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Transformers
Not the cartoon characters
What is a transformer?
A transformer is a device for converting one
voltage to another voltage.
Every time you plug a cell phone into a wall, you are
using a transformer, which “steps down” the voltage
from 120 volts to some lower voltage.
One use of transformers is in the transmission
of electrical power to your home.
Slide 26-20
Power to Your Home
A transformer on a pole
“steps down” the
voltage from 7200 volts
to the 120 volts used in
your home.
Diagram of a transformer
A transformer is
composed of 3 things
1) An iron core
2) A primary coil
3) A secondary coil
Diagram of a transformer
The primary coil has N1 (or NP)
turns (in this case, 4)
The secondary coil has N2 (or NS)
turns (in this case, 8)
The iron core transmits a
magnetic field, called “flux”
between the two coils.
The field is created just the
same way as an electromagnet,
when current flows in the
primary coil
What does a transformer do?
Suppose the primary coil
has a voltage (V1 or VP) of
120 volts in this case
The secondary coil will
have a voltage (V2 or VS) of
240 volts (also in this case)
Why is this so?
How does a transformer work?
There are twice as many
secondary coils (in this
case), so the voltage is
twice as high on the
secondary side.
Changing the ratio of
coils changes how much
the voltage changes.
Which means?
If there are MORE turns in
the secondary coil, then the
voltage goes up. This is a
“Step-Up” transformer.
If there are LESS turns in the
secondary coil, then the
voltage goes down. This is a
“Step-Down” transformer.
How can I calculate it?
The ratio of coils between
the secondary and
primary can be expressed
as a fraction, N2/N1, or
often NS/NP. (here, 8/4)
This ratio is always the
same as the ratio of the
voltages, V2/V1, or often
VS/VP.
Calculations - continued
If the ratios are always the same, then we can
set them equal to each other, like this:
N S VS

N P VP
In other words the ratio of turns is equal to the
ratio of voltages.
Calculations – continued 2
We can even solve this equation for each
variable
N S VS
NP

VP
For example, if we know the number of turns
and the primary voltage, we can calculate the
secondary voltage:
NS
VS 
VP
NP
Calculations – continued 3
Suppose we know the primary and secondary
voltage, and how many turns in the primary coil.
N S VS

N P VP
We can calculate the how many turns we need
in the secondary coil
VS
NS 
NP
VP
Checking Understanding
If the primary coil of wire on a transformer is kept the same and
the number of turns of wire on the secondary is increased, how will
this affect the voltage observed at the secondary?
A. The voltage will increase.
B. The voltage will stay the same.
C. The voltage will decrease.
Answer
If the primary coil of wire on a transformer is kept the same and
the number of turns of wire on the secondary is increased, how will
this affect the voltage observed at the secondary?
A. The voltage will increase.
B. The voltage will stay the same.
C. The voltage will decrease.
Example Problem
A 120 V AC power supply steps down the voltage to
12 volts for a cell phone charger. If there are 40
turns in the primary coil, how many turns must be in
the secondary coil?
Let’s look at the equation:
N S VS

N P VP
Now solve for secondary turns, NS
VS
NS 
NP
VP
Example Problem (continued)
A 120 V AC power supply steps down the voltage to
12 volts for a cell phone charger. If there are 40
turns in the primary coil, how many turns must be in
the secondary coil?
VS
NP
From here, we substitute: N S 
VP
12 volts
NS 
(40 turns )
120 volts
NS = 4 turns
Would another ratio work?
Of course. As long as Ns/Np = 12/120 = 0.1
Considerations
We discussed how a Step-Up transformer causes
the voltage to go up, but what happens to the
current?
Power is equal to current times voltage ( P = I x V)
(remember V = I x R ??)
A transformer can never increase the power (or
energy) because of the law of conservation of
energy.
This means that the power going in must equal the
power going out.
Considerations (continued)
That means if voltage goes up, current must
go down. If voltage goes down, the current
goes up.
We can calculate it like this: PS = PP
(secondary power = primary power)
So, ISVS = IPVP
You already know about current and voltage
calculations, so this is easy!
Just remember – Voltage Up means Current
Down (and the opposite)
Now it’s your turn
Complete the problems on the
worksheet on your own.