Make Your Own Variable DC Power Supply

25/7/21

Hello RC enthusiasts from around the world and welcome to this post. I hope you are all well. 

In this post I will be showing you how I built my own variable DC power supply, as the can be super useful in our hobby. However, they are not cheap. 

Therefore, I will be repurposing an old ham radio supply that I built, that I no longer use. The enclosure will also be reused as far as possible for the new purpose. Most of the parts removed from the old build will also be reused in this build.

 Some new parts will be required for varying the voltage and I will explain their use. The enclosure is made from 6mm fire retardant marine ply, which is also reclaimed timber.  I will be supplying the dimensions and circuit diagrams I used for this build. I will not be adding prices and where to purchase from, as these parts are readily available and prices vary greatly from country to country. A variable power supply, for myself is vital when working with RC cars, components and batteries. There are many problems you can solve and diagnose with this variable DC power supply.

I actually designed and built this last year, but I wanted to fully test the unit before sharing it's construction. Obviously I would not share my idea if it was not safe to do so. Whilst on the topic of safety, I take no responsibility for any injuries, damages or deaths caused if you decide to follow these instructions. An unprotected 240 VAC will kill an adult person instantly, FACT. I would advise that only trained, qualified and competent persons attempt to build this project. I cannot emphasise enough how dangerous AC voltage can be, even for the qualified person. Please be careful.

OK, here we go!

Above is a picture of the defunct Ham radio supply that I will be repurposing. As you can see there are 4 displays used in this build that I can salvage as well as many other components. Waste not, want not! Time to start the salvage operation.

As you can see the tear down operation proved a success for reclaiming components and cables. As you can see the enclosure is a sorry sight with all those drill holes. That problem will be addressed in the new build.

I first needed to make a new piece to replace the old front plate. This piece will be the new lid for the unit. A new front plate will now need designing to fit the dimensions of the case. I do have a limited supply of marine plywood so I need to be as accurate as I possibly can. 

 

As you can see, it is a very basic design to understand and copy. Because my unit will be situated in a very hot, dry and hostile environment, I incorporated a temperature display that can control the on & off operation of an internal cooling fan. With the temperature controller display at the top, we then have a timer and clock display which I always find useful. To the direct right of the timer is an illuminated momentary switch that toggles through the timer functions. Below the timer is the red LED ammeter display with its adjust potentiometer to the right with a corresponding red cap. At the bottom is the blue LED voltmeter with its corresponding potentiometer capped in blue. At the bottom of the new face plate are the high quality binding posts I invested in. I want this unit to last forever, if it can.

Above is a picture of what I started with, albeit the newly cut display holes. If this instrument is intended to last a long time, then it needs to have protection on the long and arduous path ahead. This was achieved be test and design. 

 The dimensions in the above image are correct. Please ignore my sequences and drawing notes. The side plates do have a drawing for instruction with added notes.

With the dimensions set, I pushed on and completed the enclosure. 

In the picture above, you can see I have painted the inside of the case grey. This is for one simple but important reason. Any unwanted material inside the case can be seen and removed easily. We do not want anything inside this enclosure other than what is required. An example of this could be a piece of cut off wire that has the potential to cause a short circuit resulting in damage/fire/injury. Now the main enclosure is nearing completion, it was time to follow the new front plate design posted earlier.

As you can see it turned out quite well and I am pleased with my results. At this point I still need to drill holes for the potentiometers, power switch, momentary switch and the binding posts. Considering I am no carpenter, joiner or cabin maker, it turned out quite well. 

I then assembled the front plate with the components needed. With all the components aligned, straight and level I was happy to move on to painting the outer case. 

When I was removing the displays, disaster struck. I was not careful enough and split a lamination layer in 2 places as can be seen above. As I earlier mention, my reclaimed marine plywood is of a limited supply so, unfortunately I cannot remake it. It has to stay. With the case completed, I started to install the wiring and components.

To power this little project, I have employed a 12 VDC, 5 Amp and rated at 60 Watts SMPS (switched mode power supply), which is more than capable for its intended use. It's  position, within the cooling aspect is critical. It needs many cubic centimetres as the heat builds up and is purged during ventilation. The PCB that controls the variable voltage and current is a buck/boost LTC 3780. It is the blue PCB in the centre of the picture below. It is a small yet effective module that can deliver 60 watts at 12 volts which is more than enough for my needs. It is also quite compact so it was the best choice for this application as there is little room at the intended site.

