Designers of electronic devices are always on the lookout for ways to prevent EMI/RFI because of the havoc it creates. EMI/RFI can cause a multitude of problems for electronic equipment—from simple interference in a radio speaker to malfunctioning of electronically controlled machinery.
EE asked shielding and suppression-component manufacturers to share how they helped a customer solve a particularly difficult EMI/RFI control situation. These applications focus on laptop computers, nuclear process controls, heart pacemakers and mail-sorting equipment.
Laminate Provides Cost-Effective Shield
The laptop computer market has exploded in the past few years and shows no sign of letting up. As the competition becomes more intense, cost savings are important, but more design improvements are needed to reduce noise.
Some manufacturers use the laptop’s plastic case coated inside with a conductive paint to provide an overall shield. This method is lightweight and effective, but costly (approximately $10 per unit) and subject to wear and oxidization.
Chomerics came up with a solution that not only prevented EMI/RFI emissions in the laptop but also reduced unit cost. The laptop’s painted shield was replaced with a Chomerics laminate of PVC/aluminum formed and fitted to specifications, wrapped around the electronics and video display, and terminated along the metallic frame of the display. The solution proved to be as effective as paint for providing EMI shielding, and cost only $2 to $4 per unit. Chomerics, Inc., (617) 935-4850.
Wire-Mesh Tape Provides Noise Immunity
A manufacturer of nuclear process controls approached Instrument Specialties when its product did not meet radiated immunity specifications. Without compliance, the company could not sell its system cabinets. The cabinets sold for $200,000 to $300,000 each and were supplied in quantities of 200 per installation.
The noisy culprit was an interface board housed in a plastic enclosure outside the cabinet. During the immunity testing phase, it stopped all communications between the cabinet and other instruments.
At first, it looked like a metal enclosure could provide the shielding. Unfortunately, the metal enclosure caused excessive heat buildup around the board, causing it to fail.
A knitted Monel wire-mesh tape from Instrument Specialties was the answer. Applied to the plastic enclosure around the communications board, the wire-mesh holes allowed the necessary airflow and prevented heat buildup. Not only did the wire-mesh tape eliminate the noise, it also averted the purchase of costly fans to cool the boards. Instrument Specialties Co., Inc. (717) 424-8510.
Sorting Out the Signal From the Noise
A manufacturer of mail-sorting equipment had a contract with the German government to replace all of the country’s sorting equipment. The company had to meet the stringent VDE EMI/RFI standards as well as those imposed by the contract.
The manufacturer enlisted outside help. One consultant suggested the equipment be housed in a metal enclosure. Unfortunately, this did not meet the noise requirements.
Instrument Specialties recommended that the company spray the inside of the existing plastic enclosure with a conductive coating and apply UltraFlex, a beryllium-copper knitted wire mesh, around the perimeter.
The wire mesh allowed the company to pass the extremely strict noise requirements of the contract. It also saved the cost of new enclosure design and allowed quick retrofitting of existing enclosures. Instrument Specialties Co., Inc. (717) 424-8510.
Getting to the Heart of Noise
High-frequency EMI causes sensitive electronic products to act erratically, sometimes in life-threatening ways. For example, you certainly would want your pacemaker to be impervious to EMI.
Sometimes it isn’t easy. A manufacturer of implantable pacemakers was struggling to get a newly designed product to work in noisy environments. The six sensing and signal lines were acting as antennas, picking up radiated EMI and conducting it into the pacemaker, where it could cause improper operation.
AVX was asked to find a solution. The result was a new multilead device with a custom capacitive feed-through filter that blocked EMI from entering the pacemaker shell. The miniature unit had a biocompatible case and lead materials and a hermetic case. The finished unit met all noise-suppression requirements. AVX Corp., (818) 767-6770.
Suppression/Shielding Products
Unit Meets ACCESS.bus
Standards for Connectors
The Shielded Data Link (SDL) connector meets the ACCESS.bus standard and is available for connecting multiple input/output devices to a single port. The SDL has gold contacts and low insertion-force mating that enables the connector to mate/unmate 3,000 times. The connector features a polarized latch to prevent mismating. It is available in right-angle, vertical and shunted versions. The plug has a strain relief for a conductor and a cable jacket. Applications include electronic equipment such as a mouse, keyboard, joystick, light pen, scanner and printer. AMP Inc., (800) 522-6752.
