In transformer power supplies, the voltage is converted by the transformer. The transformer consists of a core and a primary and secondary winding wound around the core, usually made of a copper wire. The mains voltage is reduced to the required level by electromagnetic induction, i.e. permeation of the magnetic field between the primary and secondary winding. The windings are galvanically isolated, i.e. there is no electrical connection between the windings. Depending on the number of turns of wire in each winding, the transformers can either increase or decrease the voltage.
The transformer power supplies can be unregulated or regulated.
Fig. 1. Unregulated power supply
a - transformer
b - rectifier unit (diode bridge)
c - capacitor as an output filter
Unregulated power supply (Fig. 1) includes: a transformer (a), rectifier unit (diode bridge) (b) and capacitor as an output filter (c).
First, the voltage is decreased by the transformer to a specified level. Next, the voltage is rectified by a full-wave rectifier consisting of four diodes. As a result, regardless of the direction flow of the alternating input voltage, the voltage flows in the same direction at the output. The voltage is far from an ideal constant voltage due to high ripples, however, those can be eliminated with a capacitor used as a ripple filter.
A regulated (linear) transformer power supply differs from the unregulated power supply in that it has an additional voltage regulator (Fig. 2).
Fig. 2. Regulated power supply
d - voltage regulator
Voltage regulator (d) maintains the output voltage at the same lever regardless of the power supply load or input voltage fluctuations. Depending on the ripple attenuation factor, the regulator can also smoothen the voltage. However, this is usually the task of a capacitor. Depending on the power supply class, different voltage regulators can be used, however, those are usually integrated circuits.
Fig. 3. Voltage waveform at the linear power supply blocks
a - transformer output voltage
b - full-wave rectified voltage
c - ripple free voltage
d - ideal direct voltage waveform
The higher the power supply quality, the closer the output voltage to the ideal.
Unlike switched mode power supplies, the transformer power supplies offer a significantly lower efficiency, i.e. the ratio of the output power to the input power (40-50%). The efficiency depends on the transformer design, materials used and the voltage regulator, in which a certain amount of power is converted into thermal energy. Transformer power supplies are also relatively large and heavy compared to the switched mode power supplies offering similar performance. Transformer power supplies are also more expensive. Also, at no-load condition (no loads connected), the transformer power supply can still draw up to 20% of its rated current.
The transformer power supplies are immune to overload and overvoltage, and due to simple design, they are also very reliable and can be used in alarm systems. They also produce low interference and can be used as a power source for amplifiers, e.g. for antennas.
Example 12V/100MA/S-TAT transformer power supply available in Delta offer (Fig. 4).
Fig. 4. Regulated transformer power supply 12V/100MA/S-TAT