Shielded Components

Operation of even the best electronic components can be seriously impaired by magnetic interference (EMI) from nearby components such as transformers, motors, solenoids, generators, cables carrying large AC or DC currents at power frequencies, or even from the earth’s magnetic field. For example, shielding is widely used to protect high definition cathode ray tubes from the effect of local fields induced by transformers or high voltage cables. With the current trend of placing more and more electronics in the smallest possible case the need for shielding of sensitive components becomes ever more significant.

Stray magnetic fields are a serious problem in situations involving low impedance and low frequency. A familiar example of magnetic interference is “50 cycle hum” – the leakage of power frequency energy into an audio circuit of amplifiers. A radar monitor system may require a cathode ray tube and an electronic motor in the same enclosure. Because interference from the motor may deflect the electron beam in the cathode ray tube, the ‘nuisance’ field must be reduced to a minimum. The attenuation of this field can be accomplished by physical separation or by shielding.

A magnetic shield is a ferromagnetic metal enclosure that surrounds the device as completely as possible. As good conductors of magnetic flux, shields carry the field around the device to be shielded. They can be used to keep out the field (as in cathode ray tube shields) or to keep it in and prevent it disturbing neighbouring components (as in power transformers and motor shields).