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Reply To: Transformer Riddle No.60 – Various types of transformers and their uses


    Matching transformer Electronics practitioners versed in both audio and radio frequency techniques are familiar with impedance-matching applications of transformers. Generally, the objective is to convert a resistance to a higher value in a step-up transformer or to convert a resistance to a lower value in a step down transformer. In the first case, the converted value of resistance will be greater than that associated with the primary by the square of the secondary to primary turns ratio. In the second case the converted resistance will be less than that associated with the primary by the ratio of the secondary to primary turns, squared. More generally, impedances are also convertable in this manner with transformers. Moreover, the conversion can be accomplished with autotransformers as well as with conventional two winding transformers. A familiar impedance transformation takes place in the output transformer used to match the output impedance of an audio output stage to the inordinately low voice-coil impedance of a dynamic speaker via a step-down transformer. An interesting application of impedance transformation is encountered with single-phase a.c. induction motors in which a large capacitor is needed to split the phase of the applied line voltage so that the motor starts essentially as a two-phase motor. Unless something of this nature is done, the motor will not develop starting torque. Once under acceleration, the capacitor is cut out of the circuit by a centrifugal switch. A practical problem arises because of the large capacitance required; because of cost and physical size, such a starting capacitor usually has to be an electrolytic type. These can be marginally satisfactory, but tend to have adverse ageing characteristics, high leakage current, sloppy capacitance values, limited temperature tolerance and other manifestations of unreliability. Avoiding the need for the electrolytic starting capacitor is a worthwhile design objective. Isolation Transformers An isolation transformer is designed to specifically address the problems associated with referencing its internal shields to ground. It is constructed with two isolated Faraday shields between the primary and secondary windings. When properly installed, the shield, which is closest to the primary winding, is connected to the common power supply ground and the shield closest to the secondary winding is connected to the shield of the circuit to be isolated. The use of two shields in the construction of the isolation transformer diverts high frequency noise, which would normally be coupled across the transformer to the grounds of the circuit in which they occur. The two shields provide more effective isolation of the primary and secondary circuits by also isolating their grounds. The isolation transformer adds a third capacitance between the two Farady shields, which may allow coupling of high frequency noise between the system grounds. However, increasing the separation between the two Faraday shields normally minimizes this third capacitance. Additionally, the dielectric effect of the shields plus the increased separation of the windings significantly reduce the inter-capacitance between the windings. An equivalent circuit for an isolation transformer is presented in Figure below. R1 = Resistance in Primary Windings R 2 = Resistance in Secondary Windings L1 = Primary Inductance Which Creates Leakage Flux L2 = Secondary Inductance Which Creates Leakage Flux M = Mutual Transformer Inductance C1 = Capacitance Between Primary Windings and Primary Shield C2 = Capacitance Between Secondary Winding and Secondary Shield C12 = Capacitance Between Primary and Secondary Shields Transformer with zig-zag connection: The interconnected-wye-wye connections have the advantages of the star