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Figure 3: Interference control with Pulser P11
In the case of the SF 11 demo board, a gap in the GND surface deliberately allows magnet field to
pass through the module between the signal line1 and GND. A functional error occurs - the relay
rattles. The path of the magnetic field lines is shown in Figure 3. (If this test does not lead to a
functional error, the polarity must be changed at the pulser).
The coupling can be prevented by a „shield" made of adhesive copper tape: the magnetic field is
blocked by the eddy currents within the adhesive copper tape. The magnetic field can no longer
penetrate the assembly, and the SF 11 is immune to interference.
Figure 4: Improving the immunity with a piece of adhesive copper tape
Alternatively, the line „Signal2" can be activated by switching the switch S2. This line is completely
surrounded by GND and deposited in the inner layer with GND. Almost no magnetic field can reach
around the signal line, the assembly is much more resistant to interference (Fig. 5).
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Langer EMV-Technik GmbH
Figure 5: Coupling near line signal2
5.2 SF 21 - Demo Board E-Field Immunity
If only one single cable is connected to an assembly, the result is a field image as shown in Fig. 6.
Figure 6: Field image of SF 21 - only one cable connected
The interference current flows into the module, but couples out again capacitively as an electric field
(electric flux). This decoupling is done from the GND area as well as from all connected components
and lines. The current decoupling from the signal line in Fig. 6 generates a voltage difference at
resistor „R" (This resistor symbolizes the output resistance of an IC output.). This voltage difference
can lead to the functional error.
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