Mini Lab Power Supply

Introduction

Do you need a small power supply for prototyping electronics projects, often needing to power multiple components? Do you find the existing market bench power supplies too expensive, or too overkill, because you only need low voltages? Or do you just want to build something cool for your tinkering station? Then this instructable is just for you.

Why I built this?

I designed and built this power supply because I needed a stable voltage source for my electronic tinkering, and lab bench power supplies can be quite expensive. I tried looking for instructions on building one, but didn't have the required parts (namely the XL4015, which most other instructables use). And anyway, most power supply instructables are just placing pre-made modules into 3D printed casings, which I'm not a fan of. So I designed my own, budget-friendly edition. The total cost of everything used is probably 10$.

This project is the perfect stepping stone to your electronics journey because not only will it allow you to learn many useful skills such as soldering and how circuits work, but the power supply can aid your construction of future projects! No fiddling with software packages, or copy-pasting code from anywhere. Just pure building.

Features

Powered by a DC barrel jack, the power supply provides a voltage adjustable between around 1.25V and 11V DC using the knob. There are 8 pairs of output and ground pins to power multiple components. A heatsink on the side (Image 2) prevents overheating.

⚠️ Hazards When Building ⚠️

1. The heatsink on the casing can and will get hot (see more in Step 1: How it Works). Don't touch it!
2. Soldering iron and hot glue guns are also hot. Be careful using them.
3. Superglue and epoxy hardener can stick to your skin. Exercise caution!
4. There is a mild but existent threat of electric shocks. Don't build the circuit with the power on, or touch any exposed contacts when there is power.

Note

You will also need a DC power cord (Pictured in the next step) to power the power supply (how ironic). One of these could be lying around your house, formerly used to power some old appliance. Check the sticker on the cord to see the voltage and current output. It will probably look like this:

Input: 220V~ 50Hz 7W

Output: 12V⎓ 0.8A

(The ~ symbol means AC and the ⎓ symbol means DC).

Anything under 12V is fine to use, refrain from using anything above 12V. My own one supplies 9V at 0.85A. If the cord supplies 12V, you do not need the voltage booster module. (learn more in the next step).

Let's get started!

Parts

Hover over parts in the above images after clicking them, to see the part names

1x Mini Booster Module (Pictured) (Such as this one, this is not an affiliate link)

1x DC Barrel Jack Input

1x LM317 Regulator IC (with a heatsink!!)

1x Potentiometer (ideally 2kΩ - 5kΩ) (or a variable resistor)

1x 104 Capacitor (0.1 uF)

1x 100uF 25V Capacitor

1x 220Ω resistor

1x Diode

1x indicator LED (optional)

1x 510Ω resistor (optional, if you are adding LED) **

1x Rocker Switch (optional but recommended)

1x Voltage Display Module (Optional, and pictured)

** If you don't have the exact value, that's fine. Use something between 470 - 560 Ω.

Miscellaneous Parts

DC Power Cord (<=12V)

Casing (I used an old plastic box, any casing should suffice)

3D Printed parts: Potentiometer holder and knob (see links at bottom of this step)

Heat shrink tubing

Epoxy Hardener

Female header pins (as many as you want, but an even number)

Wires

Perfboard

Paper and markers

Tools Needed

Soldering iron (and good solder)

hot glue gun and/or superglue

Multimeter (for testing)

Hot air gun / Hair dryer (for heat-shrink tubing)

3D Print Links

https://www.printables.com/model/510002-simple-potentiometer-poti-holder

https://www.printables.com/model/379608-potentiometer-knob

NOTE: These parts were not designed by me.

Above is the circuit for the power supply (photo edited for clarity). Let me explain how it works.

This section is mostly theory and fine to skip if you want to jump straight to the build. Don't worry if you don't understand some of this stuff, you can build just fine without this knowledge.

Voltage enters the power supply through the DC barrel jack. The booster module increases its voltage to 12 volts (and hence why it is pointless if the DC supply is already at 12 volts).

