Anyone who starts building electronic circuits quickly realizes that there is one piece of equipment that ends up being used practically all the time:
the power supply.
Batteries are useful, but we don’t always have exactly the voltage we need.
For one project, we might need:
3 V;
for another:
5 V;
then:
9 V;
or perhaps:
12 V.
A very useful solution for the workbench is to use a variable power supply, which allows us to adjust the output voltage according to the circuit’s needs.
And it becomes even more useful when we can also monitor:
the voltage and the current drawn by the project.
In this video, I show exactly that: a homemade variable power supply with a voltage and current meter.
What Is a Variable Power Supply Used For?
Imagine that you have just finished building a small circuit.
It was designed to operate at:
6 V.
If we only have a 12 V power supply, we cannot simply connect it to the circuit.
We need to provide the appropriate voltage.
A variable power supply allows us to adjust its output before connecting the project.
We can then select, for example:
5 V → 6 V → 9 V → 12 V
depending on what we are testing and the limits of the power supply itself.
This makes the equipment extremely versatile for an electronics workbench.
Why Not Just Use Batteries?
We can.
For certain experiments, batteries are excellent.
The problem is that when we start experimenting with several circuits, we would need different combinations of batteries to obtain different voltages.
In addition, during testing, we may want to gradually change the voltage.
With a variable power supply, we can do exactly that:
adjust → measure → test → readjust.
It is a much more practical tool for experimentation.
See My Homemade Variable Power Supply
In the video below, I present the power supply I built and demonstrate how it works, including the voltage and current readings.
Homemade Variable Power Supply — With Voltage and Current Meter
After seeing the project, we can better understand why these two measurements are so important in a bench power supply.
The Voltmeter Shows the Voltage
The first thing we want to know is:
what voltage am I applying to the circuit?
The unit used is the:
volt (V).
If we adjust the power supply to approximately 5 V, for example, the voltmeter allows us to check the actual value present at the output.
This is much better than relying only on the position of a knob.
We have an:
actual measurement.
The Ammeter Shows the Circuit’s Current Consumption
The second piece of information is even more interesting:
how much current is the circuit drawing?
Current is measured in:
amperes (A)
or, for smaller values:
milliamperes (mA).
So, when we connect a project to the power supply, we might observe something like:
5.0 V — 80 mA
or:
12.0 V — 1.2 A.
Now we know more than just the applied voltage.
We also begin to see:
how the circuit is behaving.
Why Is Knowing the Current So Useful?
Imagine a small circuit that should normally draw around:
100 mA.
You turn on the project and observe:
900 mA.
This immediately catches your attention.
There may be:
a short circuit;
an incorrectly connected component;
an overload;
a stalled motor;
a faulty component.
Therefore, the ammeter is not just there to satisfy our curiosity.
It can serve as an important:
diagnostic tool.
Voltage and Current Are Not the Same Thing
This difference is fundamental.
The power supply establishes a certain:
voltage.
The connected load determines, according to its characteristics and the circuit, how much current it will draw.
Imagine a power supply adjusted to:
12 V
and capable of supplying up to:
5 A.
This does not mean that it will necessarily push 5 A into every connected circuit.
If a particular load draws only:
200 mA,
approximately those 200 mA will flow under normal system conditions.
The power supply’s 5 A rating represents its maximum current capacity, not a current that will necessarily be applied to the load.
The Power Supply Must Support the Required Current
Now imagine the opposite situation.
We have a device that requires:
12 V and 3 A.
Our power supply provides:
12 V, but can only supply 1 A.
The nominal voltage is correct, but the power supply does not have enough capacity to power that load properly.
It may:
reduce its output voltage;
enter protection mode;
overheat;
or even become damaged, depending on its design.
That is why we must always consider:
voltage + current.
And We Can Calculate Power
Knowing the voltage and current, we can approximately calculate the electrical power in a DC load:
P = V × I
where:
P = power in watts V = voltage in volts I = current in amperes
Imagine that the display shows:
12 V
and:
1.5 A.
We have:
P = 12 × 1.5
P = 18 W.
Therefore, at that point, we are supplying approximately:
18 watts.
Why Is a Bench Power Supply So Useful for Testing?
One of its major advantages is being able to observe the circuit while changing its power supply conditions.
Imagine a project that should operate between:
6 V and 12 V.
We can start with a lower voltage and gradually increase it, always respecting the circuit’s specifications.
While doing this, we can observe:
voltage;
current;
heating;
operation.
This provides much more information than simply connecting a battery and checking:
“did it work or not?”
We Can Find Problems by Observing the Current
Suppose we are testing a small motor.
With no load, we observe:
300 mA.
We apply a mechanical load and find:
700 mA.
We increase the load even further:
1.2 A.
We are observing electrically what is happening mechanically.
The greater the effort required from the motor, the higher its current may become.
If we completely prevent its shaft from rotating, the current may reach a very high value.
This is called:
locked-rotor current, or stall current.
This shows why a power supply with a current display is so useful.
Current Limiting Is Another Important Feature
More advanced bench power supplies may allow us not only to adjust the voltage but also to set a:
current limit.
Imagine that we are powering up a circuit for the first time, and it should draw approximately:
100 mA.
We can set a relatively low limit and power the circuit with greater control.
If there is a problem and the circuit tries to draw a very high current, the power supply can limit the current.
This feature is known as:
current limiting.
Constant Voltage and Constant Current
In power supplies with current control, we often find two operating modes:
CV — Constant Voltage
and:
CC — Constant Current.
In CV mode, the power supply tries to maintain the:
set voltage.
In CC mode, when the load tries to draw more current than the established limit, the power supply reduces its voltage as necessary to maintain approximately:
the limited current.
This is a very useful feature in a bench power supply.
