The article discusses the role of transformers in electronic power supply, focusing on how they step voltage up or down through the turns ratio between primary and secondary windings. It also explains the relationship between voltage, current, and resistance in transformer operation, with practical examples for better understanding.
The energy source of an electronic power supply is alternating current. In most systems, the AC power line supplies this. Most small electronic systems use 120-V, single-phase, 60-Hz AC as their energy source. This energy is readily available in homes, buildings, and industrial facilities. Electronic power supplies rarely operate with AC obtained directly from the power line.
The first component of an electronic power supply is called the Transformer, which is the component used to step the line voltage up or down to a desired value. This article discusses the block diagram, voltage output conventions, and turn ratio for general purpose power supply transformers.
Block Diagram
The Block Diagram of General-Purpose Electronic Power Supply is given in Figure 1. Whereas, the structure of a transformer used in such circuits is given in Figure 2.
Figure 1. Block Diagram of a General-Purpose Electronic Power Supply.
Figure 2. Structure of Transformer
The coil on the left of the symbol is called the primary winding. The AC input is applied to this winding. The coil on the right side is the secondary winding. The secondary winding develops the output of the transformer. The parallel lines near the center of the symbol represent the core. In a transformer of this type, the core is usually made of laminated soft steel. Both windings are placed on the same core. Power supply transformers may have more than one primary and secondary winding on the same core, which permits them to accommodate different line voltages and to develop alternate output values.
Conventions for Output Voltages
The output voltage of a power supply transformer may have the same polarity as that of the input voltage, or it may be reversed. This depends on the winding direction of the coils. A dot is often used on the transformer symbol to indicate the polarity of a winding, as shown in Figure 3.
Figure 3. Dot Convention of Transformer
The polarity of the input and output is the same when the dots are in adjacent locations on the transformer symbol. The polarity of the input and output is opposite when the dots are in opposite corners on the transformer symbol.
Turns Ratio
The output voltage developed by a transformer primarily depends on the turns ratio of its windings. The turns ratio is the ratio of the number of turns in the primary winding to the number of turns in the secondary winding. It is represented by the following formula:
$$\frac{N_{pri}}{N_{sec}}$$
The relationship of the turns ratio to voltage is
$$\frac{N_{pri}}{N_{sec}}=\frac{V_{pri}}{V_{sec}}$$
For example, if the primary winding of a transformer has 500 turns of wire and the secondary has 1000 turns, the transformer has a 1:2 turns ratio.
$$\frac{N_{pri}}{N_{sec}}$$
$$\frac{500}{1000}$$
1 : 2
This type of transformer steps up the input voltage by a factor of 2. With 120 $V_{RMS}$ applied from the power line, the output voltage is approximately two times the input voltage (2 × 120 V).
$$\frac{N_{pri}}{N_{sec}}=\frac{V_{pri}}{V_{sec}}$$
$$\frac{1}{2}=\frac{120V}{V_{sec}}$$
$$1 \times V_{sec}=2 \times 120V$$
$$V_{sec}= 240V$$
Example 1
If the primary winding has 1000 turns of wire, and the secondary has 50 turns of wire, what is the turns ratio of the transformer?
Solution
$$\frac{N_{pri}}{N_{sec}}$$
$$\frac{1000}{50}$$
20 : 1
Example 2
If a transformer has a turns ratio of 20:1, and 120 V is applied from the power line, what is the output voltage and current?
Solution
$$\frac{N_{pri}}{N_{sec}}=\frac{V_{pri}}{V_{sec}}$$
$$\frac{20}{1}=\frac{120V}{V_{sec}}$$
$$20 \times V_{sec}=120V$$
$$\frac{20 \times V_{sec}}{20}=\frac{120V}{20}$$
$$V_{sec}= 6V$$
It is interesting to note that the current capability of a transformer is the reverse of its turns ratio. This is represented by the following formula:
$$\frac{V_{pri}}{V_{sec}}=\frac{I_{sec}}{V_{pri}}$$
A 1:2 turns ratio steps up the voltage and steps down the current capability. Therefore, 1 A of primary current is capable of producing only 0.5A of secondary current:
$$\frac{N_{pri}}{N_{sec}}=\frac{I_{sec}}{I_{pri}}$$
$$\frac{1}{2}=\frac{I_{sec}}{1A}$$
$$2 \times I_{sec}=1A$$
$$\frac{2 \times I_{sec}}{2}=\frac{1A}{2}$$
$$I_{sec}=0.5A$$
The resistance of the wire is largely responsible for this condition. A 1:2 turns ratio has twice as much resistance in the secondary as it has in the primary winding. An increase in resistance, therefore, causes a decrease in current. This is all based on the wire size of the two windings being the same.
Power supplies for solid-state systems generally develop low-voltage values. Transformers for this type of power supply are designed to step down the line voltage. A turns ratio of 10:1 is very common. With 120 V applied to the input, the output will be approximately 12 V (120 V / 10). The output current capabilities of this transformer are increased by a ratio of 1:10. This means that a step-down transformer has a low-voltage output with a high-current capacity. Supplies of this type are well suited for solid-state applications.
Transformers in Electronic Power Supply Review Questions
- The winding develops the output of the transformer.
- The winding develops the input of the transformer.
- The number of turns in the primary winding to the number of turns in the secondary winding is called the .
- A 1:10 turns ratio steps up the and steps down the capability.
- A 10:1 turns ratio steps up the and steps down the capability.
- The output voltage of the transformer with a turns ratio of 10:1 and 120V applied from the power line is V.
- The secondary current of a transformer that has as a turns ratio of 10:1 and 6A applied to the primary is A.
Answers
- secondary
- primary
- turns ratio
- voltage, current
- current, voltage
- 12
- 60
Transformers in Electronic Power Supply Key Takeaways
Understanding transformer operation is essential in designing electronic power supplies, especially for adapting AC input to the voltage and current requirements of various electronic systems.