0 – 24V Variable Power Supply

0 – 24V Variable Power Supply

Project Brief

Having a power supply for electronics work is an important thing. Without a power supply, your electronics simply can’t work at all. Yes, you may think that using regulated power adapters / wall-warts is enough but what if your device needs a specific voltage that your adapter can’t provide? This is where the variable power supply comes in.

Circuit Design

I did some research on variable power supply circuits and I found 2. One that uses LM317 and one that uses LM723. The former is more common, but the latter has more features, ability to control the current flowing compared to just max 1.5A on the former. Circuit diagrams courtesy from http://www.thebackshed.com

I also found another circuit, it uses a 2N3055 to boost the current capability up to 15 Amps and 60V, although the LM317 can go up to 37V. I think I’ll choose this one for the project because I don’t need to control current and the 2N3055 makes it possible to use this for future expanding to high currents uses.
Circuit diagram from http://www.circuitdiagram.org

Circuit Design

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Main Circuit & Product

As shown above, the circuit is fairly simple.
I mounted a heatsink on the regulator ICs to dissipate the heat. I’m also planning to put a fan nearby too. I also applied some thermal grease I bought from Sim Lim Tower for S$1. The purpose of this is to allow better conductivity between the heatsink and the IC. There is a piece of thin plastic in between to isolate the IC from the heatsink as the metal tab of the IC is connected to the output pin.
I’m also planning to put a LM7805 next to the 317 as 5V is mostly used in the digital electronics world. There is a 50V 1000uF capacitor at the input to smooth out any voltage spikes if any. I placed a power diode on the output to prevent any backflow of voltage.
For the enclosure of the power supply, I am using a 20 x 14.5 x 7.5 cm multipurpose black project box. I drilled some holes to accommodate the potentiometers and voltmeter as well as the 2.1mm centre-positive barrel jack for power in.

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Additional Heat Dissipation

A heatsink alone is not sufficient to cool down the IC regulator chips. It gets quite hot after some time (especially the 2N3055 transistor). An active cooling solution is needed such as a fan. So I went on to my favourite Electronics Store at Sim Lim and found a great deal. A 12V PC fan for S$2. Cheap right? Although the one of the screw holes had broke (top right), it is still gonna perform its job right?
I’m also planning to 3D print some grills for the fan for the exterior so that the blades wouldn’t catch the wires.

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Soldering The Circuit

The circuit is pretty straight forward. I used thick gauge wires for all of the wiring except for the potentiometer. The circuit footprint could have been smaller if I had planned the circuit beforehand.

Laser Cutting The Back Panel

The back panel is where the fan, main switch and 2.1mm power connector will be mounted. As there is a non-circular object, the switch (of which obviously I can’t drill a hole) and a large cut out for the fan, I decided to laser cut the back panel as it is much easier and cleaner-looking.
I used AutoCAD 2012 to draw everything from scratch
I chose clear acrylic (The protective sticker is not removed yet) because it looks cool when the insides of the PSU is exposed.
The total amount of time taken to laser cut the acrylic was around 5 mins.
Although the fit wasn’t really snug as the original plastic, it fits in quite well and it is partly due to acrylic being thicker than the former.
The final fits the fan and the switch perfectly.

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Final Touches

Its been 3 weeks since my last progress. Numerous school work and projects as well as the recent tests have made me busy. I didn’t have much time.
Now that the tests are over yesterday, I can now go full steam ahead.
First thing that I did was to drill some holes for the banana plug sockets, the 5K Potentiometer and the switches for the fan and supply. The red plug and the Pot require a 7-8mm hole, but the maximum drill bit the systems workshop in my school has is 6.5mm. Either I would have to go the fabricating workshop for a bigger drill bit, or I have to find a way to make the hole bigger. What I did was to take the piece and hold it up in the air and pressing it against the bit, going on circles for an even cut. (Dangerous, do not do this!)
Second thing was to saw off the pot’s knob. It was too long and the knob cover that I used would not be flush with the panel.
Third thing was to solder the switches and the resolder the 2.1mm center-positive barrel connector.
I got really shocked when I was soldering the switch. I heated up the pins, apply solder and right after that, the pin of the switch came just right off. I didn’t heat it up for too long. Tried the other side, same happened. So I had to use a different brand switch. Low quality switches…Sigh
Fourth thing was to think of how to mount the 2N3055 transistor to the board. I drilled 2 holes and put some screws in for the transistor.
The last thing was to mount the perf board to the enclosure. I didn’t have small self-tapping screw but the holes on the enclosure was small, so what I did was to use the smallest screw in the workshop, and partially screwing it in. As long the board stays, it should be ok.
I am using a Kodak photo printer power adapter that has a 2.1mm jack. It can supply up to 2.2A at 24V.

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