Anyone who has ever opened an electric toy, a remote-controlled car, or another device that uses a brushed DC motor has probably come across a small capacitor soldered directly across the motor terminals.
At first glance, this connection may seem strange.
After all, if the motor runs on direct current and the capacitor is connected in parallel with its power supply, what exactly is the purpose of this component?
This small capacitor is there primarily to reduce the noise and high-frequency signals generated by the motor during operation.
Why Does a Brushed Motor Generate Electrical Noise?
To understand the capacitor’s purpose, we need to look at how a brushed motor works.
Inside the motor, brushes and a commutator are responsible for successively establishing the electrical connections needed to keep the rotor spinning.
During operation, these contacts are constantly made and broken.
This can produce small sparks and rapid changes in current.
Therefore, even though we supply the motor with DC voltage, its operation is not electrically “silent.”
It can generate electrical noise containing high-frequency components.
What Does This Have to Do with the Capacitor?
A capacitor behaves differently depending on the frequency of the signal applied to it.
In simple terms, under steady-state DC conditions, it behaves like an open circuit.
That’s why connecting a capacitor in parallel with the motor does not mean short-circuiting the power supply. This was precisely one of the points discussed in the original article.
However, its behavior changes when rapid voltage variations occur.
The higher the frequency, the lower the opposition the capacitor tends to present to these alternating components.
That’s what makes it so useful in this application.
The Capacitor Provides a Path for High-Frequency Signals
Imagine the motor operating normally.
The direct current used to make it spin does not find a permanent path through the capacitor.
But when the brushes and commutator generate high-frequency components, the capacitor provides a low-impedance path for some of this noise.
The capacitor is connected directly between the motor’s two terminals.
In this way, some of the high-frequency components remain confined near the noise source itself instead of spreading through the power supply wires.
This is also why we usually find the capacitor soldered directly to the motor terminals, with very short connections.
The Capacitor Helps Protect the Transistor
In many devices, the motor is not operated directly by a switch.
Instead, an electronic circuit controls it using components such as transistors.
The noise and transients produced by the motor can reach these control electronics.
In the original article, I highlighted precisely this function: the capacitor helps prevent these unwanted high-frequency signals from reaching the transistor controlling the motor, contributing to its protection.
This is particularly relevant in simple circuits found in toys and small radio-controlled models.
It Also Helps Reduce Interference
There is another important consequence.
The wires connected to the motor can act as paths through which electrical noise spreads and even as small antennas.
In a radio-controlled device, this is especially undesirable.
We have a receiver trying to interpret relatively weak radio signals while, inside the model itself, a brushed motor is generating electrical noise.
Reducing this noise close to the motor helps decrease the possibility of interference with other electronic circuits.
Therefore, the capacitor is not there to increase the motor’s power. Its main purpose in this setup is high-frequency electrical noise suppression.
Why Do We Usually Use a Ceramic Capacitor?
For small brushed motors, ceramic capacitors are very commonly used for this purpose.
They are small, inexpensive, and suitable for working with high-frequency signals.
In the original article, I mentioned approximately 100 nF as a typical value for small motors operating at low voltages.
You may also find this value written in other ways:
100 nF = 0.1 µF = 100,000 pF
On certain ceramic capacitors, it may also appear with the marking:
104
This means:
10 + 4 zeros in pF = 100,000 pF = 100 nF
Does the Capacitor Have Polarity?
A typical 100 nF ceramic capacitor used in this application has no polarity.
Therefore, there is no positive or negative terminal.
This is especially convenient for motors that can rotate in both directions, as is the case with many remote-controlled cars.
The controller can reverse the polarity applied to the motor to make it spin forward or backward, while the capacitor remains connected in the same way between its two terminals.
Why Place the Capacitor Close to the Motor?
If the goal is to eliminate noise produced by the motor, it makes sense to act as close as possible to the source of that noise.
That’s why placing the capacitor far from the motor and connecting it with long wires is not ideal.
The wires themselves have electrical characteristics and can contribute to the propagation of high-frequency components.
When we solder the capacitor directly to the terminals, we keep its connections short and allow it to act exactly where the noise is being generated.
That’s why, when opening certain motors or toys, we find the small capacitor practically “hugging” the two terminals.
Does One Capacitor Solve the Entire Problem?
Not always.
A capacitor connected in parallel is a simple and widely used solution, but other noise suppression techniques also exist.
Depending on the design, we may find additional capacitors connected between the terminals and the motor’s metal housing, filters with inductors, and other protective components associated with the control circuit.
The appropriate design depends on the motor, the current involved, the electronic circuit, and the level of interference we need to control.
But to understand the principle, the small ceramic capacitor connected directly between the terminals is an excellent starting point.
A Simple Experiment to Understand Filters
This small component also provides an interesting opportunity to understand a much broader concept in electronics: a circuit can behave differently depending on frequency.
For the DC power supplied to the motor, the capacitor does not establish a permanent current path.
For the high-frequency components generated by the brushes, however, it presents a much lower impedance.
In this way, we can reduce these components without simply short-circuiting the motor’s power supply.
This is the beginning of the reasoning used in many filter circuits.
Watch the Video to Understand the Capacitor’s Purpose
In the article’s video, I show this connection and explain why we find the capacitor directly across the terminals of small brushed motors.
The next time you open a remote-controlled car and find that small ceramic capacitor soldered to the motor, you’ll know it’s there for a reason.
It’s a great example of how a component that costs very little can play an important role in the operation of an entire circuit.
And more important than simply memorizing that “brushed motors use capacitors” is understanding why the capacitor is there. It’s precisely when we understand the purpose of components that we begin to move beyond simply copying circuits and start truly understanding electronics.
To learn about other components and better understand the principles behind how circuits work, explore the Electronics Fundamentals and Components section.