How to Get Rid of Electromagnetic Interferences

I have at times encountered severe electromagnetic interferences in the cockpit, mainly on my LCD multifunction displays. They were caused by the DC motors and electromagnetic clutches that I installed to move throttle and spoiler levers.

These interferences produced white flashes on the displays, temporary double images and were – generally speaking – powerfull enough to make me wonder how long the delicate electronics that drive the displays would resist.

What follows is a list of measures I took to correct the problem. I can not say wich is the most effective. I employed them all and together they made the interferences go away completely.

Electromagnetic Clutch

An electromagnetic clutch is basically a coil. Whenever DC is applied a magnetic field is built up. Upon disconnection this field collapses and an electrical charge with reverse polarity is sent into the leads of the coil. Without ground, the voltage builds up and ultimately discharges with a spark. This spark is very likely to cause an electromagnetic interference.

Solution:A suppression diode is installed between “+” and “-” leads of the electromagnetic clutch, with the anode connected to minus. I used a standard N1N48 diode.

Electric DC motor

The brushes of an electric motor cause continuous sparking and hence electromagnetic interferences.

Solution: Three condensors, wired to ground and “+” and “-” contacts of the motor. For wiring details see this scheme. The condensor between “+” and “-” should be 100 nF, the two condensors to ground 47 nF.

I also isolated both the motor and the clutch from the aluminum clutch case. Metal transmits electromagnetic interferences. An unshielded metal surface acts as an antenna, so it makes sense to keep the area of that surface as small as possible.

Ferrite rings:

I installed ferrite rings in all DC cables to motors and clutches. As a general rule, it seems to be sufficient to wind the cabel 6 – 8 times through the ring.