My next job was to populate the front panel with the controls and displays needed for its operation as can be seen above. Now that I was happy with the component positioning and spacing, I pushed on. 

To connect the wires together I needed to create a circuit diagram.

As you can see it is quite an easy circuit to follow, with almost every connection being made mechanically.  In the above circuit diagram there are written instructions detailing the removal of the on board micro potentiometers and replacing them with conventional potentiometers situated on the front plate. 

   I have removed the micro potentiometers from the PCB. I marked there use with An I for current and a V for voltage. The 200 Ohm conventional pot controls the variable current, whereas the 300 Ohm pot controls the variable voltage. You can see the conventional potentiometers inserted into the front panel and have extension cables attached to them. These wires will be soldered onto the pads where the micro potentiometers were originally positioned. I used 5 amp 3 core flex to make the extensions, however I did eventually  remove the outer sheath insulator as is was a little bulky. 

Now the display modules in place, I needed the side panels cut and drilled. I followed the above diagram. The left and right hand side panels are almost identical albeit that the right hand side has an extra reveal to accommodate a 240VAC mains filter. I needed the side panels completed as they too will be fitted with components that need to be wired.

The above picture shows the case completed and with all the components installed. You can also see I have connected the conventional potentiometers on the front panel to the LTC 3780 pads where the micro potentiometers were originally installed. With the case complete and all components in position, it was time to start connecting everything together.

I had at that time just disassembled 2 desk top PC's so I took the power plugs and socket looms to help my build process as can be seen above. I will show this in better detail when I get to that point. 

I started the wiring using the plugs and sockets, this means in can disconnect any part very easily for maintenance. As you can see the mains voltage filter wiring going off to the front panel. There is also a return path for the mains voltage back to the switched mode power supply (SMPS). Then the 12VDC is then sent back to the front panel.

With the return wiring now installed (black plug) and the mains (white socket) input finished, I could concentrate on the front panel wiring.

The front panel took a little while to complete as I needed it as neat as possible. As I have previously stated, that space is of a premium inside the enclosure. I am more than happy with my efforts as it looks nice and tidy with nothing sticking out and it is all secured to the front panel. I flowed the cables to the display side, again this would have been a space issue if I had not. 

As you can see in the picture above that there is not a lot of space between the LTC 3780 and the SMPS. 

The only part of the enclosure I did not renew was the back panel. This was partly down to lack of materials but also it will be hidden nearly all the time, at the back. You can also see above the sockets that cannot get mixed up as they are of opposite types. 

Another angle of the wiring, showing why I flowed the cable to one side.

Above is a picture of the only fan in the unit. It actually sucks air in from the other side and over the components and exhausts from the fan. It is one of the best cooling methods I have tried and believe me, I have tried and failed with many types of cooling. This system works perfectly as it's been in use for more than 12 months in some really hot conditions. The fan I choose is a little noisy, but that is of no concern to me. 

Above are the instructions for the temperature display, giving instructions of how to wire it and how to control it.

As you can see the completed power supply above. I designed operations with  matching colour codes for ease of use. It is also narrow enough to sit easily fit on a shelf with the side entry AC to aid in that position. I can easily set the temperature to a desired level to activate the fan and purge the warm air with the top display. The timer/clock is in green and has a green illuminated momentary switch to toggle through the functions. Below that is the red LED ammeter with matching red capped potentiometer. The bottom display is a blue LED  voltmeter with a matching blue capped potentiometer. At the centre bottom is a pair of high quality gold plated binding posts, which are perfect for all types of connection.

 The 240VAC green mains switch failed during operation, so I replaced it with an LED switch finished in black. Also the green switch was illuminated by a neon light, which got really hot. However, because all the connections are mechanical, it was an easy replacement. That will be the case for any component used in this build.

Well my friends, I hope you have enjoyed this post. I also hope it helps you build your own variable power supply. They are ultra useful in a workshop or home and a must for all hobbyists. 

Until my next post, take care. 

CATXLS 25/7/21