Measurements DC meters a concepts Ammeter

Measurements

1. 1
DC Meters
still in Chapter 02

2. 2
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.

3. 3
Outlines
•Introduction: What is a meter?.
•Pmmc in a DC Ammeter.
•Ammeter Insertion Effects.

4. 4
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.

5. 5
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.

6. 6
Introduction
Figure 1: A typical
0 to 1mA ammeter.

7. 7
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.

8. 8
DC Ammeter
DC Ammeter
•placing a LOW resistance in PARALLEL
with the meter movement resistance to
increase the range of currentincrease the range of current that can be
measured by the meter.
•R
sh = shunt resistor.
•R
m = meter movement resistance.

9. 9
DC Ammeter
Figure 2: d’Arsonval used
in DC Ammeter circuit.
• Ish – current through the
shunt.
• Im – full-scale deflection
current of meter movement.
• I – full-scale deflection
current for the ammeter.

10. 10
DC Ammeter
Figure 2: d’Arsonval used
in DC Ammeter circuit.
(single range only).
• V
m
= I
m
*R
m
• V
sh
= V
m
• I
sh
= I – I
m
•

R
sh
= V
sh
/I
sh
= I
m
*R
m
/I
sh
= I
m
/(I -I
m
)*R
m
Ω

11. 11
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.

12. 12
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.

13. 13
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.

14. 14
Ayrton Shunt
Rb
Rm
Im
I
RaRc
I1
I2
I3
I - Im
+ -
1A
5A
10A
Fig. 3: An Ammeter using Ayrton shunt.

15. 15
Ayrton Shunt
Rb
Rm
Im
I
RaRc
I1
I2
I3
I - Im
+ -
1A
5A
10A
Fig. 3: An Ammeter using Ayrton shunt.
From figure;
(R
b
+R
c
)(I
2
-I
m
) = I
m
(R
a
+R
m
)
I
2
(R
b
+R
c
) - I
m
(R
b
+R
c
) =
I
m [R
sh-(R
b+R
c) + R
m)
I
2(R
b+R
c) - I
m(R
b+R
c) =
I
m
R
sh
- I
m
(R
b
+R
c
) + I
m
R
m
.
Thus;
R
b
+R
c
= I
m
(R
sh
+R
m
) / I
2
(1)
R
a
= R
sh
– (R
b
+R
c
)(2)
R
c = I
m(R
sh+R
m) / I
3 (3)

16. 16
Ayrton Shunt
•Remember:
 n = I / I
m
 R
sh = R
m / (n-1)

17. 17
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
R
m = 1kΩ, I
m = 100 A,
I
1
=10mA, I
2
=100mA,
I
3
=1A.
Check :
Rsh = Ra + Rb + Rc
always!

18. 18
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.

19. 19
Ammeter Insertion Effects
R1
E
Fig.4a: Expected current
in a simple circuit.
I = E / R
1
R1
E
Rm
Fig.4b: Expected current
in a series circuit with an ammeter.
I = E / (R
1+R
m)

20. 20
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

21. 21
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.

22. 22
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.