The low loss power supply circuit explained here employs an SCR that chooses taps on the secondary of the transformer based on the load current.
Thus it helps to dissipate reduced power on the transformer, and saves power during idle state.
The output voltage about 9 V is quite suitable to act as an input voltage for a 5 V regulator. This component, thence, operates with extremely low power dissipation.
How it Saves Power (Working)
Referring to the diagram above, when the load currents are low to medium, the SCR presents in a blocking state and remains automatically switched OFF.
The secondary transformer voltage undergoes rectification through D1, D2, D5 and D6 only. During the positive half cycle, the load current passes through D1, the load and D5.
When the negative half-wave is acting, the current flows through D2, the load and D6. In both cases, the tapped secondary voltage is about 8 V whereas a 2 V section stays idle.
As the load current progressively increases, the output voltage falls until there is not any more current flow in the Zener diode.
After that, transistor T7 turns off and consequently detaches the short circuit from the SCR’s gate, allowing it to conducts.
Immediately after that, the full secondary transformer voltage undergoes rectification through D1 to D4 whereas diodes D5 and D6 are reverse biased.
In this way the circuit powers only in response to a load current, otherwise remains switched OFF reducing unnecessary consumption and power loss.
You would have noticed the voltage across the Zener diode is the lowest during the zero crossings of the secondary voltage.
At this phase, the SCR continuously switches on immediately or just before.
This alleviates the possibility of high current pulses and other noise that are related to the switching of the SCR.
You will require a mains transformer with a 12 V secondary that occupies taps with 2 V steps to build this circuit. One 2 A rated transformer is sufficient for load currents up to 1.5 A. For an output current of 2 A, we recommend choosing a 3 A transformer.