E6D

Inductors, cores and crystals

Explain piezoelectricity and the crystal's equivalent circuit, how core permeability, laminations and saturation affect inductors and transformers.

12 pool questions1 on the exam

Syllabus: Inductors and piezoelectricity: permeability, core material and configuration; transformers; piezoelectric devices

Piezoelectric: squeeze it for voltage, apply voltage and it flexes

Piezoelectric materials such as quartz generate a voltage when stressed and flex (deform) when a voltage is applied. A crystal's electrical behavior is a series RLC branch (its mechanical resonance) in parallel with a shunt capacitance from the electrodes and strays.

Mechanical stress in, voltage out.

Series RLC branch, in parallel with shunt C. Two resonances, very close together.

RememberPiezo: stress and voltage convert both ways. Crystal = series RLC, shunted by C.

Core permeability sets inductance

Permeability is how readily a core material carries magnetic flux, and it determines inductance. Ferrite has higher initial permeability than powdered iron, so it needs fewer turns for a given inductance. Powdered iron has the better temperature stability. Non-magnetic brass pushed into a coil lowers the inductance.

Core permeability sets inductance. Brass lowers it; powdered iron and ferrite raise it. Values are illustrative.

Core	Inductance	Notable
Ferrite	highest	fewest turns needed
Powdered iron	high	best temperature stability
Air or ceramic	baseline	no magnetic boost
Brass	slightly lower	non-magnetic conductor

RememberPermeability sets L. Ferrite: fewer turns. Powdered iron: most stable. Brass: lowers L.

Cores waste power as eddy currents and fail at saturation

Changing flux drives circulating eddy currents in a solid core, heating it. Building the core from thin insulated layers keeps those loops small and cuts power loss. Push too much current and the core saturates: excessive magnetic flux, no further rise, and inductance collapses.

Changing flux drives circulating currents in the core. Thin layers keep them small.

Saturation: too much magnetic flux. Flux stops rising, so inductance collapses.

Winding current30 %

Inductance left66 %

RememberThin layers: less eddy-current loss. Saturation: too much flux.

Toroids keep the field inside; ferrite beads damp VHF/UHF parasitics

A toroid winds the coil on a ring, so the magnetic field is confined within the core instead of spreading into space like a straight (solenoid) coil. A ferrite bead slipped over a lead is lossy at high frequency and is used as a VHF/UHF parasitic suppressor at transistor amplifier terminals.

Toroid: field stays in the core. Ferrite bead: lossy at VHF/UHF, so it damps parasitics.

RememberToroid: field stays in the core. Ferrite bead: parasitic suppressor.

With no load, the primary still draws magnetizing current

Even with the secondary open, the primary draws a small current to create the core's magnetic flux. That is the magnetizing current.

No load: the primary draws just enough current to magnetize the core.

RememberTransformer, no load: magnetizing current.

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