The Noisy World of EMI Filters

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Ohmite develops solutions engineers need to meet their design and regulatory requirements for applications that include high current, high voltage, high energy and other products. These requirements often include noise suppression associated with electromagnetic waves external to the device, known as electromagnetic interference (EMI) or radio frequency interference (RFI). Ohmite provides a variety of EMI filters to alleviate the disruptions caused by EMI and RFI in many applications.  

Modern switch-mode power supplies create EMI in the form of high-frequency signals that propagate through power lines or cables. EMI also comes from natural sources and may induce unwanted currents and voltages into device circuitry in a way that disrupts the desirable current, leading to malfunction and failure. Global regulatory agencies specify how much EMI a device may emit and how much it must tolerate.  

EMI filters are circuits designed to reduce the high-frequency noise generated by electronic equipment. They block signals above a certain frequency, allowing lower frequencies to pass through. The input to an EMI filter is typically from an alternating current (AC) power source. Passive components—capacitors and inductors—configured in a resonant circuit remove unwanted noise. These circuits are the electrical resonance analogue of a tuning fork, used to pick out signals at a particular frequency. They offer a low impedance path to redirect unwanted current, or a high impedance path to prevent it entirely.  

Connecting an EMI filter to an electronic device involves placing it between the input power source and the power supplied to the end product. Because these filters only block EMI generated by conducted energy, they can be paired with a shield to block radiated EMI. Ohmite filter designs meet all North American regulatory standards (UL and CSA) as well as similar standards in Europe (RoHS). A variety of filter types can provide a range of suppression capability for industry applications according to system requirements. Ohmite provides five main types of EMI filter: IEC/inlet, single phase, three phase, dual stage, and PCB.  

The simplest EMI filters consist of an integrated filter and plug in a single unit. These inlet filters, or power entry modules, are widely used in computers, peripherals, and hardware devices. Ohmite offers modules in multiple combinations including power switches, fuses, and mounting variations. For example, the “AF Series” handles currents from 1 to 15 amps (A) and circuits up to 250 volts AC power (VAC), using a standardized International Electrochemical Commission (IEC) C14 inlet that serves the global market.  

Single-phase filters also provide a general-purpose option. They limit the amplitude of interfering voltages on single-phase AC power lines—the standard two-wire power circuit common in small electrical load delivery. Ohmite offers multiple filter series designed for single-phase circuits with current ratings up to 100 A. For example, the “AG Series” is general purpose; and the “AGM Series” meets rigorous IEC 60601-1 standards for medical applications.  

Three-phase filters block higher levels of noise and are compatible with heavy duty AC power circuits with three- or four wires. Applications include industrial machinery and motors, medical equipment, and industrial tool and test equipment. Ohmite also offers bookshelf and chassis mount designs. Their “TK series” for motor drive applications covers a maximum current range of 7 to 150 A, with voltage ratings of 440, 520, and 600 VAC.  

Dual stage filters can improve performance relative to single-stage EMI filters for attenuating a signal. They’re used in switched-mode power supplies, computers, measuring instruments, and products with very high standards of performance, or that must meet stringent regulatory standards. Ohmite offers dual mode filters for maximum current levels up to 50 A. Their “AH series” controls continuous and intermittent interference noise; and work in situations where high levels power mains carry high levels of interference.  

Finally, EMI filters that can be mounted on printed circuit boards (PCBs) offer a low-cost solution. These PCB filters are used in applications with low current and little space as is common in programmable logic controllers, electronic instruments, and PC periphery. Ohmite offers a version with current ratings to satisfy multiple applications. Their “AP Series” has ratings up to 6 A and maximum operating voltage of 250 VAC.  

The large power devices used in transportation and railway applications result in severe EMI as do closely-packed industrial machines and power lines. In the medical field, life support equipment and pacemakers can fail from EMI. Military equipment requires protection from high-powered electromagnetic pulses, a weaponized form of EMI. For aerospace, EMI impact on satellite performance can lead to abnormal noise in radio transmissions. Ohmite provides a full complement of resistor construction components and can provide a range of EMI filters to accommodate these and many other unique applications.