LearnTronics
Simple Regulated DC Power Supply
Recipe #15 — transformer, rectifier, filter, and regulator
The previous page stopped after the transformer, bridge rectifier, and filter capacitor. One more block can make the output much steadier: a voltage regulator IC.
What each section does
T1: steps the AC voltage down and provides isolation.
BR1: uses both halves of the AC waveform to make pulsating DC.
C1: stores charge and fills in much of the space between the rectified peaks, reducing ripple.
7805: continuously adjusts its internal pass transistor so the output stays near 5 V while the input remains high enough and the regulator stays within its electrical and thermal limits.
C2: is a small local bypass capacitor. Exact capacitor recommendations depend on the regulator and layout, so the actual regulator datasheet should be followed.
Another way to think of it
The transformer changes the voltage, the bridge makes all the current pulses face the same direction, the big capacitor smooths the bumps, and the regulator performs the final trimming so the load receives a much steadier voltage.
The filter comes before the regulator
The regulator should not have to work directly from the deeply pulsating output of the bridge. C1 first creates a reasonably smooth DC input. The regulator then removes much of the remaining variation.
A classic 7805 is a linear regulator. The voltage it removes becomes heat:
That is why the input voltage, load current, package, heat sink, and temperature all matter.