Unexpected Glamour Laboratory Power Supply

Along with a soldering iron, multimeter and oscilloscope, laboratory power supply is the main tool for electronic engineer, because without power, no electronic device can even turn on. Today I want to tell you a story how once, for one evening, I tried to make such a device and what came of it.

One evening, my boss called me and said that by the morning we need, at any cost, a laboratory power supply with the following parameters:

1. input voltage - 220V;
2. output voltage(1 channel) - unstable, non-adjustable 12V / 1A;
3. output voltage(2 channel) - stable, adjustable from 3.3V to 12V / 0.1A;
4. easy wiring;
5. built-in voltmeter;
6. nice body.

Well, the chief said - it means that it must be done, I quickly composed the unit scheme and picked up all components. The main snag was finding a nice body (box). The only candidate for the box that could normally accommodate a huge toroidal transformer, turned out to be an empty laundry bleach can that I found under my bed and wanted to use in the project. In the darkness the can seemed to me very beautiful: blue and black (or something like that). Cool, I thought! And I began the creation of the power supply unit.

In the morning, when I came to work and brought the craft to my boss, as in the well-known [story], turned out to be not blue & black, but rose & gold! All of us, of course, laughed together at this small incident, but decided to leave the power supply in such a way as I brought it :)

Step 1: Schematics

The power supply scheme includes 4 parts:

1. high vlotage part;
2. rectifier;
3. linear volage regulator;
4. front panel (with voltmeter and clamps).

Next, let's look in detail each of the power supply components.

Step 2: Hight Voltage Part

The high-voltage part is a toroidal transformer to which cable and power plug are connected. The cable is connected to the primary winding of the transformer(red in the image) by means of contact terminals screw, so the cable can be removed and replaced if necessary. Power plug is also connected to the cable with a screw fastening (see photo).

List of components:

1. power plug 220V;
2. cable 2x0.75 - 1 meter;
3. double screw terminal power;
4. toroidal transformer (primary 200V, secondary 12V / 30A in my case).

Step 3: Rectifier

The rectifier is built on the classic single-phase full-wave circuit, at rectifier's output there is an electrolytic capacitor of 4700uF smoothing the rectified voltage. Additionally, the unit is equipped with a 10A fuse to enhance the security of the scheme.

List of components:

1. PCB (you can use breadboard 4x5cm);
2. diode bridge - 25V / 5A;
3. capacitor 4700 uF / 50V;
4. capacitor 0.1 uF;
5. fuse 10A;
6. fuse holders;
7. screw contact terminals (2 pcs).

Step 4: Linear Volage Regulator

Voltage regulator scheme is based on the use of the classic LM317 chip made by Texas Instruments. The scheme is taken from [datasheet]. It provides voltage stabilization in the range of 1.25V to 14V (in my case).

List of components:

1. PCB (you can use breadboard 2x3cm);
2. chip LM317 (case - TO220);
3. diodes 1N4001 (2 pcs);
4. resistor 220 Ohm / 0.25W;
5. variable resistor 10 kOhm;
6. capacitor 0.1 uF;
7. capacitor 10 uF;
8. capacitor 100 uF (Tantalum in my case);
9. screw contact terminals (2 pcs).

Step 5: Front Panel

The front panel of the power supply is its most important part, it includes 3 main components:

1. self-locking contacts clamp for external devices;
2. a voltmeter for monitoring device status;
3. switch providing connection of voltmeter to the first or second output of the power supply.

I made the front panel of the bleach (stain remover) can's cap, to do this I cut out 3 holes with a sharp knife (having marked them out carefully).

List of components:

1. voltmeter [link];
2. self-locking clamp connector [link];
3. switch (3 pin or 6 pin like in my case);
4. wire 1x0.5 (red and black) - 2 meters;
5. screws / nuts / washers M3 (for mounting the clamp connector).

Step 6: Result

Well, it remains only to apply voltage and you're done!
Finally the unit was a success! It works great and it is easy connect to it any load. We have been using this unit for a few months and, so far, no problems.

For the future there are a few ideas to improve the scheme:

1. add heatsinks on the diode bridge and a LM317 chip;
2. replace potentiometer R2 by a multi-turn one;
3. replace diode bridge with separate Schottky diodes (so we can improve the thermal characteristics of the power supply);
4. I'd like very much to add a current stabilization unit and an ammeter.

Well, for the moment that's all, thanks for reading this guide, I hope you enjoyed it!