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SILICON SILIC CHIP www.siliconchip.com.au Publisher/Editor Nicholas Vinen Technical Editor John Clarke – B.E.(Elec.) Technical Staff Bao Smith – B.Sc. Tim Blythman – B.E., B.Sc. Advertising Enquiries (02) 9939 3295 adverts<at>siliconchip.com.au Regular Contributors Allan Linton-Smith Dave Thompson David Maddison – B.App.Sc. (Hons 1), PhD, Grad.Dip.Entr.Innov. Geoff Graham Associate Professor Graham Parslow Dr Hugo Holden – B.H.B, MB.ChB., FRANZCO Ian Batty – M.Ed. Phil Prosser – B.Sc., B.E.(Elec.) Cartoonist Louis Decrevel loueee.com Founding Editor (retired) Leo Simpson – B.Bus., FAICD Silicon Chip is published 12 times a year by Silicon Chip Publications Pty Ltd. ACN 626 922 870. ABN 20 880 526 923. All material is copyright ©. No part of this publication may be reproduced without the written consent of the publisher. Subscription rates (Australia only) 6 issues (6 months): $70 12 issues (1 year): $130 24 issues (2 years): $245 Online subscription (Worldwide) 6 issues (6 months): $52.50 12 issues (1 year): $100 24 issues (2 years): $190 For overseas rates, see our website or email silicon<at>siliconchip.com.au * recommended & maximum price only Editorial office: Unit 1 (up ramp), 234 Harbord Rd, Brookvale, NSW 2100. Postal address: PO Box 194, Matraville, NSW 2036. Phone: (02) 9939 3295. ISSN: 1030-2662 Printing and Distribution: Editorial Viewpoint Ferrite beads are not inductors I often see ferrite beads drawn in circuit diagrams as if they are inductors, with “Lx” designators. While many circuit designers likely realise that they are not true inductors, treating them as such could cause confusion, especially for those reading the diagrams. This might lead them to assume that a ferrite bead is just another type of inductor, when in reality, it serves a very different purpose. Ferrite beads clearly exhibit some inductance – as do most components, including wires and PCB tracks – but their operation does not rely on it. At their simplest, ferrite beads are just a piece of wire passing close to (or through a hole in) a piece of ferrite. Some of the confusion may stem from the fact that ferrite is used as a core material in high-frequency inductors and transformers. However, in those applications, the ferrite core is surrounded by multiple turns of wire to create significant inductance. In contrast, a ferrite bead typically has just one or a few turns and thus a relatively low inductance. Ferrite is a ceramic material that contains iron oxide. Like other magnetic core materials, it provides a path for magnetic flux, but only up to a certain frequency. Beyond that, ferrite becomes highly ‘lossy’, converting much of the magnetic energy to heat, due to hysteresis and eddy current losses within the ferrite material. Ferrite beads take advantage of this property to suppress unwanted highfrequency signals by dissipating their energy, effectively acting as a frequencydependent resistor rather than an inductor. Unlike an inductor, a ferrite bead does not store energy or resonate. It simply increases its effective resistance in a targeted frequency range to block unwanted signals. Ferrite beads are available with all sorts of resistances and curves, with the resistance peaking at different frequencies depending on the exact construction of the bead. At very high frequencies, the impedance of the ferrite bead drops as the parasitic capacitance across it starts to cause the signal to bypass it. While you can make a ferrite bead yourself, by passing a wire through a ferrite core, they are also available as pre-built SMD ‘chip’ devices that you can simply solder across pads on a board. Pre-formed through-hole beads are also available but are less common these days. A bead’s peak resistance can range from a few ohms up to a few kilohms, although most fall between 100W and 1kW. Many have a DC resistance well under 1W and can handle from a few hundred milliamps to several amps. However, those with a higher peak resistance usually also have a higher base resistance at DC. The impedance peak is usually between 100MHz and 1GHz and can be fairly broad, allowing the bead to block RF signals over a wide range of frequencies (to some extent, at least). Much of the impedance is real resistance, but not all. The accompanying figure from the TDK MPZ1608 data sheet should give you some idea of the behaviour of a range of different ferrite beads. So, rather than thinking of ferrite beads as inductors, it’s more accurate to consider them as a lossy impedance element that selectively dampens high-frequency signals. That distinction matters. The 2020-2024 block of Silicon Chip PDFs on USB is now available (see p95). Order the set at siliconchip.au/ Shop/digital_pdfs by Nicholas Vinen 9 Kendall Street, Granville NSW 2142 2 Silicon Chip Australia's electronics magazine siliconchip.com.au