Solid State Devices I

Benchmark Companies Inc

PO Box 473768

Aurora CO 80047

Transformers

Back

Objectives:• Definition of Transformer• Uses of a Transformer

•Step-Up•Step-Down•Isolator

Transformers

Back

Transformers

Back

Primary Secondary

NP NS

:

:

Symbol

A transformer typically has two separate windings with a Varying number of turns (N). They are the primary winding And Secondary winding

Transformers

Back

The two lines in the center of the two coils indicate this Transformer has a core. (No lines indicates no core)

Primary Secondary:

NP NS:

Symbol

Transformers

Back

There is another type of transformer called a center-tap transformer (photo). It has many uses in today’s electronics

Primary Secondary:

NP NS:

Symbol

Transformers

Back

The benefit of center-tap transformers is the availability of several voltage levels on the secondary winding.

Primary Secondary:

NP NS:

Symbol

Transformers

BackVirtually every electronic devices which uses AC has to convert the power to a usable level.

Transformers

Back

Primary Secondary:

Symbol

A transformer is a typically a pair of coils where ones magnetic field acts on the other

Transformers

Back

Primary Secondary:

Symbol

The flow of current through the primary causes current to flow in the opposite direction in the secondary.

Transformers

Back

Primary Secondary:

Symbol

The direction of current is designated by dots on either end of the windings.

Transformers

Back

RECALL current flows through the primary winding, a magnetic field is produced that causes a current to flow through the secondary winding.

Primary Secondary:

Symbol

NP NS:

Transformers

Back

The current flowing through the primary winding causes the current to flow in an opposite direction through the secondary.

Primary Secondary:

Symbol

NP NS:

Transformers

Back

The current direction is indicated by the dots on each coil. Having the dot on the same end tells us the output is in phase.

Primary Secondary:

Symbol

NP NS:

Transformers

Back

Output Voltage is directly proportional to the number ofTurns in the secondary winding (Ns) with respect to the Primary windings(Np). This is called the Turns Ratio.

Primary Secondary:

Symbol

NP NS:

Transformers

Back

The relationship is defined as follows:

Ep/Es = Np/Ns

Primary Secondary:

Symbol

NP NS:

Transformers

Back

Example: Primary winding has 20 turns and the Secondary Winding has 40 turns. The result is the input voltage is Increased by a factor of 2 . 10/V(out) = 20/40 >> 400/20 = V(out) = 20 volts. This is a Step-up Transformer

NP NS:

Transformers

Back

Likewise, if a Primary winding has 40 turns and the Secondary Winding has 20 turns. The result is the input voltage is decreased by ½. 10/V(out) = 40/20 >> 200/40 = V(out) = 5 volts. This is a Step-down Transformer

NP NS:

Transformers

Back

Sometimes, there is a need to isolate voltage source from the Circuit. This can be done by using a transformer with a 1:1 Turns ratio

NP NS:

Transformers

Back

NP NS:

You should understand that there is no net power lossIn an ideal transformer. That means that: Power (primary) = Power (secondary)

Ip*Ep = Is*Es

Transformers

Back

NP NS:

Moving Ip and Is to one side of the equal sign and Ep and Es to the other yields:

Ip/Is = Es/Ep (Inversely Proportional)

Current (in) Current (out)

Voltage (in)Voltage (out)

Transformers

Back

NP NS:

Transformers and the secondary circuit have a loading Effect on the source voltage that is proportional to the Turns ratio

Np/Ns x R1 = RL

Transformers

Back

Example: Primary winding has 20 turns and the Secondary Winding has 40 turns. If R1 = 10KOhms the Load Resistance (RL) = (20/40)x10k = 5kOhms. Note the difference in expected current needed to drive the circuit.

NP NS:

Transformers

Back

Summary on TransformersFormulas:

Vin/Vout = Np/Ns

Iin/Iout = Ns/Np

Pin = Pout

Np/Ns x R1 = RL

The Formulas above of are based on the assumption thatThe transformer is ideal and no power loss occurs in the

Transformer.

TransformersSummary

Back

Transformers take a waveform and scale the waveform according to the turns ratio of the primary coil with respect to the secondary coil. If the turns ratio is greater than 1 the waveform is increased, if the ratio is less than one, the waveform is decreased.

Sine waveinput

Scaled WaveformTransformer

End of Presentation

Back