DIY two-channel 120W DC power supply

DC power is the basic need for any electronics project to start. Though batteries are good for small projects and also recommended due to the stable supply of current. But with increased power demand for more sophisticated projects batteries don’t seem a very good option to work with. A good bench supply or power adapter is handy and can be easily procured, but there is no match for custom-tailored design, specifically suited for the need. This is the story of a DIY two-channel 120W twelve volts fixed DC power supply.

Starting up

Why?

After the whole day of work, the workstation gets messed up in the evening filled with small wire pieces, which can be dangerous if cleaned by hand. But at the same time purchasing a separate small vacuum cleaner only to be used for the corners and workstation, doesn’t seem wise decision. So, why can’t we use our car vacuum cleaner at these places, the only challenge was, it works on DC, while household supply is AC. When I started planning for this I wanted to extend its capability to be used in my project also as a fixed power supply, and I don’t have to purchase another adapter that I was planning to buy. So to summarize we needed an AC-DC converter which can

1. Power 120W car vacuum cleaner, and
2. Act as a stable 12V power supply for 2-3A current.
3. Also, both the outputs will never be used together.

Required skills

• Basic soldering.
• Basic Knowledge of the heating effect of current, the polarity of capacitors and power supply.
• Knowledge of electrical safety. This is very important, as the projects involve 250V/120V AC supply hence safety measures should be taken always to avoid any unpleasant accident.

Components and tools

The list of components provided here is not final, but it is as perused in this project. There is no comparison for human ingenuity, and requirements might also vary, so does the components and tools.

The Essentials

1. 150W power Brick (Meanwell LRS-150-12 AC/DC, A little overhead is always good as a rule of thumb keep overhead between 20-30%)
2. Automobile auxiliary power outlet/Cigar lighter socket (to plug vacuum cleaner)
3. Banana socket for bench power supply needs.
4. AC Switch
5. Heat Shrink tubes, Different sizes / Electrical tape
6. Wire (>1 sq mm core, In this project 4 sq mm cable is used as it was lying around while I was working on this project in Isolation)
7. Capacitor (Electrolytic, 2200 uF x 2, 100 uF x 4, As a rule of thumb, choose electrolytic capacitor voltage rating to be more than twice use case voltage. In this case 50V capacitors were used.)
8. Ferrite Bead (10A current capacity, or 6A x 2 as used due to availability)
9. Common-mode Choke ( 10A)
10. Inrush Current limiter (Imax = 10A, or 3A x 3 as used in this project due to availability)
11. Soldering kit
12. Screwdrivers
13. Enclosure
14. Screws

Good to Have

1. Basic Multimeter with DC and AC measurement capabilities
2. Contactless temperature Sensor
3. Power tools. Follow safety precautions while using power tools.
4. Glue Gun
5. Zip-ties

DIY Final Design

After I planned to make it, I fell for the pre-castigation cycle only to think out, should I use direct wire soldering, or make a complete PCB for the design. Finally, I ended up making the wire soldering version. Being in pursuit of perfection and beauty, I postponed the project for many months. But Later I realized it’s not worth making PCB for this project. Also, I didn’t want any other feature in the power supply so it’s better off without PCB.

Casing

The idea is simple. The casing should be compact, robust, and safe. I have chosen an 8-inch x 6-inch electrical switch box without cutout for switches. The reasons for selecting an electrical switch box are cost and fire safety. Switch, and connectors are mounted on the opposite sides. We know its not a good arrangement, but EMI is not a great concern here, functionality is.

Circuit

The Story Behind

1st ignorant attempt

So, as soon as the idea struck my mind, the simple reaction was Ah! I have the AC-DC converter ( Meanwell LRS-150-12 AC/DC ) that I have purchased for some other project, I just have to connect the wires into sockets. And it will be done. Soon enough I checked the power supply rating. It is 150W 12V, and the input is 250V/120V switch-selectable working with frequency 50-60Hz both. All criteria are fulfilled. The power supply also has a builtin circuit for overcurrent protection, which seemed to be an added advantage. To verify I just connected the wires, plugged in the socket and switched the setup on, but to my surprise, it didn’t start. There was no power.

Here the multimeter came in handy. From some initial checks, it was confirmed that the AC plug wire which I have repurposed from an old extension cord has breakage in between. AC power itself is not available to the converter. It was a quick diagnosis. Without spending any minute, wire change was done. Switched it on again, plugged in the car vacuum cleaner and I can see the indicator led glowing, confirming that I am having the DC power. A quick check on the voltage reading confirmed to be 12V. All seemed okay at this moment, so just switched on the vacuum cleaner. The issue was far from over, the cleaner was trying to start but without success. This was on a repeated cycle. Probing the voltage shown that when the vacuum cleaner tries to start voltage drops to as low as 1V and as soon as it stops and before going into another cycle voltage was getting back to 12V. Looking at this my first thought was that, it might be the issue with the converter. I had never checked this power supply. It was lying down for a year so it might be bad. I packed everything back as I have to rush for the office. During the day whenever I was idle, the power supply was in my mind. Before marking it as e-waste I wanted to open it up and try to find the root cause. While thinking about the debugging plan, I suddenly realized, what if the overcurrent protection circuit is kicking in and switching off the supply.

As soon as I returned from the office, I got the setup out, adjusted the trim register to get 8V output and switched on the system. It was working at lower power. While the system was on, I increased the voltage back to 12V and now the vacuum cleaner was working in full power and there were no re-start cycles. This concluded the dry run of the first attempt indicating the issue of very high inrush current.

Learnings

• When designing a power supply, not only look for the regular power requirement but also consider the high inrush current demand.
• A soft start where power supply ramps up its voltage from low to high will limit the inrush current demand.
• Inductive loads such as motors have the tendency to consume very high current at start.

None of the learnings are new. I ask all the above points as questions to my team if these things are taken care of. All these are basic engineering know-how, which I have missed when I did a project in a nonorganized environment in a short time frame. So, In my opinion, the actual learnings from the 1st attempt are

• Failures are important, they prepare you and don’t let you forget the important points when you are working in a stressed environment or with very tight schedules. It is better to fail at DIY projects and prepare yourself for critical projects.
• There is credit in testing idea, you might get surprize from places which are never considered, and its a boon. When you are working on an important project there is less chance of surprise.

Update

Based on the learnings from the 1st attempt, some components were added rather than a direct wire connection. The circuit diagram is provided in the “DIY Final Design” section already.

• Ferrite bead to limit the sudden change in current.
• Inrush current limiter for soft start. Although the max current is 9A from 3 parallel inrush current limiters which is lower than required, so voltage setting is also reduced to 11.9V to avoid a meltdown.
• Capacitor bank for stabilization
• Common mode choke to suppress noise transmission from power supply to load side and vice versa.

In the current case, I didn’t want to change the power supply so voltage was reduced, but in the ideal case for the inductive load such as motor, it would be better to have a 50% overhead power.

Finally, this small project is complete and we are able to use the vacuum cleaner in the home. The project might be small but learnings are immense, not only in the engineering aspect but also in the life lessons aspect. It was a fun and adventure ride, a roller coaster for sure and the outcome was sweet. Be part of creator guild and keep creating, keep inspiring.