We had earlier in 2 different posts discussed about a variable power supply using LM 317. But in this post we discuss clearly about the working and designing of the LM 317 power supply in detailed.

Block Diagram

This circuit, like all voltage regulators  must  follow the same general block diagram


Here, we have got an input high voltage AC going into a transformer which usually steps down the high voltage AC from mains to low voltage AC required for our application. The following bridge rectifier and a smoothing capacitor to convert AC voltage into unregulated DC voltage. But this voltage will change according to varying load and input stability. This unregulated DC voltage is fed into a voltage regulator which will keep a constant output voltage and suppresses unregulated voltage ripples. Now this voltage can be fed into our load.

Firstly let us discuss about the need for the smoothing capacitance.As you know  the out put of the bridge rectifier will be as follows


As you can see, although the waveform can be considered to be a DC voltage since the output polarity does not invert itself, the large ripples Continue reading