However, it is important not to confuse CC for Constant Current with the Portuguese abbreviation CC for corrente contínua (direct current); the context indicates which meaning is intended.
A Homemade Power Supply Needs Clearly Defined Limits
Building our own power supply is an excellent learning experience.
But we need to understand the limitations of the circuit being used.
A power supply has limits for:
voltage;
current;
power;
temperature.
If the regulator circuit can supply a certain maximum current, we cannot simply install a 10 A display and conclude that our power supply can now provide:
10 A.
The meter only:
measures.
The power circuit determines the power supply’s capacity.
The Transformer Also Has Limits
In power supplies that use a mains transformer, the transformer has its own specifications.
It cannot supply unlimited power.
If we have a certain voltage and a certain current capacity, we also have an approximate:
power capacity.
In addition, after the transformer, there may still be:
rectification;
filtering;
regulation.
Each stage must be properly sized.
From Alternating Current to Direct Current
In a traditional mains-powered linear power supply, we may find a sequence similar to:
electrical mains
↓
transformer
↓
reduced AC voltage
↓
rectifier
↓
pulsating DC
↓
filter capacitor
↓
DC voltage
↓
regulator
↓
adjustable output.
Each stage has a specific function.
This is what transforms the energy available from the mains into a voltage suitable for our small electronic circuits.
What Are Rectifier Diodes Used For?
After reducing the AC voltage, we need to convert it into a unidirectional voltage.
We can use:
diodes.
A very common configuration is the:
bridge rectifier.
It uses diodes to produce an output current with a defined polarity from the AC voltage applied at the input.
But this output is not yet perfectly smooth DC voltage.
The Capacitor Helps with Filtering
After rectification, we normally use a:
filter capacitor.
It stores energy when the voltage rises and supplies some of that energy when the rectified voltage decreases.
The result is a reduction in:
ripple.
We then have a voltage that is much more suitable for subsequent regulation.
And Finally, We Adjust the Voltage
After rectification and filtering, we can use a regulator circuit to obtain the desired voltage.
Depending on the design, this can be achieved with:
linear regulators;
transistors;
switching converters;
or other topologies.
Each solution has advantages and limitations.
A linear power supply, for example, can be very useful for certain applications, but it may dissipate a considerable amount of heat when there is a large difference between the input and output voltages.
Why Can the Regulator Get Hot?
Imagine a linear regulator receiving:
15 V
and supplying:
5 V at 1 A.
We have approximately:
10 V
across the regulator.
The power dissipated will be approximately:
P = (15 − 5) × 1
therefore:
10 W.
That is a considerable amount of heat.
That is why we often find:
heat sinks
in linear power supplies.
A Heat Sink Is Not Just a Piece of Metal
Its function is to facilitate heat transfer from the component to the surrounding environment.
We can think of it as:
semiconductor
↓
package
↓
heat sink
↓
air.
This creates a very interesting connection between this project and the study of:
power and temperature.
A power supply is not just an electrical project.
It also has:
thermal aspects.
The Enclosure Is Also Important
When building a homemade power supply, we need to think beyond the circuit.
There is also the:
physical construction.
We need to organize:
connectors;
meters;
adjustment knob;
switch;
cables;
ventilation.
A well-organized build makes the power supply much more convenient and safer to use on the workbench.
Pay Special Attention to Mains Electricity
There is an important difference here compared with many low-voltage projects.
If the power supply is powered directly from the electrical mains, there are internal points that may operate at:
dangerous voltages.
This requires appropriate precautions regarding:
insulation;
fuses;
cables;
connections;
enclosure;
grounding, when applicable.
We should not treat the mains-connected side in the same way we treat a small battery-powered circuit.
For beginners, a safer alternative for experiments is to work with an external isolated DC power supply and build only the low-voltage regulation stage.
A Power Supply Changes the Way We Work at the Bench
Before having a variable power supply, many experiments end up starting like this:
“where is that 9 V battery?”
or:
“do I have a 6 V power supply somewhere?”
With a bench power supply, the approach changes:
adjust the voltage
↓
connect the circuit
↓
observe the current
↓
perform the test
↓
change the conditions if necessary.
It becomes an instrument used in practically every other project.
The Display Turns the Power Supply into a Diagnostic Instrument
This is perhaps the most interesting aspect of the power supply presented in the video.
We do not just have:
energy going out.
We also have:
information coming back to us.
The voltmeter tells us:
what the power supply is delivering.
The ammeter helps us understand:
how the load is responding.
This allows us to use the power supply not only to power circuits but also to:
understand them.
An Essential Tool for Learning Electronics
Building a variable power supply is one of those projects that remains useful long after the experiment is finished.
You build it once and then use it to test:
LEDs;
motors;
amplifiers;
transistor circuits;
Arduino;
electronic modules;
audio projects.
And with every new circuit connected, the voltage and current displays help answer two fundamental questions:
what voltage am I supplying?
and:
how much current is my circuit drawing?
When we begin answering these questions with measurements instead of assumptions, our workbench stops being just a place where we build circuits.
It becomes a place where we can:
experiment, measure, diagnose, and learn electronics.
Want to Learn More About Electronics in Practice?
A bench power supply is directly related to many fundamental electronics concepts:
voltage;
current;
power;
diodes;
rectification;
capacitors;
filtering;
regulation;
heat dissipation.
These same concepts appear constantly when we build and analyze audio circuits.
In my Applied Electronics for Audio Course, I cover these fundamentals in a practical way so that you are not limited to copying schematics: the idea is to understand what is happening in each part of the circuit and know how to use measurements to find problems. https://www.alexbaroni.com.br/pv_1h_linear/pv_audio_elet/
To continue exploring free content about power supplies, regulators, rectification, filtering, converters, and workbench projects, visit the Power Supplies section.