How Regular Power Supplies Work

You may recall from your Physics class that AC voltage can be altered (stepped up or down) using a transformer. This won't work with DC, however, which is why DC power supplies normally use switching to alter voltage. The voltage is rapidly turned on and off, and the 'average' of this wave pattern forms the new voltage. This method is called Pulse Width Modulation (PWM).

However, our cheaper version uses a linear regulator called the LM317 (more on it below). They're super precise with voltage, and instead of switching, they remove all the excess energy as heat to lower voltage. You can see relevant calculations below.

How the LM317 works

The LM317 is a popular and versatile adjustable three-terminal positive voltage regulator. Its primary function is to provide a constant, stable output voltage that can be adjusted by the user. The LM317 achieves this through an internal reference voltage of 1.25V, which is maintained between its output (VOUT) and adjust (ADJ) pins. The user sets the desired output voltage by using an external voltage divider: a fixed resistor and a variable resistor (a potentiometer in this case). The LM317 works by automatically adjusting its output to maintain that precise 1.25V difference across the fixed resistor. The final output voltage is determined by the formula:

V-out = 1.25V × (1 + R2/R1​​) + (I-ADJ​ × R2​)

where R2 is the potentiometer, R1 is a fixed resistor (220Ω in our case) I-ADJ​ is the small current flowing out of the adjust pin, which is typically so small that it's ignored in most calculations.

A 10K potentiometer is quite large for this build, since the max output voltage can be achieved when the pot is at the 1.5 kilo-ohm mark, and the rest of the rotation is pointless. Sadly I did not have a smaller potentiometer so I had to use this one.

The other components

The diode provides reverse current protection between the ADJ and VIN pins of the LM317, which can occur when the input voltage is removed while a capacitor on the output is still charged. The 104 capacitor is used to smooth the input voltage and the 100 uF capacitor smooths the output voltage. The 510-ohm resistor limits the current going to the LED to its optimal current rating, around 20mA.

Why the heatsink is MANDATORY

When the LM317 linear regulator decreases voltage, the excess voltage is dissipated as heat. How hot will it get? We can answer that using the LM317's thermal resistance, which is 65℃/Watt. (This is from the datasheet for the Texas Instruments LM317T. Some manufacturers have values as low as 50 C/W and as high as 80 C/W). If we assume we're supplying 500mA (0.5A):

Temperature at 10V supply:

Power = IV = 0.5 x (12-10) = 1 Watt, so about 65 ℃. The LM317 can bear 125℃ so this is tolerable (but unbearably hot for the case.) This is the best case scenario, however. When outputting 1.25V:

0.5 x (12-1.25) = 5.4 Watts, so around 350 ℃. Way too hot! This is why the heatsink cannot be ignored. I've placed it in open air to provide rapid cooling.

Enough theory! Let's get building.

This part of the project is quite flexible. If you want, you could design and 3D print an enclosure. Otherwise, any box will suffice (preferably plastic to prevent components short circuiting).

I used an old screwdriver case and a heated knife to cut holes for the DC input, power switch, knob, output pins, and LED. I also used black chart paper and a marker to design a scale for the knob and markings to label the + and - of the output. The +/- labelling is important if you don't want to accidentally reverse connect and fry your components! The scale is also really important, so you know what voltage you're setting to (assuming you don't have the 3 digit screen).

We will build the circuit according to the schematic pictured in the above step.

1. Check the pin specifications of the barrel jack. Normally, The center pin is positive and the surrounding coaxial is negative. Be extra sure by using a multimeter to check the voltage after connecting the barrel jack to the power supply, as I've done in the picture above. Some barrel jacks could have inverted polarity, and this could damage your components!

1. Get the parts 3D Printed using the links above. They're not essential, but will make everything a whole lot easier. I don't have my own 3D printer but a friend of mine printed them for me.

1. Also check the pin-out of the LM317 in the picture above. Read your exact chip's manufacturer and model number (might need a magnifying glass to do this!). Mine is an LM317T, TO220 model (found by the shape, see image) by ST. The best thing to do is lookup the exact pin-out the manufacturer states, as this is the most reliable. Also, don't worry, different LM317 models by different manufacturers should perform the same.

