Noise suppression resistors are wirewound resistors that are especially designed to reduce radio frequency interference (RFI) caused by high electrical voltage discharges generated by automotive ignition systems.
The resistor element is a resistive wire, which is wound in a single layer on a fibreglass or ceramic core. Metallic caps, electrodes and/or axial wire terminals are fixed to the ends of the resistive core to assure a good mechanical and electrical contact. A coating protects the resistive element against moisture, mechanical shock and is able to withstand high temperatures These products can be moulded with epoxy resin, thermoplastic or thermo set plastic materials.
RFI reduction method
As example, automotive ignition systems produce high voltage pulses and these pulses make the sparks that ignite the mixture air/gasoline in the combustion chamber. These pulses have a very rapid current change that generates an electromagnetic field around the ignition system. In addition pulses of current at the spark plug ionises atoms that during the de-ionisation process releases a wide range of radio waves. These waves with random frequencies generate the radio frequency interference (RFI) and disturb many electronic devices like engine management systems, radio/TV broadcasting and communication systems.
According to international legislation, these disturbances must be reduced to acceptable levels. One of the best solutions to minimise this noise interference is to introduce in the ignition system a noise suppressor resistor that can be placed in the rotor of the distributor, at the ends of spark plug cables, or fully integrated in the new ignition systems technologies such as the pencil coil.
NSR parameters
A noise suppression resistor (NSR) is a special wire wound resistor where the ohmic value (R), along with the self inductance (L), gives its frequency response behaviour, changing the impedance level (Z) with the change of the frequency. This impedance is responsible for damping the RFI or noise suppression. The main parameters to define a NSR are the ohmic value/tolerance and the self-inductance variables such as number of windings diameter and length of the resistive core. The final design for these products depends on the application and can be developed under request.
Typical characteristics available are: resistance, 1 kΩ to 15 kΩ; resistance tolerances, ±10%, ±15%, ±20%; working voltage, 25 kV to 40 kV; inductance level (2 MHz), 5 µH to 72 µH; operating temperature, -40 to 200°C.
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