Ferrite Bead Symbol
Definition: The Ferrite Bead symbol represents a passive two-terminal EMI suppression component that attenuates high-frequency noise on power supply and signal lines by presenting a high impedance (resistive loss) at MHz frequencies while passing DC and low-frequency signals with minimal attenuation, as defined in IEC 60317 (ferrite materials) and depicted in schematics as a filled rectangle or a series inductor symbol with a ferrite core marking, with designator FB or L.
Also known as: ferrite choke, ferrite core bead, EMI filter bead, power line filter, bead inductor, ferrite filter.
What the Ferrite Bead symbol means
The Ferrite Bead symbol denotes a passive EMI (electromagnetic interference) suppression element placed in series with a power or signal trace to absorb high-frequency switching noise, preventing it from propagating through the circuit or radiating as conducted or radiated emissions. Unlike an ideal inductor that stores energy (reactance), a ferrite bead is designed to dissipate high-frequency energy as heat, presenting a predominantly resistive impedance in the target frequency range (typically 10 MHz to 1 GHz).
In PCB schematics and power supply designs the ferrite bead symbol appears on VCC power rails entering noise-sensitive analogue or RF sections, on USB data lines to suppress common-mode noise, and on crystal oscillator supply pins. The two terminals — A (input/anode side) and B (output/load side) — are connected in series in the current path to be filtered.
How to identify the Ferrite Bead symbol
The Ferrite Bead symbol is drawn in one of two ways: as a filled black rectangle (the IEC convention for a ferrite core element in series with a line), or as a standard inductor symbol (a series of humps or loops) with a thick bar above or below indicating the ferrite core material. Some EDA tools use a labelled rectangle block with pins A and B. The component is always two-terminal (series element) with no polarity. The designator is FB (ferrite bead) in modern PCB design practice, sometimes L when used alongside inductors.
Function in a circuit
A ferrite bead presents low impedance at DC and low frequencies (its DC resistance, or DCR, is typically 0.1–1 Ω) and increasing impedance at higher frequencies. The impedance is predominantly resistive (loss) in the noise-attenuation frequency range: energy is dissipated in the ferrite material as heat rather than reflected back to the source. This resistive damping suppresses high-frequency resonances and prevents noise from propagating between circuit sections. Ferrite beads are characterised by their impedance at 100 MHz (e.g., 600 Ω at 100 MHz) and rated current, and are selected based on the noise frequency range and required attenuation.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60317 and IEC 62333 (ferrite components) govern ferrite material specifications. The EMC-related application of ferrite beads in PCB design falls under IEC 61000-4 (immunity) and CISPR 25/CISPR 32 (conducted/radiated emissions). The schematic symbol follows IEC 60617-04 (passive components) using a filled rectangle for the ferrite-core-loaded element. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 / ANSI Y32.2 does not define a dedicated ferrite bead symbol; it is represented as an inductor symbol with a ferrite core indicator (a line above the inductor coil symbols) or as a labelled two-terminal block. EIA/ECA standards govern ferrite bead performance characterisation. |
| Key difference | IEC 60617 uses a filled rectangle for a ferrite bead; ANSI/IEEE schematics often use an inductor coil symbol with a ferrite bar. The IEC rectangle more clearly distinguishes the ferrite bead (lossy) from an inductor (reactive). In practice, many EDA tools use a hybrid — three humps with a filled core bar — accepted by both conventions. |
Terminals / pins
| Pin | Name |
|---|---|
| a | A |
| b | B |
Typical values
Impedance at 100 MHz: 30 Ω to 2500 Ω (common values: 100 Ω, 220 Ω, 600 Ω, 1000 Ω). DC resistance (DCR): 0.05 Ω to 3 Ω. Rated current: 0.1 A to 6 A (0402 to 1210 package sizes). Package sizes: 0201, 0402, 0603, 0805, 1206, 1210. Operating temperature: −55 °C to +125 °C. Example parts: Murata BLM series, TDK MMZ series, Würth 742 series.
Where the Ferrite Bead symbol is used
- VCC power rail filtering on PCBs: ferrite beads in series with VCC entering analogue or RF sections prevent digital switching noise from contaminating sensitive circuits
- USB data line EMI filtering: ferrite bead arrays or common-mode chokes on D+ and D− lines suppress differential and common-mode noise per USB 2.0 / USB 3.0 EMC requirements
- Microcontroller oscillator supply filtering: single ferrite bead on VDDA or VCCA pins isolates the crystal oscillator supply from digital power noise
- LED driver output filtering: ferrite beads on LED PWM lines reduce radiated emissions from high-frequency switching
- Automotive ECU power entry filtering: high-current ferrite beads (1–3 A) on 12 V and 5 V supply rails suppress load-dump and alternator ripple per CISPR 25
- Bluetooth and Wi-Fi antenna supply: ferrite bead between the digital supply and the RF transceiver VCC isolates radio circuitry from MCU switching noise
Example
In an ESP32-based sensor board, a 600 Ω / 100 MHz, 500 mA ferrite bead (FB1) is placed in series between the 3.3 V power rail and the ESP32 VCC supply pin. Pin A of the ferrite bead connects to the main 3.3 V rail; pin B connects to the ESP32 VCC. A 100 nF bypass capacitor is placed at pin B to complete the LC-type filter. This prevents the Wi-Fi transmitter switching current (80 mA pulses at 250 kHz and harmonics at tens of MHz) from coupling noise back onto the power rail and into the analogue sensor supply.