1. If you don't already, learn the electrical symbols to make reading the schematic easier. (Such as here). I've refrained from the more complex symbols to keep it easy to read.

This is a simple step covered completely in the video above.

I made the stupid mistake of using some wires to tie up the potentiometer (will refer to it as pot from now on) to the 3D printed bracket. As the video shows, its much easier to use the pot's built-in screw nut and washer. The knob also fits on nicely. The specific model I chose is good because it has a bar sticking out, making the scale easier to read. Some other pot knob models require dual color printing to show where the knob is pointing.

This is probably the easiest circuit part. The barrel jack's output should be connected to the input of the booster module, with a switch to allow you to turn the power off. The final circuit part should look like Image 2.

I also added a tiny wooden block onto my switch's bottom (Image 3) to make it easier to glue in. You can use any ol' piece of junk lying around your house.

The rubber looking stuff you see over the circuit joints is called heat-shrink tubing. It's an inexpensive simple rubber tube, except it shrinks to around half its size when heated strongly. It can help insulate joints to prevent short circuiting, and also make joints stronger! Choose tubing that's less than double the diameter of the wire/joint. It's best to have a large kit of these so you can use different sizes.

My favorite workflow style for steps like this is:

1. Slide some heat-shrink tubing onto the wires
2. Wrap the wires around the component leads
3. Solder the wires onto the leads
4. slide the heat-shrink onto the joint
5. Use a hot-air gun (or a hair dryer, like I did) to shrink the tubing, forming a tight seal over the joint
6. Place the circuit part into your casing (wise to check the voltage once before permanently gluing it in, like image 4!)
7. Use superglue to attach it all to the casing

Get ready for a cycle of soldering, heating and gluing. The resulting part-circuit I got looks quite clean, for my low standards at least.

I made the mistake of doing this step after adding the potentiometer (step 7), which made my workroom a little tight. Hence why I've moved this step up, as I'd recommend doing this first.

This is another easy step. Link the output pins on each side together (don't link the opposite sides!) and then run a wire from them. I added duct tape and hot glue to firmly secure my wiring in place.

It might seem weird why we're mounting these before assembling the main circuit. For me, I think it'd be easier if all the exterior components were done, and then I could simply link up their wires within the guts of the power supply.

After my success with the previous tiny wooden piece, I decided to use more for the pot mount. By gluing 2 together, we get a 9.5mm thick piece (Pic 2), which is really well sized. After soldering wires to the pot (Pic 4) (we only need two pins, the middle (called the wiper) and one of the side pins, you can use either), I glued the wood onto the mount and blew hot air onto the heat-shrink.

Caution: I'd advise you DON'T do it in the order I did, and remove the pot from the mount before heating the heat-shrink. This is because the hair dryer is hot enough to literally warp the plastic frame (especially PLA). Do as I say, not as I do! I then used hot glue to firmly fix the potentiometer in place.

Mounting the LED is easy. Just make sure you add your 510Ω resistor, and some heat-shrink to isolate the LED's pins from each other. Then, simply insert it into wherever you decided it will go on your casing. Click on the first picture, and the notes will show where I've attached the LED. Pic 3 shows it externally.

On a side note, its 2 AM and I'm pretty tired, so I'll be continuing the rest tomorrow.

Finally, the part that some of you have been eagerly awaiting, and some of you (like me) have been dreading.

I'm no professional solderer, and thus kind of fearful of building circuits. Some quick tips I'd like to give here before I continue, as these are invaluable for beginners:

1. ALWAYS TIN YOUR TIP. You can see what that is here. I've already ruined a tip because I didn't tin it well, and it quickly oxidized and burnt away the tip.
2. Get something to clean your tip, like (preferably) brass wool (not steel wool! It's too abrasive and will destroy the tip) or wet sponge. Cleaning the tip is important.
3. Get some kind of helping hands, like I have in the pictures. It can really make a big difference, unless you have two people with heatproof hands and no other obligations to help you solder.

My soldering joints aren't exactly exquisite, so if I were you I'd refrain from using them as an example!

