There are a lot of convenient and inexpensive ways to get get variable voltage from a fixed voltage DC power supply. The previous post detailed one way with an adjustable boost module. Many variations of these inexpensive modules are commercially available.
This post covers how to build a slightly different unit with an LM2596 step down DC buck converter module. The main difference with this unit and the adjustable boost module in the previous post is that this unit does not support increasing voltage. It is strictly for stepping down voltage. But I think it’s a good unit and definitely worth having in my toolbox. This post describes the steps on how to build it.
I bought this unit at Amazon. There was an option to buy it with the heat sink and mounting hardware but I ordered it without them. That was a mistake. It was a hassle tracking down all the parts from other suppliers. I wound up having to locate one supplier for a small quantity of standoffs (M3 x 5mm female + 6mm male) with nuts, plus another supplier for the screws (M3 x 5mm) and another supplier for the heat sink with thermal adhesive (11mm x 11mm x 5mm).
Luckily it all came together in the end. Even if I did buy the unit at Amazon with the heat sink and mounting harware option, the unit still did not include the screws. I would have had to get them from another supplier anyway. That’s just a heads up if you go shopping for one of these units. I haven’t found any that include the screws.
Just like in the previous post, this unit was assembled in a clear plastic box with a hinged lid. It was convenient so the voltmeter would always be visible plus I would have access to the controls, wiring and connectors.
This unit is wired up exactly the same as the in the previous post. There are individual positive and negative banana jacks for input and output plus a DC power jack for an AC adapter input.
This unit is very similar to the adjustable boost module with an adjustment screw on a trimmer pot to set the output voltage. It also has a button to switch the voltmeter display from input to output voltage. The input range is 4 to ~40v and the output range is 1.25 to ~37v at 2 amps (normal and stable), 3 amps maximum.
Before getting started I wanted to check the input and output voltages with a multimeter.
When the voltmeter displays the input voltage a red LED lights up. The input voltage checks out.
A green LED lights up for the output voltage. Unlike the adjustable boost module there is no calibration function included. Luckily it checks out okay.
The photo below shows the parts needed for the build – back row: step down DC buck converter module, mounting hardware, plastic box (3 9/16″ x 2 9/16″ x 1 1/8″) front row: 2 black banana jacks, 2 red banana jacks and a 2.1 mm coax panel mount DC power jack.
Below is the wiring diagram for the build:
Next I drilled the holes in the plastic case. All the holes were drilled with a sharp 1/8 inch drill bit and reamed to a larger size where needed with a taper reamer. This was the safest way to drill the holes because the box was made from styrene plastic, which is very brittle and cracks easily.
The markings were made for the holes by measuring out the locations on plain white paper. The paper was trimmed with a utility knife and affixed to the box panels with a roll on glue tape dispenser and repositionable adhesive (available from a stationery store). The markings for the base holes was affixed to the underside of the box.
The markings for the side panels was affixed to the inside of the box.
I placed the box over a small block of wood to drill the holes in the base.
I used a smaller block of wood to drill the holes in the side panels.
Shown below are the finished holes in the base.
Below are the holes for the input side panel. After the holes are drilled, simply peel off the paper and it comes off clean.
I used a taper reamer to gradually enlarge the side panel holes one at a time and periodically checked the fit of each component. The job went very fast and easy. It is much safer than using a larger drill bit. The plastic is very brittle and may crack.
The hole shown below is finished after reaming.
After all the holes are drilled and reamed I mounted all the parts to make sure everything fit.
Remove all the parts and cut some leads from 24 gauge stranded wire. I used red and black wiring for positive (red) and negative (black) connections. The center post in the 2.1mm DC power jack is for the positive wire.
The next step was to connect the input and output leads to the screw terminal blocks on the circuit board, then mount the circuit board to the box with screws. Shown below is the finished assembly. It is ready to use.
After assembly I checked the DC power jack by connecting a 12v AC adapter. The voltmeter displays the input voltage.
The last step was to check the banana jacks with a DC bench supply. The voltmeter displays the input voltage.
The unit is ready to connect to a device or circuit board. Adjust the output voltage with the screw on the trimmer pot. Just make sure not to exceed the current capacity of the unit or the power supply.