Transformers Objectives Explain mutual inductance Describe how a transformer is constructed and how it works Explain how a step-up transformer works Explain how a step-down transformer works Discuss the effect of a resistive load across the secondary winding Discuss the concept of a reflected load in a transformer Discuss impedance matching with transformers Explain how the transformer acts as an isolation device Describe a practical transformer 13 February 2004 Transformers 2 Professor Andrew H. Andersen 1
Coefficient of Coupling The coefficient of coupling (k) between two coils is the ratio of the lines of force (flux) produced by one coil linking the second coil (φ 1-2 ) to the total flux produced by the first coil (φ 1 ) The coefficient of coupling depends on the physical closeness of the coils and the type of core material on which they are wound Construction and core shape are also factors influencing coefficient of coupling 13 February 2004 Transformers 3 Mutual Inductance When two coils are placed close to each other, a changing electromagnetic field produced by the current in one coil will cause an induced voltage in the second coil because of the mutual inductance Mutual inductance is established by the inductance of each coil and by the amount of coupling between the two coils. To maximize coupling, the two coils are wound on a common core 13 February 2004 Transformers 4 Professor Andrew H. Andersen 2
Examples of Transformers with Different Cores 13 February 2004 Transformers 5 Transformer Packages 13 February 2004 Transformers 6 Professor Andrew H. Andersen 3
Source voltage is applied to the primary winding The load is connected to the secondary winding The core provides a physical structure for placement of windings and a magnetic path so that the magnetic flux lines are concentrated close to the coils The Basic Transformer 13 February 2004 Transformers 7 Types of Transformer Cores Typical core materials are: air, ferrite, and iron Air and ferrite cores are used at high frequencies Iron cores are used for AF and power applications 13 February 2004 Transformers 8 Professor Andrew H. Andersen 4
Operation of a Transformer 13 February 2004 Transformers 9 Direction of Windings The direction of the windings determines the polarity of the voltage across the secondary winding with respect to the voltage across the primary Phase dots are used to indicate polarities 13 February 2004 Transformers 10 Professor Andrew H. Andersen 5
Turns Ratio Turns ratio (n) is defined as the ratio of the number of turns in the secondary winding (NSEC) to the number of turns in the primary winding (NPRI) N n = N SEC PRI 13 February 2004 Transformers 11 Step-up Transformers A transformer in which the secondary voltage is greater than the primary voltage is called a step-up transformer The ratio of secondary voltage (VSEC) to primary voltage (VPRI) is equal to the ratio of the number of turns in the secondary winding (NSEC) to the number of turns in the primary winding (NPRI) V SEC N SEC = V PRI N PRI N SEC V SEC = V PRI N PRI 13 February 2004 Transformers 12 Professor Andrew H. Andersen 6
Step-Down Transformer A transformer in which the secondary voltage is less than the primary voltage is called a step-down transformer The amount by which the voltage is stepped down depends on the turns ratio The turns ratio of a step-down transformer is always less than 1 13 February 2004 Transformers 13 Transformers 13 February 2004 Transformers 14 Professor Andrew H. Andersen 7
Loading the Secondary Winding When a load resistor is connected to the secondary winding, there is a current through the resulting secondary circuit because of the voltage induced in the secondary coil This secondary current causes a current in the primary I SEC N PRI = I PRI N SEC N I SEC = I PRI N PRI SEC 13 February 2004 Transformers 15 Impedance Matching When a source is connected to a load, maximum power is delivered to the load when the load impedance is equal to the fixed internal source impedance One application of transformers is in the matching of a load resistance to a source resistance in order to achieve maximum transfer of power This is termed impedance matching Transformers designed specifically for impedance matching usually show the input and output impedance they are designed to match 13 February 2004 Transformers 16 Professor Andrew H. Andersen 8
Matching Transformer The reflected-resistance characteristics provided by a transformer are used to make the load resistance appear to have the same value as the source resistance 13 February 2004 Transformers 17 DC Isolation A transformer does not pass dc, therefore a transformer can be used to keep the dc voltage on the output of an amplifier stage from affecting the bias of the next amplifier The ac signal is coupled through the transformer between amplifier stages 13 February 2004 Transformers 18 Professor Andrew H. Andersen 9
Power Line Isolation Transformers are often used to electrically isolate electronic equipment from the ac power line 13 February 2004 Transformers 19 Utility Distribution 13 February 2004 Transformers 20 Professor Andrew H. Andersen 10
Multiple-Winding Transformers Multiple-winding transformers have more than one winding on a common core. They are used to operate on, or provide, different operating voltages 13 February 2004 Transformers 21 Multiple Winding Transformers 13 February 2004 Transformers 22 Professor Andrew H. Andersen 11
Multiple Winding Transformers 13 February 2004 Transformers 23 Auto Transformers In an autotransformer, one winding serves as both the primary and the secondary. The winding is tapped at the proper points to achieve the desired turns ratio for stepping up or down the voltage 13 February 2004 Transformers 24 Professor Andrew H. Andersen 12
Transformer Losses 13 February 2004 Transformers 25 Professor Andrew H. Andersen 13