As an electrician, I know how a power supply play a role in working of electrical instrument. The power supply is the first thing you have to think of. It has to conformity with your uses. I made a ATX power supply from old computer PSU. It's powerful, safety, nearly perfect for many uses but it has a really big problem. My mainly object in electronic is design audio amplifier then PWM and switch mode power supply are making too much noise to the output due to their operation. I added many big filter capacitors to my ATX power supply and it just enough to make it "acceptable". But most of all, I need a small, powerful, multi-purpose as my lab bench power supply. A perfect lab bench power supply. Lately I came up with my design and built it.

Step 1: Designing

To design a power supplly, I was looking for some information online. Dave John, a professtional electrian, is a good guider to start. According to him, a good power suplpy should have:

- Absolute 0v to 6v range: Okay, a LM317 can do it (1.25v reference? Easy. Just 2 or 3 diode in series then problem is solved).

- 0 to 1A constant current adjust: Ok, we have my DC current limiter already.

- Control of voltage and current: A potentiometer is good enough for me.

- Low noise (linear): it means using voltage regulator (This is the most important thing you have to pay attention to if you work with audio amplifer). It makes this project a lot easier ;) Lazy people like this.

- Single supply input: use a single supply transformer or one side of dual supply transormer.

I'll take these idea and put it in my design. I also have some idea for the function of my power supply. It should aslo has fixed voltage output like ATX power supply, also have 2 seperated output selector like the multi-power supply Dave holding. Then I think I will combine all of them in my design.

One important thing you also need to know and it also make my power supply is unique: I make an 36VAC output. Why? For many reasons:

+ I always want an AC supply voltage to keep me away from dangerous wall outlet 220VAC (only 36VAC and limited power)

+ It is isolated from wall output and "safe" for an accidenttly touch (not both terminals at the same time)

+ Useful to test some small electrical appliances like charger, light bulb,... especially when it shorted inside or danger of leakage voltage

Also don't forget to arrange your component to make a good layout

Step 2: Components & Tools

My full list of component is in the photo but my final decision is cut some part in my project. All components are:
- For "source" part:

+ Power cord

+ A 18V 3A dual power supply transformer - rated at 108VA

+ A swicth, a fuse holder and a F 1A 250V (fast burn type) fuse

+ 3A bridge rectifier

+ 1x1000uF 50V capacitor, 2x100uF 25V capacitor, 1x104 ceramic capacitor (100nF)

- For "voltage" part:

+ KA7805, 7809, 7812, 7815, 7905, 7909, 7915 ICs, each type 1 IC. 2xKA7812

+ 1xLM317,1x 1k ohm resistor, 2x diode and potentiometer, value is depend on your desire voltage

+ 3xTIP41, 2xTIP42, 5xheat sink, heat paste. This is optional for high output current, high power transformer

- For "general" part:

+ A box

+ A cooling fan and 2 male header

+ 5 pairs of binding post

+ 3xDPDT toggle switch with 3 positions

+ A perf board or make your own pcb board

+ Wires in many color + Heat shrink tube. Optional, I use hot glue instead

+ 2xLEDs and 2x 1.5k ohm resistor

Tools are: soldering iron, soldering wire, wire stripper and hot glue gun

Step 3: Build It

Layout was draw after I soldered. I normal think up a layout when positioning component. I don't know how to use software so I draw it by hand.

SW= switch

BP= binding post

X= hole of heat sink

PTM= potentiometer

Black dot=out by wire

Blue dot=normal join

Pencil mark=board border