Ferrite Bead
Inductor
A tiny ferrite cylinder or SMD chip that turns high-frequency noise into heat while passing DC almost unhindered. Behaves like a frequency-dependent resistor, not a pure inductor.
Schematic symbol
Inductor / coil
How ferrite bead appears in a circuit diagram.
Types & variants
Key specs
Impedance @ 100 MHz
ΩThe headline rating, e.g. 600 Ω — impedance the noise sees.
Rated current
AAbove this the ferrite saturates and stops working.
DC resistance
ΩKeep low on supply rails to avoid voltage drop.
Impedance curve
Which frequency band it actually suppresses.
Markings
SMD beads look identical to resistors with no useful marking — keep them in labelled tape. Clamp-ons print a part number.
Standard values
120 Ω, 220 Ω, 600 Ω, 1 kΩ @ 100 MHz are the everyday chip-bead values.
How to choose
Match the impedance peak to your noise frequency and check the current rating at DC bias — impedance drops sharply as load current rises.
Pinout & package
2 terminals, not polarised. 0402–1206 SMD, or clamp-on halves for cables.
Example circuits
- Bead + capacitor π-filter on an IC supply pin
- Clamp-on core on a USB cable to stop radiated noise
- Isolating a noisy digital rail from an analog one
Common failures
Cracked SMD beads go open (rail dies); saturation from too much DC current quietly makes them useless; wrong frequency choice does nothing at all.
How to test
Continuity — a good bead reads under 1 Ω at DC. It should NOT read open. Actual noise suppression needs a scope or spectrum view before/after.
Substitutes
A small inductor (~1 µH) plus damping, or a low-value resistor if the rail current allows — but a bead of the right curve is usually the correct part.