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 things1) An iron core2) A primary coil3) A secondary coil

Diagram of a transformerThe 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 - continuedIf the ratios are always the same, then we can set them equal to each other, like this:

In other words the ratio of turns is equal to the ratio of voltages.

P

S

P

S

V

V

N

N

Calculations – continued 2We can even solve this equation for each variable

For example, if we know the number of turns and the primary voltage, we can calculate the secondary voltage:

P

S

P

S

V

V

N

N

PP

SS V

N

NV

Calculations – continued 3Suppose we know the primary and secondary voltage, and how many turns in the primary coil.

We can calculate the how many turns we need in the secondary coil

P

S

P

S

V

V

N

N

PP

SS NV

VN

Checking UnderstandingIf 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.

AnswerIf 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:

Now solve for secondary turns, NS

P

S

P

S

V

V

N

N

PP

SS NV

VN

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?

From here, we substitute:

NS = 4 turnsWould another ratio work?Of course. As long as Ns/Np = 12/120 = 0.1

PP

SS NV

VN

) 40( 120

12turns

volts

voltsNS

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 turnComplete the problems on the

worksheet on your own.