When we disassemble an old electronic device, not everything necessarily needs to be discarded. Many devices contain components and circuits that can be reused in other electronics projects.
FM radios are a good example. Inside them, we can find speakers, transformers, potentiometers, electronic components, and, depending on the device, a power supply that can be studied and reused.
In this project, I disassembled an FM radio and took advantage of the opportunity to examine its power supply, understand how it works, and discuss some ways to improve it using capacitor filtering and voltage regulation.
What Do We Find When We Disassemble an FM Radio?
Opening an electronic device is an excellent way to learn electronics in practice.
In a radio, we can identify different stages responsible for specific functions:
- power supply;
- radio signal reception;
- tuning;
- audio processing;
- amplification;
- speaker.
In this case, our main interest is the power supply.
It is responsible for providing the circuit with the electrical conditions required for all these stages to operate correctly.
How Does a Power Supply Work?
The basic function of a power supply is to provide the appropriate voltage and current to the circuit.
In devices powered directly from the electrical mains that use a traditional linear power supply, we may find stages such as:
electrical mains → transformer → rectification → filtering → electronic circuit
The transformer reduces and isolates the mains voltage, while the diodes perform rectification, converting the alternating voltage into a pulsating voltage with a single polarity.
But this voltage is still not perfectly steady direct current.
This is where an extremely important component comes into play: the filter capacitor.
What Is the Capacitor Used for in the Power Supply?
After rectification, the voltage fluctuates.
An electrolytic filter capacitor stores energy when the voltage rises and supplies some of that energy to the circuit when the voltage falls.
In very simplified terms:
AC → rectification → pulsating voltage → capacitor → DC voltage with less ripple
This residual fluctuation is commonly called ripple.
The better the filtering is sized for a particular load, the smaller this variation tends to be.
This is especially important in audio circuits because problems with the power supply can appear as noise or hum in the sound.
Does a Larger Capacitor Improve the Power Supply?
This is an interesting question because the answer should not simply be “the bigger, the better.”
Properly increasing the filter capacitance can reduce ripple under certain conditions. However, the capacitor must be selected with several circuit factors in mind.
In addition to capacitance, we must pay particular attention to its voltage rating.
There are also consequences for the initial charging current, rectifier, transformer, and other components.
Therefore, we should not simply find the largest capacitor available and install it in the circuit.
What matters is understanding why that capacitor is there and how its value affects the behavior of the power supply.
Filtering Is Not the Same as Regulation
This is a very important concept.
The capacitor can improve filtering, but that does not necessarily mean we will have a perfectly regulated voltage.
They are different functions.
We can think of it this way:
Filtering → reduces voltage ripple
Regulation → attempts to maintain the output voltage at a certain value despite variations within the circuit’s intended operating conditions
Therefore, if we want to turn a simple reused power supply into a more suitable power source for another project, it may be useful to add a voltage regulation stage.
How Do We Regulate the Power Supply Voltage?
There are several ways to do this.
A classic solution in linear power supplies is to use a voltage regulator.
Depending on the desired voltage, we can use fixed or adjustable regulators.
However, there is an important condition: the regulator must receive an input voltage compatible with its operation and must operate within its current, power, and temperature limits.
Simply adding a regulator without analyzing the circuit is not enough.
We need to consider:
input voltage → desired voltage → load current → power dissipation → temperature
In some cases, a heat sink will also be necessary.
Measuring the Power Supply Before Modifying It
Before reusing any power supply removed from a device, I would start with measurements.
Using a multimeter, we can check the available voltage and better understand the circuit.
This is much more useful than simply looking at the transformer or components and assuming that a certain voltage will be present.
Practical electronics constantly involves this process:
observe → measure → understand → modify → measure again
This allows us to verify whether our modification actually produced the expected result.
Can I Use the Radio’s Power Supply in Another Project?
In some cases, yes, provided its characteristics are suitable for the new circuit.
We need to know at least:
- available voltage;
- current the power supply can provide;
- quality of the filtering;
- need for regulation;
- characteristics of the new load.
A power supply that worked perfectly inside a radio is not automatically suitable for any other device.
The original load was part of the design of that power supply.
When we change the application, we need to check its operating conditions again.
Be Careful When Working with a Device’s Power Supply
There is one particularly important precaution here.
When disassembling a radio powered from a wall outlet, we may encounter parts of the circuit connected to mains voltage.
Even after the device has been switched off, certain capacitors may remain charged for some time.
Therefore, this type of experiment requires proper knowledge and precautions. For beginners, it is much safer to carry out initial experiments with low-voltage power supplies that are already isolated from the mains.
The goal of reusing components should never justify working unsafely.
See the FM Radio Disassembly and Power Supply in Practice
In the video below, I show the radio disassembly and use its existing power supply to explain these concepts, including ways to improve its filtering and work with voltage regulation. Video in Portuguese, use subtitles from YouTube.
Why Does Disassembling Equipment Help You Learn Electronics?
There is a big difference between studying what a transformer, diode, or capacitor is in isolation and finding these components working together inside a real device.
When we disassemble a radio and identify its power supply, we begin to see a logical sequence:
transformation → rectification → filtering → powering the circuit
And from there, we can start asking questions.
Why does this capacitor have this value? What is the voltage after rectification? Is there ripple? What happens when we connect a load? Can the output be regulated?
These questions turn a simple disassembly into an electronics learning experience.
Learn Electronics by Observing Real Circuits
One of the best ways to develop electronics knowledge is to connect theory with what we find inside actual devices.
A power supply removed from a radio can help us understand transformers, diodes, rectification, capacitors, filtering, ripple, and voltage regulators.
Once these concepts become clear, we begin to encounter them again in amplifiers, pedals, receivers, microcontroller projects, and many other devices.
This is how components stop being isolated elements and begin to form functional blocks that we can recognize in different circuits.
Want to Go Further with Electronics Applied to Audio?
Since this project starts with a radio and directly involves powering audio circuits, it also provides a good bridge to other content about amplifiers, power supplies, noise, shielding, and electronics applied to audio available here on the site.
In my Electronics Applied to Audio Course, the goal is precisely to approach electronics in a practical way and understand the circuits found in real-world applications.
To continue exploring free content about rectification, filtering, voltage regulators, adjustable power supplies, and other power supply circuits, also visit the Power Supplies section.