The following points highlight the nine main types of transformers used in electronic equipments. The types are: 1. Isolation Transformers 2. Current Transformers 3. Auto Transformers 4. Pulse Transformers 5. Audio Transformers 6. Radio Frequency Transformers 7. Intermediate Transformers 8. Rectifier Transformers 9. Constant Voltage Transformers.
Type # 1. Isolation Transformers:
In these transformers, the primary and secondary coils have the same number of turns, which means that the primary and secondary will have the same amount of voltage. This type of transformer is used to provide isolation between the primary circuit and the secondary circuit. It is often used where one of the circuits is grounded and other is not.
Service technicians frequently employ such transformers while working on DC receivers. The transformer isolates the chassis from the power line, reducing the possibility of accidental shock.
Type # 2. Current Transformers:
Current transformers are used for the accurate measurement of large alternating current. This can be achieved only when the transformers are properly designed and constructed and then utilised within their ratings.
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The current transformer must be designed so that the no load current is small compared with the transformer load current. Use of an impedance greater than rated will cause the voltage on the primary and the no-load current to increase. The load current will then be a smaller multiple of the no-load current, and the accuracy of the current transformer will be reduced. Therefore care must be taken in designing this transformer to obtain minimum ratio and phase errors.
It is important to keep the secondary of a current transformer loaded or shorted as very high destructive voltage can develop if it is operated with the secondary open circuited.
Type # 3. Auto Transformers:
In an auto transformer, the same turns used for both primary and secondary windings. If all the turns are used as the primary, as shown in Fig. 8.2(a), the transformer has a step down ratio. If all the turns are used as the secondary, as shown in Fig. 8.2(b), the transformer has step up ratio. Generally, the auto transformer is constructed with several fixed taps or a movable tap to permit variation of secondary voltage.
These transformers are used where saving of copper is required. They are smaller in size and weight and cheaper than an equivalent isolation transformer.
Type # 4. Pulse Transformers:
A pulse transformer is a special kind of wide band transformer, designed to accommodate rapid and decay of pulse times with minimum of distortion particularly of video pulses. A normal transformer may introduce distortion into a square pulse by slowing down the rise and decay times.
The electrical requirements of a pulse transformer are, first that the winding must be tightly coupled to minimize leakage inductance, and thereby to ensure that the output pulse will have a fast rise time and, second, that the insulation be good enough to provide the isolation required.
Type # 5. Audio Transformers:
Audio transformers are used to connect transducers to amplifiers, for inter-stage coupling, to change signal levels, to match impedance, to invert signal polarities and to provide electrical isolation.
Ideally audio transformers should have a flat response throughout the audio range. In practice however, the response tends to fall off at the low and high ends of audio spectrum because inductive reactance is small at these frequencies. This can be corrected by increasing primary turns which will increase reactance. However, the number of secondary turns must also be increased to maintain given voltage ratio. This stray capacitance results in the poor high frequency response, as these high frequencies are bypassed through the low shunt capacitive reactance.
Type # 6. Radio Frequency Transformers:
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Radio frequency transformers are usually air core types, but powdered iron cores may be used depending on the signal frequency. In general, step-up ratios are desirable because of the increased signal amplitude they provide. Skin effect losses and distributed capacitance become significant at radio frequencies.
These effects can be minimized by the use of a special ‘Litz wire’. RF transformers are more often shielded, to reduce the undesirable coupling and feedback effects of stray capacitance. RF transformers are used for inter-stage coupling and for coupling a signal to or form an antenna.
Type # 7. Intermediate Transformers:
Intermediate transformers are similar to RF transformers but are designed to operate at a fixed or intermediate frequency equal to the difference between the RF signal and local oscillator frequency in super heterodyne receivers.
Type # 8. Rectifier Transformers:
Rectifiers are among the most common applications for transformers. They increase or decrease ac voltages for rectification, filtering etc. They operate from a narrow band of sine wave frequencies or from a fixed wave form. The transformer secondary voltage and current ratings are calculated from the output dc requirements.
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The main objective is thus establishing the required dc voltage at the required current most optimally. They are used in traction devices, electroplating battery chargers, electronic and communication system, computer power supplies and X-ray equipment.
Type # 9. Constant Voltage Transformers:
It is essentially, a saturable device which maintains a constant output voltage irrespective of the changes in the magnitude of the input voltage. It differs from a conventional transformer by having large leakage inductance, a saturating shunt inductance, and a large capacitance in parallel with the load.