Variable Power Supply

 Purpose:

I recently began using transistors in my projects! While transistors are awesome devices for controlling the flow of electrons, they often have very wide operational tolerances that make it important to test-bias transistors instead of relying on computation alone when incorporating them into a circuit. While transistor biasing is a somewhat complex topic that I'll save for another time, just know for now that biasing a transistor involves applying specific voltages and restricting currents to different pins to control how the transistor behaves. 

Long story short, I need to be able to precisely control voltage at greater voltages with much more oomph than my Arduinos or Analog Discovery 2 (AD2) can provide. I will use two OPA551 power operational amplifiers and two cheap 18VDC fixed-power supplies to take 0-5V signals from my AD2 to control the op-amps' output from -15 and +15 volts. By using two supplies and two amplifiers, I can have two independently controllable 15V power supplies or I can combine their voltage potentials to achieve +-30 volts.

The Circuit:

I will build this circuit twice and tie Vm to ground. This will allow all of our voltage control to be done by just changing the voltage supplied to Vp1 and Vp2.

The OPA551 Operational Amplifier:

Check out the Datasheet (Texas Instruments)

Materials:

1. (x2) fixed-voltage wall plug power supplies (I'm using two 18V AC to DC switching supplies)

2. (x2) 2.2mm barrel plug to bread board adapters

3. (x2) OPA551 high-voltage, high-current operational amplifiers

4. (x2) 3.3kΩ resistors

5. (x2) 10kΩ resistors

6. A digital lab / +- 0-5V power supply such as an ADALAM2000 or Analog Discovery 2

7. A breadboard, jumper wires, and some header pins

Procedure:

1. Wire up the circuit from figure 1, twice. Each 18V wall power supply will ahve its own operational amplifier, Vp, and Vout. Vm is grounded to the voltage applied to Vp is equal to the voltage difference between the positive and negative terminals of the op-amp

2. Connect digital lab's 0-5V power supplies.

a) V+ and V- to Vp1 and Vp2, respectively

b) Oscilloscope 1+ to Vout1 and 1- to ground

c) Oscilloscope 2+ to Vout2 and 2- to ground

d) Digital lab ground to op-amp ground

3. Test the circuit with a voltage sweep and map Vp inputs to Vout to generate a key for which values of Vp result in which values for Vout.

In the screenshot below, you can see that +5V and -5V were produced on Vout1 and Vout2 from Vp inputs of 1.656V and -1.656V.

Results:

Graphing useful Vout values show a linear relationship between Vp and Vout. The resulting graph is a useful reference to find the right Vp values for a desired Vout when using this power supply in the future.

Now that I've build this variable power supply, I can step-down the two +-18V fixed power supplies to a variable voltage within the range [-15, +15V] with enough current to power all the breadboard components my heart desires. Max current outputs are driven by the OP551 since its ratings are lower than the 18V power supplies output of 18 watts. This power supply will be incredibly helpful when a variable volage is required for biasing circuit components as well as many other projects in the future!