This is variable DC power supply circuit, or we can call it adjustable DC power supply which give us a 1.2V-25V output voltage. The power supply schematic above is taken from the datasheet of regulator LM338K manufactured National Semiconductor. The circuit uses IC LM338K as the voltage regulator, so we will get the stable and regulated output. The regulator IC LM338K provides 5A output current max, and the voltage from 1.2 to 32V, it also featured the integrated thermal and short-circuit protection. Maximum voltage at input and output of the regulator is 35V .
The transformer current value depends your needs. For example, if you need 120W power output then you’ll need 5A transformer. 5A x 24V (full power) will give you 120W output power. Since this power supply circuit described to give us 1.2-25V DC output, then you must have transformer with 24V AC output.
For the diode, you have the choices whether use 4 diodes of single bridge diode. If you are have plan to use the PCB design in this post, then you have to use bridge diode. The value of diode is depend on the current value of transformer. For the example, if the circuit uses 3A transformer the you must use minimum 3A diode. The higher current value of diode, the price is more expensive of course… 🙂
The following images are the PCB layout design (bottom) and the component placement on the top. Don’f forget to use heatsink for LM338K to prevent overheat. I think every electronics hobbysts have high level of creativity, may be you’ll face some problems during the practice and you’ll solve the problem by yourself.. 🙂