Miniature Filter Has
0.075″ Diameter Case
The MiniGuard Filter has a 0.075″ dia case, a 60% reduction in size from the company’s previous smallest filter. The unit has a hermetic glass-to-metal seal and utilizes a nonsolder construction that tolerates installation temperatures to 300°C. The voltage range is 50 VDC to 100 VDC. Effective filtering is 100 MHz to 25 GHz. The capacitance range is 10 pF to 4,000 pF. The filter meets MIL-F-28861 testing requirements. AVX Corp., (818) 767-6770.
Foil and Film Laminates
Provide 60-dB Shielding
A line of shielding laminates is adhesive-bonded with copper or aluminum foil and polyvinyl chloride (PVC) film. The conductive foil layer provides 20 dB to 60 dB of EMI shielding. PVC is offered on one or both sides of the metal foil. The foil can be exposed selectively for grounding purposes. Applications include PCB ground planes, cross-talk shields and display shields on computer notebooks. The shields are available in 1.4-mil copper, 2-mil aluminum and 3-, 6- and 10-mil PVC. Chomerics, Inc., (617) 935-4850.
Suppressor Accommodates
Cable Diameters to 0.520″
The Cable Snap Series of bisected toroidal RFI suppressors is offered in four sizes to accommodate cable and cable-bundle diameters to 0.520″. Each size is offered five mounting configurations: in variable diameter grip, standard, adhesive base, press fit bare and hardware-mount base. Data frequencies pass unimpeded while insertion loss occurs on interfering frequencies to 1 GHz. FerriShield, Inc., (212) 268-4020.
Wire-Mesh Tape Shields
Cable Assemblies
ElectroMesh is double-layer knitted wire-mesh tape for shielding and grounding electrical and electronic cable assemblies. It is a 0.005″ dia tin-plated, copper-clad wire with 10 to 12 openings per inch. The tape is 0.020″ thick, available in widths from 0.25″ to 1.75″ and supplied in 50′ rolls. Instrument Specialties Co., Inc., (717) 424-8510.
Laminates Control EMI in
Keyboards, Shielded Rooms
The Electroshield line of laminates protects electronic products such as keyboards, CRTs, cellular phones and shielded rooms from EMI/RFI. It is available with or without a pressure-sensitive adhesive. The copper and aluminum foils are laminated to a polyester film, and resist punctures and abrasions. They are available in electrically conductive and nonconductive versions. Lamart Corp., (201) 772-6262.
Coaxial Transient Suppressor
Protects Against Lightning
The PTC Series Coaxial Transient Suppressor protects equipment from damage by lightning and electromagnetic pulses. The series is available in a variety of connector styles including N, BNC, SMA, TNC and UHF. The transient current capacity is 20 kA, and insertion loss is rated at 1,000 V. NexTek, Inc., (508) 486-0582.
Power-Line Filters Meet
FCC and VDE Requirements
A series of EMI/RFI power-line filters meets international safety-agency requirements. The filters also help customers meet FCC and VDE emission standards. The series consists of many standard configurations for power-line connectors and multifunction modules as well as block types and medical versions. Qualtek Electronics Corp., PPC Division, (216) 951-3300.
Power-Entry Module
Incorporates RFI Filter
The CE Series of power-entry modules incorporates an RF filter, a power inlet that meets IEC 320 requirements, a fuse holder and a series/parallel voltage selector. The modules also hold two domestic- or foreign-type fuses in a standard or IEC 601-1-type fuse drawer. The double-pole, double-throw contact arrangement allows dual primaries to be split or combined using the voltage selector. Amperage ratings of 1 A, 2 A, 4 A or 6 A at a voltage rating of 250 V are available. Schurter Inc., (707) 778-6311.
Transient Voltage Suppressor
Rated to 330 V
The PQI-1115W and PQI-1115WD STABILINE® Power Quality Interfaces divert and attenuate power disturbances before they reach the circuitry of home, business and industrial electronic equipment. The multistage suppression and filtration design offers the UL 1449 performance rating for transient suppression of 330 V. It provides bidirectional protection from source or load power disturbances. Both models are fire-rated with an ABS plastic housing with NEMA 5-15P plug and plastic outlet locator-pin assemblies. The PQI-1115WD has two RJ11 jacks with transient suppression. Superior Electric, (203) 585-4500.
Copyright 1995 Nelson Publishing Inc.
June 1995
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