DIY Variable Lab Bench Power Supply

Max Imagination

Issue 60, July 2022

Instead of buying a commercially available lab power supply for his workbench, Max made one with commonly available parts and some maker know-how.

We first discovered Max’s DIY power supply while scrolling through Instagram. What caught our eye was how Max had used various electronics modules to make an all-in-one current limited 0-36V power supply with digital display, 12VDC port, dual USB ports, and dual Li-ion charger. Max has even made it portable.

We caught up with Max to learn more about it.

Well done for successfully building your own bench power supply, Max. First, please tell our readers a little about yourself, and what got you interested in electronics.

Hi, makers! I'm Max, a 16-year-old electronics hobbyist maker & YouTuber who comes up with various technically-involved projects, from practical devices/gadgets such as DIY Security Cameras, Garden Irrigation systems improving and assisting the quality of living, to making a handful of fun projects like flying RC Plane and boat models to enjoy on weekends.

Up until now, I've been homeschooled all my life and still learning. This form of education not only has taught me well in a multitude of ways but has also enabled me to have more time for being in touch with making.

From the earliest days I remember, I liked to build. Whether it was constructing things with LEGO or making PVC slingshots to shoot targets since I was a small kid, it's been a gradual but rewarding journey of always creating.

Later on, I moved on to more advanced projects for my level and started taking an interest in taking apart old toys/devices, and figuring out how the stuff worked, then trying to wire together something of my own. This is where I first got hooked on the journey of learning about electronics and becoming an advanced maker. I started my YouTube channel "Max Imagination" as a way to share my projects with my viewers and at the same time to teach others to make what I've built through step-by-step tutorials. From here, as far as I believe, things took off.

What motivated you to build your own power supply instead of buying a commercially available one?

The key thing that motivated me to build my own Lab Bench Power Supply unit was knowing how pricy it may turn out to be getting my hands on a decent, reliable, stable one that would do the job. $100+ or more in USD for a factory-made PSU just didn't seem to cut it, so I set to building my own one for about half that price. Plus I wanted to challenge myself with a new complex project and customize it to my own needs.

In your opinion, why should electronics enthusiasts own a variable power supply, and the importance of current limiting.

The significant advantage as a maker of owning one of these Power Supply units is the ability to set your desired voltage for whichever circuit you're tinkering with or device you'd want to power without any mishaps or the struggle in obtaining a different power source that isn't as stable if one were to not have a Lab Bench Power Supply handy.

The function of current limiting or setting current is crucial when having to analyze or test a circuit to be able to measure the current a circuit pulls (that it's dissipating the right amount of current as needed), so you avoid exceeding too many amps, or worse, yet, a short circuit. A decent Power Supply normally has reverse polarity and short circuit protection features such as my homemade one that you can build too. Knowing this, you can get a good idea why the power supply is seen as a must-have workbench appliance to own for a DIY maker.

Agreed. It’s important not to let the smoke out of the circuit you’re building. What are the output specs of your power supply, and what are the various outputs you have?

Focusing more on the DIY Power Supply's specs, it is a pretty versatile unit with a bunch of different outputs and inputs around it. The unit is fed 120V AC mains voltage as the primary source of power from the back with a switch to toggle the AC current, the unit also has a DC input on the side to power the whole thing by battery, making it portable.

About the DC side and outputs, the large toggle switch switches between powering the DC side of the circuit either from the AC/DC converter or straight from a battery. At the front, you've got a single AC outlet for appliances, the main two connectors (positive and negative (neg being reference)) with a variable output up to 36VDC, 12V DC Barrel jack, and dual 5V USB mobile device charging ports. Looking on the left face of the unit, it even has a Li-ion 18650-type cell (battery) charger tray with charging status indicator LEDs.

Finally, on the front, the DC-DC Digital Numerical Control Adjustable 0-55V Step-down converter which directly links with the two RCA connectors below, giving you variable power.

Thermistor activated cooling fan

Nice! It sounds quite versatile. Battery powered is a nice touch for making it portable. We noticed that you have a cooling fan as well. Is that heat-activated?

Yes, the Power Supply unit (PSU) even has a heat-activated cooling fan. To break it down, there's actually a DIY analog temperature-activated switch that triggers the fan to start and stop which works based on sensing the change in temperature on the switching power converter board inside. It uses a voltage divider network of a regular resistor and an NTC Thermistor which changes in resistance value when there's a change in temperature. Connected to an N-Channel MOSFET which is given resistive feedback via the voltage divider network to either turn the transistor on or off, toggling the PSU's internal cooling fan which is connected to this temperature-controlled switch circuit.

That certainly saves on unnecessary noise while you’re working. Can you dive a little deeper into how your power supply works and the different parts and modules you have used.

The DIY Lab Bench Power Supply unit works to the liking of many makers' power needs. To operate it, you first flick on the AC rocker switch on the rear, then flip down the front DC toggle switch (flipping it up would be for DC input) to power on all of the DC components around including the small power supply module with the display. On the display, you're welcomed to a neat little power indicating interface displaying the set voltage, current, and overall power from the product of the two. The module even has a power selection menu to further set more advanced changes, and you can even set different power presets that get saved for the next time you want to power something under that setting. Pressing on the "set" button allows you to click the rotary encoder dial to hover over digits of the voltage and current limit values at the very top of the display, then rotate the dial to adjust those values. Pressing the power button below the rotary dial turns on the output with the set voltage and current values to start powering circuits!

DVD/CDROM Enclosure

It looks like you have used surplus electronics for the enclosure. Was that to keep costs down?