Now onto the circuit. I'd recommend cutting a piece of perfboard first, and checking where you'll be fitting it inside your casing. Try closing the casing to see if everything will fit properly. This was tricky for me because my pot was getting in the way. Before adding the regulator, make sure you remember it'll have a heatsink attached to it too!

There's not much further I can guide you in this step. Assemble the circuit onto the perfboard using the schematic in step 1. You can use jumper wires, the components' own leads, or pure solder to form the connections. The pin connections are: (The schematic makes this much clearer)

1. Diode from OUT into VIN (See the last image in this step for further help)
2. 220-ohm resistor between ADJ and OUT
3. 0.1uF Capacitor between VIN and GND
4. 100uF Capacitor between the output terminals (Check the polarity, or it'll explode!)
5. Potentiometer pins between ADJ and GND
6. Output (+) 12V of the boost module into VIN
7. Indicator LED (with 510-ohm resistor) connected to the 12V boost module's (+) and (-) .

(VIN, ADJ, OUT are LM317 pins. GND (pronounced 'ground') is the negative terminal of the boost module).

If I had gotten this earlier, I wouldn't have had to do any of the power supply calibration and scale. But I bought this after the project was halfway done, and just couldn't pass up on adding this to my little power supply. I've edited the Supplies list and the schematic to reflect these changes.

The wiring for this 3-digit voltage display module is easy. Just connect the wires of the display to the output voltage of the power supply (after the filtering capacitor, otherwise it'll show inaccurate readings), taking care for polarity. This display saves me the trouble of having to double check the voltage output with my multimeter because of my trust issues!

Note: Below 2.5 V or so, the screen does go blank. The scale is important for these parts (although nothing I ever build really does go below 3.3V anyway). Also, I ended up removing the indicator LED, since the screen is the indicator now, and I didn't want too much excessive current draw.

This part is entirely optional and can be skipped without any major consequence. However, it's best you include at least one, either the screen or the indicator LED.

I resorted to some really crude methods to get this stage done, such as hammering into the heatsink (Image 3). Which is why I'd advise you get a heatsink with a screw hole! Step 1 already highlighted how important it is we add a heatsink.

For this part, you'd better ensure your LM317's back face is facing the hole you cut out for the heatsink (as in Image 1). Then, hot-glue the circuit board in place (take care not to damage components! I broke my board's diode once). I also glued around all the wire connections on the circuit board to prevent any accidental short circuiting.

Now for the heatsink. Do NOT cut corners on this step and try to do something like hot-glue or superglue the heatsink. This will prevent proper heat transfer and cause the LM317 to overheat and fail, which will be an absolute pain to replace. Do it once, right! Acceptable mounting methods are:

1. Screw tightly (best option)
2. Paper binder clip, held tightly
3. heat resistant epoxy (ensuring the metal tab maintains contact with the heatsink)
4. Thermal glue

If you have some old CPU thermal paste, it would be best to apply that too.

I must also warn you here, the LM317's heat dissipation tab is linked to its V-OUT pin. Do not let the heatsink touch any other wires, or it will short circuit!

I'm going to use epoxy hardener in a bit to connect the heatsink to the plastic case, to prevent it melting.

You can see my final wiring setup in Image 2. The exposed section of wire is common ground. This is really the last major step (the rest are just finishing touches).

Do one final multimeter test before we seal the magic box up. Some minor discrepancies may arise between readings, which is safe to ignore. Just as in the above image, the readings 5.14 and 5.18 V are close enough that we can assume everything is fully operational.

Time to break out the art-and-craft supplies! I'm no artist, but I do kind of admire what I cooked up here.

The scale is cosmetic at this point now that the screen is here (I added a VOLTAGE sign above my screen), but I kept it anyway. I also marked the V+ and V- (GND) output pins to make sure I don't confuse them anytime in the future!

Congratulations! You finished building a Mini Adjustable Lab Power Supply ! (Should we call it MALPS?)

You can see mine in action in the video above (Don't give too much voltage if you have a smaller LED, or else it'll turn into a temporary firecracker).

If you have any questions regarding any of the steps or components to build your own Malps, drop them in the comments. I'll try to answer them all!

Happy tinkering! 💟