Series Regulated DC Power Supply

Power supplies are required everywhere, from industries to every household whether in urban areas or villages. They form the root of every digital as well as analog electronic item. Here, the construction of a conventional power supply using electronic components like diodes, BJTs FETs etc is demonstrated. The approach is easy to follow and can be used in many other applications. The chosen output voltage is +5V, since majority of the digital circuits nowadays require this for operation. To prevent any damage to the circuit and the load it is driving, protection techniques have been incorporated.

The power supply starts with a transformer – bridge – peak filter network cascaded with a voltage regulator network. The regulator network includes LM 723 and additional circuitry which helps us in achieving DC 5V. The LM 723 serves as 2 operational amplifiers cascaded with each other, which are under negative feedback

produce desired results with the help of external impedance. The 1st op-amp has a temperature independent band gap reference (generated within the IC) of 5V connected to the non inverting input,At the inverting input the output voltage is fed back. The negative feedback attenuates the unregulated signal  and gives the output by reducing the ripple in the waveform. The 2nd op-amp simply acts as a buffer and results in bringing down the output impedance and increasing the voltage regulation. The circuit so far can only source a limited amount of current, thus,we use an external pass transistor  (2N3055) in Darlington configuration with the output transistor. This increases the output current capability to nearly 1A.

Add-on protection circuitry has to be used to detect accidental shorting of supply terminal with ground, since the consequential surge of current is potentially dangerous to the supply as well as the load. Hence we use a voltage divider at the current limit - current sense transistor base so that we are able to choose at what value of output current, fold back circuit becomes active. Fold back circuit steals the input base current from the Darlington circuit, which reduces the output current. After forming the supply, it is tested on passive load as well as active load. We observed that the output curves closely followed the ideal trend which confirms that the circuit is functioning well and is ready for practical use.