DIY Variable Workbench Power Supply

After constructing ATX based power supply, I try to build a variable power supply. There are also many design around the net. Some are simple, Some are complex. Some factors such as power factor, size are not critical for me.

– I have a big 28V step down transformer in hand. So, I focused on linear design rather than SMPS.

– I also have a small SMPS module with +5, +15, -15 V, 25W rating with 19~35V input which salvaged from old power supp.Like this one, by combinding this, this supply can be used for both fixed and adjustable.

– Long ago, I also constructed a LM317 base linear regulator circuit. But the problem of LM317 is it support only 1.5A max and minimum offset control voltage is 1.25 V. So, it cannot drive high load and cannot set the voltage below 1.25V.

I need a linear regulator with 0~30V adjustable and about 10A load. Find out the web and finally I saw this page by ON6MU.

It support higher current, 0~30V and used LM317. I am inspired his design and try to construct this.

First, I have old bad power supply. This chassis has switches, 7-segments display and fan outlet. As usual, remove bad power module first.

Then, mark and drill, attached all required outlet. I print a sticker to cover original chass front labels. Speaker tap is for adjustable voltage output while other are fixed output. A variable potentiometer is for adjusting output voltage.

Third, power transistor 2N3055 are installed with a heat sink. Be care! this transistor collector (body of TR) should be isolated from heat sink. Several plastic washers and sil sheets are used for this.

If not clear how to install power TR and heat sink, check this image.

Test to ensure no continuity between all collectors and heat sink with the multi-meter before installing in the chassis.

Connect base and emitter in parallel. Connect all collectors with 0.5 ohm (5W) resistors.

Measure, drilled and seated , Installed all components.

Rough connection diagram is as follow.

There is no power out for 12V fan. So, I installed a 7812 regulator to power the fan.

Cabling is finished.

Testing …and finally there is a new variable + fixed power supply in my bench.

For the next step, I am considering to add current control circuit. Still tune.

Credit to : ON6MU


DIY Workbench ATX Power Supply

I love DIY and home-brewing. As an electronics engineer and hobbyist, the first equipment in my workbench is a stable, multi-voltages power supply. Formerly, I used wall-wart or small DIY power supply with mostly 78xx linear regulator. As for now, I think to build a workbench power supply.

There are many DIY workbench power supply designs around the net but I need a simple and stable for the first. Since I inspired by PC power supply base design, I started to build the one. Using power supply as power source is a good idea, no need to brother the design complex and quality plus low budget.

First, I find out good articles for reference.

Second, I find an old ATX PC power supply.

Then, strip apart all.

I find old 12V battery chassis. Good, this one has buildin switches, fuse and taps.

Measure, marking and drill holes for tappings. Install output taps.

Seat ATX power circuit, cut the cables and solder to each outlets.

The wiring diagram is as follows. I used 2 x 4.7 Ohm resistor with small heat sink as dummy load. Without it, the supply cannot be maintained stability.

The connection diagram is as follows. Interesting, my supply is labeled as -5V output (500 mA). I cannot find it in ATX pinouts. It (pin-20, white)  is labeled as reserved in wiki page. After checking, this pin-20 is connected with a 5V zener diode is circuit. It is really a simple zener supply. Anyway, it is a good idea to put -5V pin-out although it is small ampere.

Oop! Don’t forget to connect fan power pins also. If not, your supply will heat it up. Note the original fans supply pins before take off. Usually, its a small red-black pairs cable.

Cover the case. I attached the fan at the right side and drill some holes for ventilation.

I removed label sticker from original ATX case and stick it over new supply. So, I can easily check the rating of each output. More, print out  and stick labels for each taps. Finally, this is it. There is a new workbench power supply in my workbench. It is happily running now.