Adjustable 24V, 4A, Power Supply

    This is a project on which I have been working on for years and never gets finished. Or rather I should say, a project on which for years I have not been working on and that is why it never gets finished.
    The idea was to build an adjustable power supply for the lab which would go up to about 24 V and to do it using scrap and recycled materials. The problem is that I already have more adjustable power supplies than I need and so I have little incentive to finish this one. Also, with the years, I have come to dislike making the printed circuit boards. So I do the design but then never actually build it.
   
Specifications

  • Output voltage continuously adjustable from 0 V to 24 V DC
  • Remote voltage sense capability (Sense internal/external)
  • Output current limit continuously adjustable from 0.4 A to 4 A
  • Output current can be limited continuously or output shut down (Limit/cut)

    Remote sensing means there are two additional wires which sense the delivered voltage at the load and compensate for any voltage drop along the cables which carry the delivered current. This improves voltage regulation at the load considerably but requires two additional wires for the sensing. A switch allows internal sensing at the output terminals for simpler operation when remote sensing is not required.
    The way the current sensing circuit is designed means it cannot be adjusted to limit current under 400 mA. If lower current limit is needed this can be achieved by modifying the circuit somewhat but it makes it more complicated and I am not sure I need it.
    I like to have a switch which lets me choose between limiting the output current continuosly (useful for charging batteries), or shutting down the output if the current limit is reached (useful for protecting equipment being repaired).
    Another thing I like to have in the power supplies I build is a pushbutton switch which multiplies the Amps scale by a factor of 10. That way one can momentarily press the button and get a much more precise reading of current. By making the switch a pushbutton one cannot forget to turn the function off and risk the instrument being damaged when a large current is put trhough it. In this case and for now I am not installing this function because I am using the instrument's shunt resistor to sense the current for the electronic control system and I would have to change several things. I might do this in the future.
   
Building it
    I used two 12 Vac transformers found in the trash and which belonged to a halogen lighting system. I have connected the outputs in series so that I get 24 Vac which is then rectified by a bridge and smoothed by a couple of capacitors. I have secured the bridge and the capacitors directly to the case as can be seen in the photos.
    I made the case with flat aluminum sheet which I scored, bent and cut myself. The components which are bought new I have mainly bought in China when I go there and are incredibly cheap. The two instruments (V & A) on the front panel are in this category. Power supply The Volts instrument is calibrated to 50 V and I will have to change the series resistor so it shows 30 V at end of scale and I need to change the scale itself which may be a tad more complicated. I have made the new scale using Inkscape.
    In all the power supplies and other lab instruments I make I like to place AC mains output bases wired directly in parallel with the incoming power input. This way I can use any in the back of the unit to daisy-chain instruments and I am never short of places to plug instruments into. The ones in the front are useful for plugging in temporary tools or equipment under repair. In this unit I had plenty of space so I put two in the front and two in the rear.
Power supply Power supply Power supply back Power supply front Power supply front

    I printed a template of the front and then taped it over. As I drill holes and place components I cut the paper away. For some unknown reason the green and red LEDs appear white in the photo even though their colors are perfect in reality.
Power supply

    Recovered from the junk I had two 2N3055 mounted on a heatsink and I mounted the assembly on the rear of the case. Probably a single 2N3055 could have handled the load but using two means they can handle the load and the heat much better. The assembly had both 2N3055 in parallel but I fitted a couple of emitter resistors to better balance the load. I made the resistors with a short length of resistive wire and I fitted them directly on the assembly itself. TR2, BD135 is mounted inside the case so that it uses the metal case itself as a heatsink.
    At this point most of the mechanical work is done and all that needs to be done is build the control circuit. The circuit I am working with is this one. It is just a preliminary design and has not been tested or adjusted. The values of some components still need to be adjusted. TR1 through TR4 form the voltage limiting circuit and the output voltage is adjusted using P1. TR5 through TR9 form the current limiting circuit and the current limit is adjusted using P3. The red rectangle represents the board where the components will be mounted. Everything outside the rectangle is mounted on the case. All wires on the board carry very low currents and all wires which carry high currents are mounted on the case.
    I am building the circuit on perfboard and it is working well.

    All wires go to a connector on the board and this way it is easy to disconnect and work on the board. The connector is made with a standard 16 pin DIL socket and a plug that fits into it. The numbers on the schematic identify the pin number on the socket. This is, of course, just for my own help and does not affect the circuit at all.