Thinking "outside the box", making the PSU (Power Supply Unit) out of certain recycled materials is a good way to lower the total project cost. As for the shell of the unit, I used two DVD/CDROM driver cases from old computers, each being one-half of the overall case when put together.

With an angle grinder, cutting the appropriate spaces for components to poke out through the shells, then the painting, these shells make for a decent PSU case. Preparing the shells was made much easier with an angle grinder. Without one, cutting out spaces would have been a real challenge. Don't have an angle grinder? You may borrow one from your neighbor or instead drill sequenced holes in the shells with a drill machine until you get the desired cutouts. If you do use an angle grinder for this, safety first! Be sure to wear proper safety goggles when working with such machines. Stand a bit off to the side in case the grinder tool slips towards you.

This method of making the case certainly lowered the cost of the project, instead of having the shells made by external part makers and providers. Sometimes things as such can be worked around when one hasn't got a machine such as a 3D printer or a special machine that's capable of doing sheet metal fabrication. A 3D printed case will also work well, that is of course if you have access to using a 3D printer.

Feet made using Nerf Gun heads

Nerf dart heads are a novel way to make feet for your enclosure. Reminds us of the LEGO term NPU for Nice Parts Usage. We see you also made spacers from a plastic tube. What other tricks did you use like this?

In regard to various different tips and tricks when making such Power Supply, you may take the rubber heads off of those Nerf gun darts to use for the feet of your unit, your creativity may find another way of stabilizing the PSU on some form of anti-slip feet or rails.

If you lack a certain feature that you know your power supply should have (in my case - the fan's grill) e.g. I did not have any PC fan grills laying around, so.. I made one! Yes, that is right! I bent 2mm hardware store-grade steel wire to shape and soldered it all together making a fan guard/grill. Buying one could have been easier, however, I didn't manage to find those being sold separately at my local hardware store at the time. Once again, speaking of working around obstacles to progressing with the project!

Does your supply's wiring internally seem like it could get into being a real mess? No-fuss! Organize close connections between specific modules and connectors onto perforated prototyping boards (a.k.a blank perf PCBs).

Also, you might want to suspend modules like the power converters onto sitting on top of plastic spacers such as cut-offs from an ink pen which bolts may go through as well, this can prevent module terminals from shorting on the metal case below.

Thinking of ways to protect the integrity of both the structure and paint job on your supply can be taken into account by also using washers on screw-tightened parts around the unit.

Aside from already having drilled holes in the wooden rear face of the enclosure for air to travel out, don't feel bad about leaving some already existing open tabs and holes on the enclosure of your PSU as air vents too, this puppy needs to breathe!

A few great MacGyver-like tips. Obviously, your build involved wiring mains power, which is reserved for qualified electricians in Australia. Could an enclosed AC-DC power supply be used in these circumstances?

When it comes to the safety in dealing with mains voltage, never touch or work on your circuit with the AC cord plugged into the board, it is also important to ground the SMPS Converter board to the metal case to avoid potential shocks. When choosing the right AC/DC converter, it is recommended that you buy the specific 36Vout 5-Amp board used in this project, however, you might already have something as such laying nearby from an old appliance, just check that the board's output power specs are up to a standard of anywhere around 24V-50V DC 3-10A, try going for an SMPS power converter (Switch Mode Power Supply).

Since I already had most of the needed parts lying around as well as some which were salvaged, the only two things I had spent money on were the AC to DC (120VAC - 36VDC) Switching Power Supply Converter board and the DC-DC Digital Numerical Control Adjustable 0-55V Step-down converter (Power supply module). Both of these can be bought for under $50 US. The total cost of recreating this project may turn out completely different for you depending on what you already have and don't yet have.

Dual 18650 Li-ion Charging Slots

What prototyping did you need to do, and what challenges did you need to overcome?

What seemed to be the most challenging part about this project was coming up with my own modules assembled from bare components themselves. Understanding things such as why my thermistors kept getting burnt out from the odd mistake in hooking a voltage divider network wrongly to the MOSFET which makes up the analog heat-activated fan switch. You're gonna find small troubles like these along the way in almost any complex project you'll work on. However, it is a great learning curve to pick up from the odd mistake and to keep persisting.

Using PSU as a Voltmeter

We noticed that your project also doubles as a voltmeter/ammeter?

Additionally, thanks to these two specific purchased modules mentioned above, this PSU is fully protected from short circuits and reverse polarity connections between any of the outputs. That is to say that both have the same protective features making it a secure power supply. Of course, I just had to try shorting the output leads, just for the sake of it! Another thing about the power supply that makes it unique is that with the main power converter, looking at the display, you can probe the output leads to even measure voltage and current just as you would on a multimeter.

Is there anything we haven’t yet covered that our readers should know if they plan to make one for themselves?

A piece of advice to those of you who are considering recreating this project, or making something like it, is to first think of your workbench layout and decide on what features of these mentioned you'd want to keep or add more to the power supply based on your power needs. The idea here is to get creative and be open to adjusting it to your working environment, feel free to make a power supply like none other. A couple of more things would be like building your supply in a way that you can always take the lid off, open it up to fix a certain part of it, making it more accessible.

Internal Electronics and Wiring
SMPS AC to DC step down converter

Think of ways to protect various components that are most vulnerable to being damaged. Internally, you may support each and every module with supporting posts or brackets to make sure the entire unit can live up to the wear and tear of plugging things in and out of the ports.

With all of that in mind, you should be well set to make your own powerful little box of a machine! You shall enjoy playing around with the possibilities of what you can power with it.

Scroll to the bottom of this page to watch my video instructions for this PSU

Excellent. Thanks for going into great detail about your project, Max. We look forward to seeing what other amazing things you build in the future.