Magnetic Immunity Testing of Magnetically Susceptible Device

Magnetic Immunity Testing of Magnetically Susceptible Device
Magnetic Immunity Testing of Magnetically Susceptible Device

Many electronic devices, products and components utilise magnetic properties and principles in order to operate or function. Devices incorporating cathode ray tubes (CRT) and components such as Hall Effect Sensors just to name a few can experience faults or loss of function when exposed to even low-level magnetic fields. Below is just a brief list of some devices which may be susceptible to magnetic fields that may require EMC Testing.

Examples of Magnetically Susceptible Devices

Hall Effect Sensors: A Hall Effect Sensor is a transducer that responds to a magnetic field by varying its output voltage. They can be incorporated into many different types of products and can allow for many different operations such as speed detection/regulation of disk drives, proximity switching used in some fuel level indicators and current sensors.

As the function of a Hall Effect Sensor is magnetic in nature it is vulnerable to malfunction caused by magnetic interference from electronic devices or magnetised metalwork.

Closed loop hall effect current sensor

Cathode Ray Tubes: For many years Cathode Ray Tubes were the most common form of electronic displays used in many products from television sets to radar displays and even oscilloscopes hence the common nickname CRO (cathode ray oscilloscope). Cathode Ray Tubes use an electron emitter to send electrons towards a fluorescent screen while focusing and deflecting coils are used to control the electron beam in a way similar to a magnetic lens.

A strong magnetic field can easily disrupt the function of these inductive coils and cause the image or colours displayed to be heavily distorted. This is something most older people experimented with as children, placing magnets on the family television set much to their parent’s dismay! In some cases this malfunction may be permanent; in other cases, the malfunction can be rectified.

cathode ray tube magnetic fields

Magnetic storage devices: When most people think of magnetic storage devices they would probably think of “old” technology such as magnetic tape or floppy disks however many newer hard drive storage devices such as HDD (hard disk drives) still store data magnetically. A traditional spinning hard drive is the basic non-volatile storage system and essentially consists of a read/write head on an arm that stores/access data electromagnetically on a metallic plate coated with a magnetic coating.

Strong permanent or fluctuating magnetic fields can corrupt the stored data by distorting or erasing the magnetically recorded information on the metallic plate.

laptop hard drive exposed to magneticsElectro-dynamic Microphones:  Electro-dynamic microphones most commonly fall into two categories: moving coil microphones and ribbon microphones. In both cases, acoustic signals are converted into electrical signals using magnetic properties. In a moving coil microphone, acoustic signals vibrate a diaphragm attached to a movable coil, which in turn vibrates. A permanent magnetic surrounds the vibrating coil causing a very small alternating current to be induced onto the coil which is analogous to the acoustic signal. A ribbon microphone works on a similar principle, however, rather than a membrane vibrating a movable coil, an extremely thin metallic membrane is used as the electrical conductor. This thin metallic membrane is mounted between two permanent magnets so that it can vibrate freely analogous to the acoustic signal and thus produce an electrical signal.

In both moving coil microphones and ribbon microphones, a strong magnetic field can be induced onto the coil or metallic ribbon which may manifest itself as noise or interference of the wanted acoustic signal. Ribbon microphones are much more sensitive to acoustic signals (lacking the heavy metallic coil for the diaphragm to move) and therefore much more prone to magnetic interference than moving-coil microphones, however, interference can still be problematic. In some very rare cases, an extremely strong magnetic field can pull on the extremely thin and fragile metallic ribbon to such an extent that physical mechanical damage can occur, though this is very unlikely. The most likely scenario is unwanted noise interfering with the wanted acoustic signal.

dynamic microphone magnetic immunity

ribbon microphone magnetic immunity

Pacemakers and implantable cardioverter-defibrillators (ICDs): Pacemakers and ICDs are devices that monitor and control abnormal heart rhythms. They are implanted in the chest area of a patient and monitor the electrical impulses produced by the heart. Electrical impulses are used to prompt the heart into a normal synchronous rhythm when an abnormal arrhythmic heartbeat is detected.

In some extreme cases magnetic interference can cause the implanted device to malfunction; either erroneously administering an electrical impulse when none is required or failing to detect an abnormal heartbeat and thus failing to produce the potential life saving electrical impulse when it is required. Technology such as Magnetic Resonance Imaging (MRI) can have serious effects on an implanted cardio device and even hand-held airport security metal detection should be avoided for long periods of exposure directly over the implant. While newer pacemakers and ICDs are shielded or have safeguards in place to protect against interference, new technologies create a potential for interference that the safeguards were not designed for.

pacemakers magnetic susceptibility

Hearing aids: Many newer hearing aids are made with what is referred to as Telecoil (T-Coil). A T-Coil is essentially an inductive loop antenna that acts as the first half of a transformer. The second half of the transformer circuit is provided by a varying magnetic field produced by a listening assistance system.

As one can imagine, any unwanted or unintentional varying magnetic field of sufficient magnitude can act in a similar manner to the intentional varying magnetic field produced by the listening assistance system. This would interfere with the intentional signal and hinder the operation of the T-Coil by causing distortion of the acoustic audio signal or reducing the remote bi-directional link.

cochlear implant

Conclusion about why Magnetic Immunity EMC Testing is important

Magnetic field immunity testing may be critical for certain products or when a product may be exposed to particularly high magnetic field environments. As the manufacturer, product designer or importer it is your responsibility to determine if your product incorporates any magnetically susceptible devices. If so then you may want to read a little further about how magnetic immunity testing is performed in our Electromagnetic compatibility testing guide.

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