Rx10 range, V=1,5 volt. Determine the current through the meter.b) Show that this same current flows through the meter movement when a 200 resistor is measured on the Rx10 range.

SolutionWhen the ohmmeter is cis set on the Rx1 range, the voltage across the potential combination of resistance is culate as


The currnt through the meter is

When the ohmmeter is set on the Rx10 range, the voltage across the potential combination of resistance is culculate as


The currnt through the meter is

The Multimeter

Fig. 4. Multimeter SUNWA code YX-360TRNMeter DC, by M. Rahmad 23

The multimeter or volt-ohmmeter-miliammeter (VOM) is such an instrument. It is a general purpose test instrument that has the necessary circuit to meassure ac and dc voltage, direct current, and resistance.

Parts of multimeter1. Pointer (to determine the scale )2. Zero calibration (0 V and 0 mA)3. Output additional terminal 4. Switch selektor range5. Negative Terminal (GND) COM/ for black probe6. Led indicator/Continuity7. scale8. Probe9. Zero ohm calibration10.Range of measurement11.Positive terminal


Multimeter care 1. Battery check periodically. 2. If the pointer does not move when the probe

connect of the resistance measurement range, check the fuse or probe, if broke replaced

3. Do not let the multimeter crashing or fall. 4. Use the measurement range appropriate to

measure electricalAplication ohmmeter1. To tests electrolytic capacitor2. for continuity checks3. to check diode, transistor


The EndThank you


Documents

Dc Meter Instrumen-2