RFI Suppression Circuit

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Locomotives 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 or disable it. Simply snipping one of the leads with remove the circuit from the equation.

This will reduce current drawn from the decoder, and improve performance. 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.