Introduction: Small Variable Power Supply
There are plenty of power supplies available on the Instructables. Compared to the others, this one is:
- Small size and compact
- Simple, made of modules and parts ready to use
- No special tools necessary
Parameters are following:
- Input voltage AC 230V (or 110V - transformer dependant)
- Output voltage DC 0 to 30V, adjustable by precision 10 turn potentiometer
- Output current max DC 3A
- Current limit adjustable roughly from 0.1 to 3A with LED indication
- Digital panel meters for voltage and current measurement
- Dimensions 20cm x 17.6cm x 7cm
I have made some power supplies for my electronic bench in the past. Their parameters are lower than 20V and 1A, so new one is big improvement. As a main module used, is Hiland low cost power supply module, delivered in kit. Finding this kit on the market I decided to purchase it without any intention. Hiland module was lying in my bench a few month and finally I decided to play with it and test it. This kit is high quality, reliable with good parameters and therefore become inspiration for this project.
Step 1: Parts
Key part is main transformer 230V to 24VAC. Hiland board require input 24VAC. DC voltage is not recommended and by Hiland user manual could destroy module. I am not sure about mentioned argument. The only problem with DC could be lower output voltage. (24VAC after diode bridge is giving more than 30VDC, and in contrast 24VDC is giving less than 24V on output ).
I have used transformer from old electronic instrument (printer) with output 24V, power about 70W. For full use of Hiland power supply´s maximum current, it should be around 100W. That means, in my case, output current 3A is not reached for voltages above 20V. Dimensions of transformers core, which determinates power, are about 66x55x32 mm.
Used parts are:
- Plastic box, project case desk instrument Excellway EF01
- 0-30V, 2mA-3AAC power supply original Hiland
- Small fan 12V or 24V, 60x60mm, example of fan . I use 12V fan from my old electronic parts store.
- Aluminium heat sink 60x150x25mm Silver
- Precision wirewound 10 turns potentiometer 10k
- Green LED panel meter 0 to 5A ammeter
- Blue LED panel meter 0 to 99.9V voltmeter
- Output connectors: Banana Female Sockets 4mm, 2 pieces.
- Power switch rocker switch This switch do not need to be illuminated. (Instrument indicate switched-on state by light of both meter modules.)
- Any power adapter 230VAC to 12VDC, more than 3W, I use charger from old mobile.
- Main transformer 230V/24V, power about 80W, as written above.
- Soldering iron
- Jig saw
- Saw blade
Step 2: Schematics
User manual for power supply Hiland is available on e-shop page of the seller here . From user manual is taken attached electronic scheme. This scheme and PCB itself can help during soldering all components on the board. From my experience, I can advice, to check all components very carefully. In my case I did check twice, but I have made small mistake anyway. Do not solder both potentiometer and limit LED on the board and solder just connectors. Original one turn potentiometer is used for current only. In this case resolution of current adjustment is enough. Voltage potentiometer is replaced by 10 turns precise potentiometer. Potentiometers and limit LED are soldered to wired connectors included in kit. By soldering both potentiometers be careful and solder them such a way, that increasing value on each pot is in clockwise direction. Current potentiometer is without pin numbers, but pin numbers are in the same position as it would be soldered directly to PCB, concerning attached numbering in the picture. Voltage potentiometer numbering is signed on pot, but numbers of both potentiometer on schematics are not correct, there is swap pin2 and pin3.
On the board Hiland is not soldered 24V regulator and socket for fan. Both places are marked by cross. The reason is powering fan from other source, power adapter and to avoid touching regulator 24V to cooling heat sink.
Scheme of Small power supply is on picture. It contains all modules, components outside modules and all wires between them. For powering meters modules, there is necessary to use another power supply to prevent from short circuit connection. It is the only way how to connect both modules to the Hiland power supply board and to the extra power source for them. I use old power adapter for mobile phone. Adapter is connected to 230VAC on one side an deliver 12VDC on other side. From this adapter is powered 12V fan as well. Total current consumption is about 200mA and could be chosen any adapter with power more than 3W.
Ammeter module is connected in back minus line. Current from minus output connector is fed through this module and therefore is plus pin of current for this module, connected to minus output connector.
Step 3: Construction
Bigger parts: transformer, fan, heat sink with Hiland board and power adapter are mounted on bottom part of plastic case. On front panel are attached switch, voltmeter, ammeter, banana connectors, both potentiometer and LED diode. On left side of bottom plastic are original vent air holes. This holes dedicate placement other components. Parts with higher cooling requirement are around vent holes: fan with heat sink and main transformer. Transformer is attached to plastic by 4 screws with metal angle joint. There are some original mounting feet on plastic bottom side and they need to be removed, because hampers. I made it by separate saw blade. It could be slightly bend.
Fan, cooler and Hiland board are mounted together in one assembly. Heat sink should be shortened on the longest side to about 12cm and than drill some holes through it according picture. Holes are necessary for air flow, because there is not enough space above cooler. Fan is attached to heat sink with four screws and standoff studs. Hiland board is mounted to the cooler through power transistor directly without isolation, because no other part of board is connected to cooler. Put some thermal conductive grease between transistor and heat sink. Than mount board to aluminium by two screws and metal angle joints. Finally, all assembly is mounted to plastic bottom cover by four screw and standoff studs.
Actual placement of bigger parts is the only possible placement, concerning air vents holes on the left side. Cooling air is fed around and through heat sink, around transformer and over board by fan, which sucks air from outside. To maintain this flow direction, I drill some set of holes on right upper part of enclosure.
Adapter is mounted at the end. There are two possibilities: on front left side and right back side. It depends on its shape and size. The best way to mount it, remain adapter in his original cover. I put it on left side near fan. Same space should be left for front panel parts. In my case I made this space by partly removing cover from adapter.
Plastic front panel need to be modified by drilling some holes and windows in it. I use small jigsaw. All parts are mounted directly to front panel except potentiometer for voltage. I use metal adapter for attaching it 5mm off front panel such away, that its big screw is covered.
Before attaching parts to front panel, I put on it, graphic label printed on photography paper. Label is made in Sketch, Windows 10 software. I apologize for mistake which I did in word "VARIABLE", but I am too lazy to do it again.
Main power cable is feed trough hole in back panel and fixed with screw clamp inside plastic box. This cable is soldered to main switch. On one of cables from main switch to primary winding of transformer, there is soldered fuse. Fuse and cable is covered in heating shrink tube. All other points with high voltage are isolated in tubing or by silicon rubber.
Step 4: Adjustment
After connection to the power, check voltage 24V on the input of Hiland board and 12V on output of adapter. If voltages are OK adjust Hiland board.
Hiland board adjustment is simple:
- Connect precise digital multimeter (able to display negative voltage) to output connectors and measure voltage.
- Adjust pot 100K (right front side of board on attached picture) exactly to 0 V.
Next adjustment on Hiland board is not adjustment really. It is calibration of Limit potentiometer with currents values. No instrument is needed. Check a few currents by attaching resistors to output connectors and write current value around potentiometer. You can see attached resistor in the picture. In this case there is displayed voltage 12.3 V, resistor 4.7 ohm and current 2.56 A. The mark on potentiometer indicate position below 3 A. In this case limit Led does not light, but increasing voltage causes LED lights. I did not make scale around potentiometer very accurate, because in most electronic laboratory experiments, it is not necessary. Usually there is enough, to adjust current limit roughly.
Thanks for reading.
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