Z-World Customer Support Center: Technical Reference/Defeating EMI

Defeating EMI

Radiated noise can be a problem for electronic equipment in some applications. A high level of noise immunity may be required in some situations. If a Z-World controller will be used in an environment that is susceptible to noise emissions, such as a heavy industrial environment, additional shielding and/or filtering may be required.

The following information may help you in solving electromagnetic inteference (EMI) problems.

Enclosures and shielding

1. The purpose of an enclosure is to:

Prevent emissions from radiating to the outside world.
Prevent susceptibility to external devices which are radiating.

2. Keep enclosure apertures (openings) as small as possible.

The area of the aperture is not as critical as the length. Seams and joints start to look like a slot antenna.
Try to keep length of openings less than 1/20 wavelength of the highest frequency of interest.

Wavelength approximation (air): l = 300/F (Mhz)

(where l = Wavelength (in meters) and F = frequency)

3. A cast metal enclosure is best to use since it does not have a structural joint.

Seams of fabricated enclosures should have continuous welds.
Mask paint from screw or pin holes when fastening different panels together.
EMI gaskets improve the conductive path between panels.

Use a gasket material with conductive properties that are comparable to the enclosure material.

Conductive paint can be used to provide RF shielding of plastic enclosures, and can improve shielding of metal enclosures.
EMI shielding tapes are very effective in reducing emissions, but are not recommended as a permanent fix.

Cables

1. Transmission lines can radiate noise.

Use shielded cables when possible, and ground cable shields at both ends to their respective equipment.
Use twisted pair shielded cable when appropriate.
Avoid parallel runs of signal and power cables.
Run cables away from apertures in shielding and close to conductive grounded structures.

Grounding

1. Systems should incorporate at least two separate grounds (not including safety ground). An electronics ground for the circuits and a chassis ground for hardware (racks and cabinets).

Connect system grounds only at the primary power ground.
The geometry of earth leads is very important.
Keep ground impedance low by keeping ground straps as short as possible.
Use a fat or thick ground conductor to minimize its RF impedance.
A braided ground is better then a solid conductor.
Mask paint from conductive surfaces when attaching ground leads.

Filtering and protection devices

Use a Power line filter if your equipment is susceptible to conducted emissions or surges on the AC line voltage.

Use a Low Pass Filter when applicable to filter out higher frequencies.

Ferrite suppressors are very effective in damping out emissions on cables.

The type of transient protection device required is dependent upon the application. Table 1 lists some common EMI transient parameters. Table 2 lists specifications of various transient-suppression components.

	Rise Time	Fall Time	Peak Voltage	Peak Current
Lightning	1-10 us	50-1000 us	6 kV	10 kA
ESD	0.7-1 ns	60 ns	15 kV	>16 A
Fast Transients	5 ns	50 ns	4 kV	N/A
Switching Inductive loads	1.2 us	50 us	4 kV	N/A

Table 1

	Turn-on Time	Leakage Current off	Capacitance off	Voltage Clamping	Current On
MOV	50 ns	5-250 uA	10-60,000 pF	14-1200V	4A - 60 kA
TVS diode	<1 ns	0.5-10 uA	10-10,000 pF	3 - 440 V	up to 50A
TVS thyristor	2-5 ns	50 nA	50 pF	25-270 V	> 3 kA
Zener diode	2-5 ns	1-1000 uA	100 pF	3-275 V	> 20 kA

Table 2

The above information is not intended to solve every EMI problem. Electromagnetic Interference can be very complex and will vary significantly between applications.

References

EMI Gone Technical

Joe DiBartolomeo, Circuit Cellar,

Issue 91 Feb. 1998 & Issue 93 April 1998.

EMC for Product Designers

Tim Williams, Butterworth Heinemann, 1992.

Electrical Interference Handbook

Norman Elise, Reed Educational and Professional Publishing Ltd,

Second Edition 1998.