DC Meters

still in Chapter 02

Warm-ups

• stand-up 3x.• sit down and relax.• listening to a song by Halim Yazid titled:

“ingat sokmo hok mok pesan”.• take a deep breath.

Objectives

At the end of this chapter, the students should be able to: explain the meaning of electrical meters. explain the purpose of shunts across a

meter. calculate the value of shunt resistors

across a meter.

Outlines

• Introduction: What is a meter?.

• Pmmc in a DC Ammeter.

• Ammeter Insertion Effects.

Introduction

What are meters?• Meters are used to measure current and voltage. • The meter most likely encountered will be a

single low range meter such as 0 - 1 mA full deflection meter of the D'Arsonval type.

• The d'Arsonval type meter works on the principle a coil of wire to which a pointer is attached is pivoted between the poles of a permanent magnet.

Introduction

What are meters?

• When current flows through the coil, it sets up a magnetic field that interacts with the field of the magnet to cause the coil to turn.

• The meter pointer deflects in direct proportion to the current. This meter is called an ammeter.

Introduction

Figure 1: A typical 0 to 1mA ammeter.

DC Ammeter

DC Ammeter

• a device used to measure current.

• put in series/ parallel with the circuit.

• very common in lab.

• use unit Ampere (A)/ mA.

• used the principle of the d’Arsonval meter movement with slight modification.

DC Ammeter

DC Ammeter

• placing a LOW resistance in PARALLEL with the meter movement resistance to increase the range of current that can be measured by the meter.

• Rsh = shunt resistor.

• Rm = meter movement resistance.

DC Ammeter

Figure 2: d’Arsonval used in DC Ammeter circuit.

• Ish – current tru the shunt.• Im – full-scale deflection current of meter movement.• I – full-scale deflection current for the ammeter.

DC Ammeter

Figure 2: d’Arsonval used in DC Ammeter circuit. (single range only).

• Vm = Im*Rm

• Vsh = Vm

• Ish = I – Im

• Rsh = Vsh /Ish

= Im*Rm /Ish

= Im/(I -Im)*Rm Ω

DC Ammeter

Example 2-1

Calculate the value of the shunt resistance required to convert a 1-mA meter movement, with a 100-Ω internal resistance, into a 0- to 10-mA ammeter.

DC Ammeter

Example 2-2

A 100-A meter movement with an internal resistance of 800-Ω is used in 0- to 100-mA ammeter. Find the value of the required shunt resistance.

Ayrton Shunt

• Also known as the universal shunt.

• Used on a multiple –range ammeter.

• It eliminates the possibility of the meter movements being in the circuit without any shunt resistance.

• It maybe used with a wide range of meter movements.

Ayrton Shunt

Rb

Rm

Im

I

RaRc

I1

I2

I3

I - Im

+ -

1A

5A

10A

Fig. 3: An Ammeter using Ayrton shunt.

Ayrton Shunt

Rb

Rm

Im

I

RaRc

I1

I2

I3

I - Im

+ -

1A

5A

10A

Fig. 3: An Ammeter using Ayrton shunt.

From figure;

(Rb+Rc)(I2-Im) = Im (Ra+Rm)

I2(Rb+Rc) - Im(Rb+Rc) = Im [Rsh-(Rb+Rc) + Rm)

I2(Rb+Rc) - Im(Rb+Rc) = ImRsh- Im(Rb+Rc) + ImRm.

Thus;Rb+Rc= Im(Rsh+Rm) / I2 (1)

Ra = Rsh – (Rb+Rc)(2)

Rc = Im(Rsh+Rm) / I3 (3)

Ayrton Shunt

• Remember:

n = I / Im

Rsh = Rm / (n-1) ; n ≥ 20.

Ayrton Shunt

Rb

Rm

Im

I

RaRc

I1

I2

I3

I - Im

+ -

1A

5A

10A

Fig. 3: An Ammeter using Ayrton shunt.

Example 2.3:Compute the value of the shunt resistors for the circuit. Given that Rm = 1kΩ, Im = 100 A, I1=10mA, I2=100mA, I3=1A.

Check :

Rsh = Ra + Rb + Rc always!

Ammeter Insertion Effects

• A frequently overlooked source of error in measurements.

• All ammeters contain some internal resistance.• inserting an Ammeter in a circuit always

increases the resistances of the circuit, therefore reduces the measured current in the circuit.

• The error caused by the meter depends on the value of resistances in Ammeter and also the measured circuit.

Ammeter Insertion Effects

R1

E

Fig.4a: Expected current in a simple circuit.

I = E / R1

R1

E

Rm

Fig.4b: Expected current in a series circuit with an ammeter.

I = E / (R1+Rm)

Ammeter Insertion Effects

Example 2.4:A current meter having an internal resistance of 88Ω is used to measure the current trough Rc in figure below. Determine the percentage of error due to the ammeter insertion.Given that Ra=Rb=Rc= 1kΩ

Ra

E = 5V Rb Rc

Summary

In this chapter, we have discussed about :

• introduction to electrical meters.

• shunt resistor in a single-range Ammeter.

• universal shunt in multiple-range Ammeter.

• calculation of shunt resistors.

• ammeter insertion effects.

Conclusion

• Also known as the universal shunt.

• Used on a multiple –range ammeter.

• It eliminates the possibility of the meter movements being in the circuit without any shunt resistance.

• It maybe used with a wide range of meter movements.

Evaluation

• Quiz.