RFI Suppression Circuit

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Summary: European models are supplied with a Radio Frequency Interference (RFI) suppression circuit, comprised of a LC (Inductor-Capacitor) circuit across the motor brushes. High frequency signals (electrical noise) emitted by the motors can interfere with household television and radio reception. Other countries may also require the same RFI supression. This is not a requirement in North America.


See the Video.

How to Identify Radio Frequency Interference Suppression


The LC circuit can be a capacitor with resistor across the motor brushes, or inductors in series with the motor and a capacitor across the motor brushes, as shown in the schematic. The capacitor provides a path for high frequency signals to flow across the motor, and the inductors block them. To a high frequency signal, the capacitor is a short circuit, while an inductor acts as a large resistor. Giving high frequency signals a short path eliminates their propagation along the wires in the locomotive, which will act as an antenna. They may also be installed on the plug used on the decoder interface, which is removed when a decoder is installed. In that instance, it will be removed with the blanking plug when you install a decoder.

Inductors (chokes, coils) can resemble a resistor in construction, the difference being they are usually larger in diameter at the ends compared to the body.

Which is great, except the PWM used for motor control is also impacted. Square waves contain a lot of harmonics, and they see the LC circuit as an additional load. This circuit can also interfere with the BEMF the decoder may want to measure.

The RFI circuit can be easily identified. They can consist of

  • Resistors and Capacitors connected across the motor brushes
  • Capacitors and Inductors
    • The inductor can resemble a resistor, or appear as a copper wire wound on a core, in series with the motor.
  • Capacitors with ferrites on their leads

Capacitors are small disk like components with two leads, resistors and inductors look similar, resistors using a series of colour bands to identify their value where inductors use numbers. Some inductors may also use colour bands.[1]

Properties of Inductors and Capacitors

Quick Electronics Lesson:

  • When used on DC inductors appear as a short with minimal resistance.
    • The impedance of an inductor increases with frequency: A one Henry inductor (coil) at 60 Hz has an impedance of 377Ω. At 10,000 Hz (10kHz) it is 63kΩ.
    • A decoder using a PWM frequency of 40kHz will see a load of 250kΩ.
      • A 1H inductor would be very large. The inductors used in a locomotive will be in the microHenry (µH) range.
  • Capacitors behave the opposite: At DC it will be an open circuit. As the frequency increases its impedance will decrease.
    • At 60Hz, a one microFarad capacitor appears as a 2.6kΩ.
    • At 40kHz, the same capacitor has an impedance of 4Ω.
  • By combining the capacitor and inductor, careful selection of their values can result in an LC circuit where at a specific frequency (resonance), its impedance is 0Ω.
    • Using the above examples, resonance occurs at 160Hz. Reactance of each component is 1000Ω at resonance.
    • Since the two reactances are always of opposite signs, when summed the result would be j(1000 + (-1000)) = 0.

The properties of these components can alter the shape and amplitude of PWM signal presented to the motor, as well as distort/attenuate the BEMF voltages produced by the motor. Since their behavior is determined by frequency, it will have different results as the motor speed is increased.

Why Does the Motor Require this Circuit?

As model trains are classified as toys, specifically designed for the use of children of the age of 8 years or under, EU regulations require RF suppression across the motor brushes. This is done to reduce any interference the motor may create. Technically, these manufactures, being toys, would not include a Digital Command Control decoder. The regulations do not differentiate between a toy (for children) and a model meant for adults.[2]

Some manufacturers install them by default to avoid compliance issues with product which may be shipped to the EU. Manufacturers which sell product globally often do this.

Fitting a Decoder

Pulse Width Modulated Signal

When fitting a decoder, check for the presence of the suppression circuit across the motor's brushes. If it is present, remove (if possible) or disable it. Simply snipping one of the capacitor leads removes it from the equation. If other components (such as a resistor or inductor) are in series with the motor leads, they should be removed or shunted using a wire connected across the component.

At a minimum remove the capacitor across the motor brushes. The inductors are in series with the motor, and will have minimal effect on the PWM signal from the decoder. The inductor is in the 5 microHenry range, and at 10 kHz will have an impedance of 300milliOhms. At PWM frequencies this increases to about 1.25 ohms. The inductor will create problems with BEMF, as it will impede changes in current. Therefore, they should be shunted or removed.

The capacitor is the issue, as its impedance decreases as frequency increases. This allows current to be shunted across the motor, without doing any work.

Removal will reduce current drawn from the decoder, and improve performance, especially with BEMF and speed curves. This is applicable to high frequency PWM, normal PWM decoders will not be affected by RFI suppression circuits. Removal will cause RFI without a decoder installed. The decoder's output will provide the RFI suppression.

Some decoder manufacturers may not recommend removal of the RFI circuit, read their instructions for guidance. They were designed with this issue in mind.

Zimo instructs the installer to remove the capacitors and inductors. The inductors can be bypassed by shunting them with a wire if required.

European DCC Decoders

Many European DCC manufacturers offer manufactures which comply with EU regulations, and as such will work with the suppression circuit installed. It is important to select a multifunction decoder meant for installation in the EU, and the final installation shall comply with EU rules. Read the instructions supplied to determine if the suppression circuit is needed or not. Some decoders may have RFI suppression built in.


See the Video.

tag at the top of the page directs the user to the video. Move the "video demo" tag to top of page (if necessary), and delete this line.

Manufacturers of devices that use DC motors must eliminate RF/EMI (electronic) noise generated by the arcing at the motor brush-commutator contacts.

External Links

How to Read an Inductor

  1. https://www.basictables.com/electronics/inductor/inductor-color-code
  2. Some products will state on the package "For ages 14+", which in a legal sense exempts the product from regulations applicable to toys intended for younger children.