RFI Suppression Circuit

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Summary: Locomotive models destined for sale in the EU are required to have Radio Frequency Interference circuitry integrated into the motor circuit. Other countries may also require the same RFI supression. This is not a requirement in North America.

How to Identify Radio Frequency Interference Suppression

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.


The LC circuit can be a capacitor with a small inductor, or a ferrite bead connected across the motor brushes. This provides a path for high frequency signals to flow back to the other motor brush. To them, it is a short circuit. Giving them 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

Which is great, except the PWM used for motor control is also impacted. Square waves contain a lot of harmonics, and they will see the LC circuit as a short cut back to the motor driver.

Fitting a Decoder

When fitting a decoder, check for the presence of the suppression circuit across the motor's brushes. If it is there, remove it if possible or disable it. Simply snipping one of the capacitor leads removes the circuit from the equation.

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 capacitor is the issue, as its impedance declines as frequency increases.

This 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 should cause RFI with a decoder installed. The decoder's output will provide RFI suppression.

  • Some decoder manufacturers may not recommend removal of the RFI circuit, read the instructions for guidance.

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

Many European manufacturers offer manufactures which comply with EU regulations, and as such will work with the suppression circuit installed.