Key facts
- A ferrite bead is a lossy passive two-terminal series element: it dissipates high-frequency noise as heat rather than reflecting it, presenting predominantly resistive impedance (not reactive inductance) in the target MHz frequency range.
- The two pins are A (input, x=0,y=10) and B (output, x=40,y=10); the bead is non-polarised — it can be connected in either direction without affecting performance.
- Ferrite beads are characterised by impedance at 100 MHz (e.g., 600 Ω @100 MHz) and rated DC current; selecting a bead requires matching the noise frequency range and ensuring rated current exceeds the maximum DC load current.
- The IEC symbol is a filled rectangle; the ANSI symbol is an inductor coil with a ferrite core bar; the designator is FB (ferrite bead) in PCB design, or L when listed alongside inductors in a BOM.
- Unlike an LC filter, a ferrite bead + bypass capacitor forms a lossy low-pass filter; the ferrite bead's resistance in the MHz range provides damping that prevents the resonance peak that occurs with a pure inductor.
- Ferrite beads are frequency-selective: a 600 Ω @100 MHz bead may present only 1–5 Ω at 1 MHz and 1500 Ω at 500 MHz — impedance vs. frequency curves in the datasheet must be checked to confirm suppression at the target noise frequency.
- High-current applications require ferrite beads rated above the maximum DC load current; overloading a ferrite bead saturates the ferrite core, dramatically reducing its high-frequency impedance and defeating its EMI suppression purpose.
Frequently asked questions
What does the ferrite bead symbol mean in a circuit diagram?
The ferrite bead symbol represents a passive two-terminal EMI suppression component placed in series on a power or signal line to absorb high-frequency noise. It appears as a filled rectangle (IEC) or an inductor with a ferrite core bar (ANSI/IEEE), with pins A (input) and B (output), and is non-polarised.
What does a ferrite bead look like in a schematic?
In IEC-convention schematics the ferrite bead is a small filled black rectangle in series with the signal line, labelled FB. In ANSI/IEEE schematics it looks like an inductor symbol (three or four humps) with a thick bar above the coil indicating the ferrite core. Both representations have two unlabelled terminals (A and B) and no polarity marker.
What is the difference between a ferrite bead and an inductor?
An inductor stores energy as a magnetic field and presents reactive (inductive) impedance; it can cause resonance with capacitive loads. A ferrite bead is designed to be lossy — it dissipates high-frequency energy as heat — presenting predominantly resistive impedance in the MHz range. The ferrite bead suppresses noise by absorption; the inductor filters by reflection. This makes the ferrite bead more effective for broadband EMI suppression without resonance issues.
How do I choose the right ferrite bead?
Select a ferrite bead based on three criteria: (1) impedance at the noise frequency (e.g., 600 Ω at 100 MHz for general decoupling), (2) rated DC current greater than the maximum load current (to avoid core saturation), and (3) DCR low enough that the voltage drop at full DC current is acceptable. The manufacturer's impedance-vs.-frequency curve confirms suppression at the target frequency.
Where should a ferrite bead be placed on a PCB?
Place the ferrite bead in series with the power or signal trace as close as possible to the noise source or the sensitive load — not in the middle of a long trace. For power rail filtering, place it between the main supply and the sensitive IC supply pin, with a bulk capacitor before the bead and a bypass capacitor (100 nF) after the bead at the IC pin.
What standard defines the ferrite bead symbol?
IEC 60617-04 (passive components) uses a filled rectangle for a ferrite-core series element. IEEE 315-1975 / ANSI Y32.2 does not define a dedicated ferrite bead symbol; ANSI schematics use an inductor symbol with a ferrite core bar. The designator FB (ferrite bead) follows PCB industry convention rather than a formal standard symbol.
Can a ferrite bead be used on digital signal lines?
Yes, ferrite beads are commonly used on digital signal lines (USB D+/D−, clock lines, CAN bus) to suppress high-frequency noise and reduce radiated emissions. For differential lines (USB, Ethernet), a common-mode choke (two windings on a common ferrite core) is preferred because it suppresses common-mode noise while passing differential signal with minimal distortion.
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