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.

http://users.belgacom.net/hamradio/schemas/6-20amps_regulatable_powersupplies_on6mu.htm

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

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