Small Variable Power Supply With Selectable Current Limiter

Firstly apologies not posting a complete 'instructable' on this build of mine, which is for a small variable voltage power supply, but it is my first project and it wasn't something that I was 100% sure would even work when finished. It was more of to satisfy my own curiosity that I could follow, adapt and build something successfully and which could be of some use to me. Also there are many other simpler and easier builds to follow on this website than the one I've attempted.

Step 1: The Inside of the Finished Power Supply.

Anyway now that I've finally completed building my first project which has been successful, I more than happy to share my experience with others, even if it only for the sake of showing off.

The design that I built was actually taken from a Practical Wireless publication called 'Practical Power Supplies' from 1985! It has several different power supply designs from 'A Battery Eliminator', a 'Dual Power Supply' and even a '30 Amp Power Supply Unit', plus some very interesting articles on Transformers and other power supply related items.

I had to be careful on which designs I selected from the magazine as they were either too simple and would be of little use to me or would have been a very ambitious (and expensive) build. Another issue I had to take into consideration is the availability of some of the components, don't forget the design I would be following was from 1985. Some were either rare and expensive or just no longer available and I don't have the knowledge or expertise to be confident in substituting these components for equivalents or significantly altering the circuit layout. My requirement was for a supply that would be suitable for powering some of the small projects that I intend to build, with a design that I could understand and follow, that would not required any major changes to due to issue with component but would still be challenging for me. The power supply I decided on building 'Auto cut-out Power Supply' by R.A. Penfold, it provides between 3 to 14 volts, with a maximum current of 650 mA. It has an auto cut-out current limiter which is selectable between 50, 100 and 500 mA. Construction Rating was 'Beginner' with an approximate cost of £12 (1985 prices - the project box alone cost more than twice that).

The only major change I made was to include a volt meter on the front panel, the original design just required a scale to be marked around the control knob of the potentiometer. Due to the lack of space this also required the mains supply switch to be moved to the back and was incorporated with a standard panel mounted fused kettle socket. A minor change was then required to the track layout of the PCB as the bridge rectifier had a different pin-out to the one originally required.

The only concern I had was producing my own PCB which meant handling corrosive liquid, which was not something I had done before however I actually had more problems with transferring the circuit design to the copper clad board, which took several attempts before I was happy with it. I am also very grateful to others on this website for the guidance they have given on this subject.

Although it has taken months for me to get all the bits together and build my power supply (years if you take into account that I had original bought the magazine new with the intention of building a power supply for my ham radio equipment!), I was is no hurry. Also I do realise it would have been cheaper and easier to buy a variable power supply from an electronic store it however wouldn't have been as much fun or entertaining.

I hope my first build is of interest to others even though it's not a full 'instructable'. If any one is interested I could add the actual circuit diagram and / or the track pattern of the PCB.

Future projects will probable include me using the Raspberry Pi or Ardinuo, unless I can find some old Everyday Electronics magazine with some more antiquated build projects for me to try.

Step 2: Component List, Circuit Diagram, PCB Layout & Wiring Diagram

After receiving a couple of requests I've now uploaded the list of components, circuit diagram, track pattern of the PCB with component placement and the 'point to point' wiring if the panel mounted components.

As mentioned the actual Bridge Rectifier (D1) used had a different pin-out to the one shown on the component placement diagram. The '+' pin and top '~' pin were connected as shown on the PCB layout but the '-' pin and bottom '~' pin were switched so this part of the track was changed to take this into account.

The BRY39 (CSR1) was relatively expensive and I spent some time trying to find an alternative. However I ended up paying £7.19 for this one component, which was nearly as much as I paid for the transformer, as this is what controls the cutoff for the current limiter.

There was a simple addition of a volt meter on the front panel as a clear indication of what the output voltage was, which was preferable to just making marks around the control knob. The only other change I made was to remove the rotary mains switch (S1) and used a switched and fused kettle plug on the back of the case instead.

The build it self went fine, other than my concern over producing my own PCB which was another expensive endeavor. For future builds I may have to rely on using strip or Tripad boards. However I did spot an error on the original 'Point to Point' wiring diagram in the magazine which was that the wired link between the 'Push-to-make' switch (S2) and the 's.p.d.t. mini-toggle' switch (S4) should have also shown a link back to the PCB to the track connecting to R6. I have added this on the picture I've uploaded to avoid any confusion.