Class:
E
Name:
18e
(
)
Date:
18e
Measuring voltage ratio of a transformer
with CRO
Apparatus
You will use the CRO to measure the voltage ratio of a transformer
and compare it with the turns ratio.
• 1 pair of double
C-cores with clip
• 1 10-turn coil
• 1 25-turn coil
• 1 low voltage
power supply
(d.c./a.c.)
• 1 CRO
Procedure
1
(a) Make up a transformer consisting of a 10-turn (primary) coil, a
25-turn (secondary) coil, a pair of double C-cores and clip.
(b) Connect one lamp each to the primary and secondary coils.
(c) Apply 1 V a.c. from a low voltage power supply across the
10-turn coil (Fig 18e-1).
power supply
secondary coil
(25 turns)
primary coil
(10 turns)
time base switched off
Fig 18e-1
Compare the brightness of the two lamps.
The lamp connected to the secondary coil is brighter than the lamp connected to the
primary coil.
2
(a) Connect the CRO across the primary coil (10-turn coil).
(b) Switch off the time base.
(c) Measure the length of the vertical trace on the screen.
© Oxford University Press 2006
New Physics at Work (Second Edition)
45
18e
Note: If a dual-trace
CRO is used, connect
channel 1 across the
10-turn coil and
channel 2 across the
25-turn coil to display
the primary and
secondary waveforms
at the same time.
The specimen results
were obtained with Y-gain
setting = 1 V/cm.
Class:
3
Name:
(
)
Date:
(a) Next connect the CRO across the secondary coil (25-turn coil).
(b) Measure the length of the vertical trace on the screen.
Results:
Length of (primary) vertical trace
lp =
3.0
cm
Length of (secondary) vertical trace
ls =
7.0
cm
Vp
lp
=
=
Vs
ls
2.33
Np
=
Ns
2.50
Voltage ratio of transformer
Turns ratio of transformer
Discussion
If the power supply is switched to 1 V d.c., what would happen to the
two lamps? Explain briefly.
The lamp connected to the secondary coil is off but the lamp connected to the primary
coil is on. It is because no current can be induced in the secondary coil if the current
in the primary coil is a smooth d.c.
For a.c. operation, compare the voltage ratio with the turns ratio of
the transformer. Account for any difference between the two ratios.
The voltage ratio is slightly smaller than the turns ratio. It is because the transformer
used is not a practical transformer with high efficiency.
The voltages in the primary and secondary coils of a transformer
depend on
turns ratio
of the coils.
For an ideal transformer:
primary voltage
=
secondary voltage
46
New Physics at Work (Second Edition)
number of turns in primary coil
number of turns in secondary coil
© Oxford University Press 2006