Introduction: Mini Adjustable Voltage Regulator

About: Electrical Engineer

The goal was to make a variable power supply and make it as compact as possible. This is what I made. I know there are many other voltage regulator projects out there, but I wanted to make mine to suit my needs. This instructable assumes you know how to solder and basic knowledge of how to align circuit components in a tight configuration. Let me know if you have questions, or try for help also.

The items listed below are what I used, or something similar. It isn't necessary to use exactly the same components that I have used, but I am giving you a few links that might help. I also didn't use the capacitors like shown, because I only use it to power things temporarily, and typically don't care if there is some fluctuation.

Bill of Materials

  • Dual 9v battery terminal (this one is parallel connected, I need series connected, just to get higher voltage range, but if you only need up to 9v, then this would work. If you can't find a series connected version, you can easily modify this one to be series)
  • If you use battery terminals like I've shown, you will need some prototype board, similar to this, but I highly recommend buying the dual terminals above because it created more work to solder them properly:

  • Mini Digital Volt Meter
  • A potentiometer with a knob, the LM350 recommends R2 = 5kohm along with the R1 = 240ohm, but I used R2 = 1000ohm and R1 was a 10-turn 20kohm pot (If I ever rebuild this, I would definitely look for a physically smaller one, but still 10-turn, however this is what I had on hand). I show other potentiometers, but those were very high values like 500kohm and 2Mohm, they might have worked, I just chose not to use them.
  • 1000ohm resistor (if you use a 20kohm pot)
  • LM350 3A Voltage Regulator
  • Toggle Switch something like this is what I used:

  • Small alligator clips
  • Some wire
  • Solder
  • Heat shrink
  • Duct tape or something to apply to the back of the volt meter as insulation (short circuits are usually bad)
  • Epoxy (I used 5 minute type, but others could be used)

Note: If you use the dual battery terminal listed, then you would not need the PCB

Step 1: Schematic and Battery Terminal Prep

For the circuit, be sure you check the datasheet of whatever voltage regulator you are using. I chose the LM350 because it has higher current output, but I've used LM317 in the past. If you plan to drive something with higher current needs, you might need a heat sink.

You will need to cut wires and cut open the plastic sheath around each battery terminal, if you use the individual 9v battery terminal with wires. If I were to build this again, I would use the dual terminal type on the first page, and modify it to be series rather than parallel like that link shows.

Then you need to cut the prototype board to fit roughly the size of the two batteries together.

Solder the terminals onto the prototype board as shown, and add a short wire beneath the terminals on one end (as shown the red wire in the 4th picture). This will put the batteries in series to give 18v, when you connect the other terminals. Keep track of which is + and - for correct operation.

Step 2: Voltage Regulator Placement and Wiring to Voltmeter and Potentiometer

I placed the voltage regulator on one edge so that I could bend the input terminal and make it stick through to the positive battery terminal. Then you just bend it to touch the terminal from the other side and heat it up and apply solder.

Then you cut the resistor shorter and bend one end around so it can be soldered to the regulator terminals as shown. I planned ahead and applied heat shrink tubing once I had the wires soldered, but don't do it too soon unless you know where to make the next connections.

Refer to the schematic to understand where the volt meter needs to be connected. The red wire is the power for the meter itself, so I connected it to the 18v, and the black wire to the ground point which will be after the switch.

I think I missed taking a few pictures for the rest of the wiring, but if you follow the schematic, you will see the battery ground is connected to one terminal of the switch, then another wire will connect to a terminal of the potentiometer (see yellow arrows on 6th picture).

Then the adjust terminal of the regulator will have a wire soldered to it which will go to the one terminal on the potentiometer. And the third terminal on the potentiometer is unused. Be sure you check that you have adjustable resistance. If you connect the two on the "ends" of the potentiometer, it will just be the max resistance and not on the wiper. The potentiometer I used had its own schematic showing what each pin 1, 2, and 3 are.

The voltmeter black ground will connect to the same terminal of the switch that the potentiometer is, and the red positive wire will connect to the bent terminal of the regulator, which is the Input voltage. The white wire will connect to the Output of the regulator. I made each wire length fit to the distance and location of each connection, keeping enough slack to allow easy assembly.

Be sure you add duct tape or something as an insulator on the bottom of the voltmeter.

Step 3: Final Assembly and Epoxy

At this point, all the wires should be soldered. You can carefully attach the batteries and turn on the switch and measure the voltage from the Output to the Ground terminals and turn the potentiometer, the voltmeter reading should change.

If it makes it easier, solder some wire (see the silver wire) to the Ground terminal of the potentiometer and the other wire (copper) to the output of the voltage regulator and solder the wire to the alligator clips (you might need to redo the heat shrink if you already did that earlier.

If this test was OK, then you can move on to epoxying everything together. No secret or best method, just hold the pieces where you want them to be, then find some way of temporarily holding them together. I used Scotch tape and a Quick-Grip clamp or something, then applied some 5-minute epoxy with a toothpick and just waited until it was dry, then rotate the assembly and add more epoxy as needed to ensure it holds tight. Then remove the tape, clamp, or whatever temporary holding method you used.

Step 4: Short Video Showing Voltage Changing

The minimum output is based on the voltage regulator's internal reference voltage (look at the datasheet formula), and the maximum output is based on the batteries connected and how much energy they have left (as well as the maximum specified in the datasheet, in case you connected much higher voltage).

Hope you enjoy making this.

Step 5: Example Usage

Just showing an example of how I use this adjustable power supply. Here I have a small humidity sensor and another volt meter display to read its output. I set my main unit at 5.0v and the sensor is reading 1.8v. By the datasheet, that means about 40% RH. This made it easy for me to do a quick check of multiple locations, without needing to build a permanent circuit. Eventually, I will just build a permanent circuit or buy a small unit to display the current humidity.