Technical Reference
The core is the magnetic heart of every inductor and transformer. Its material determines permeability, saturation, frequency response, and loss characteristics. This guide covers the three core materials we work with and how to choose between them.
Fundamentals
A magnetic core concentrates and channels magnetic flux generated by current flowing through a wire coil. The core material amplifies the inductance of the coil by a factor equal to its relative permeability. A core with a permeability of 10,000 produces 10,000 times the inductance of an air-wound coil with the same geometry.
Material choice affects every electrical parameter: how much energy the component stores, the frequency range it operates in, how much heat it generates under AC excitation, and the current level at which the core saturates and stops functioning as designed.
Key Properties
The material's ability to concentrate magnetic flux. Higher permeability means more inductance per turn, reducing the number of turns needed. Ranges from 2,000 (ferrite) to over 100,000 (1J85 Permalloy).
The maximum magnetic field the core can carry before it stops responding linearly. Silicon-iron saturates at 1.6 to 2.0 Tesla. Ferrite saturates at 0.3 to 0.5 Tesla. This determines maximum operating current.
Energy dissipated as heat during AC operation. Core loss includes hysteresis loss (from the B-H loop area) and eddy current loss (from induced currents in the core). Lower loss means higher efficiency and less thermal stress.
Ferrite is a ceramic compound made from iron oxide (Fe2O3) mixed with manganese, zinc, or nickel oxides. The material is pressed and sintered at high temperature into a dense, brittle structure. Ferrite is the most widely used core material in electronics.
Composition: Iron oxide ceramic (MnZn or NiZn formulations)
Ferrite's electrical resistivity is roughly a million times higher than metallic core materials. High resistivity blocks eddy currents, which are the primary source of core loss at high frequencies. This makes ferrite the natural choice for any application operating above 10 kHz.
MnZn ferrites are optimized for frequencies from 10 kHz to about 2 MHz. NiZn ferrites extend usable frequency into the 10 MHz range and beyond, with lower permeability but higher resistivity.
Our Ferrite Core Specs
We work with ferrite cores from 0.878" to 1.424" OD (22 to 36 mm), including manufacturer part ZW-43610-TC. Permeability ratings across our production range from 750 to 15,000. All cores are RoHS compliant.
Grain-oriented 3% silicon-iron is a metallic alloy engineered for high saturation flux density. The silicon content reduces hysteresis loss and increases electrical resistivity compared to pure iron. Grain orientation aligns the crystal structure along the rolling direction, maximizing permeability in the preferred flux path.
Composition: 3% silicon, 97% iron (grain-oriented electrical steel)
Silicon-iron is electrically conductive. In a solid chunk, alternating magnetic fields would induce large eddy currents and waste energy as heat. Tape winding solves this by building the core from thin strips of material separated by insulation.
Our silicon-iron cores use 0.007" (9 mil) tape thickness. Thinner laminations mean smaller eddy current loops and lower loss. The trade-off is that thinner tape is more expensive to process and results in a lower stacking factor (less magnetic material per unit volume).
Our Silicon-Iron Core Specs
We produce tape-wound silicon-iron toroidal cores from 0.8" to 2.25" OD (20 to 57 mm). Tape thickness: 0.007" (9 mil). Core heights from 0.256" to 0.500" (6.5 to 12.7 mm). Available with epoxy coating (0.015" thick) or powder coat finish. Also known as Microsil. All cores RoHS compliant.
1J85 is a nickel-iron alloy containing approximately 80% nickel and 20% iron, commonly known as Permalloy or Mu-metal. It delivers extremely high initial permeability, making it the go-to material for applications requiring maximum sensitivity and signal accuracy.
Composition: ~80% Nickel, ~20% Iron (Ni-Fe alloy)
In current sensing applications, the transformer must accurately reproduce a scaled-down version of the primary current. Any loss of magnetic coupling between primary and secondary reduces accuracy. A core with permeability above 100,000 achieves near-ideal coupling at very low current levels.
This ultra-high permeability also means fewer turns are needed to achieve a given inductance, which reduces winding resistance and the physical size of the component. For precision measurement circuits, the difference between a 15,000 permeability ferrite and a 100,000+ permeability 1J85 core is the difference between 2% accuracy and 0.1% accuracy.
Our 1J85 Core Specs
We source 1J85 (Permalloy) toroidal cores with epoxy coating. Currently in production for precision current sensing applications with OD ranging from 0.8" to 2.5" (20 to 63 mm). Permeability verified at 100,000+ per unit. All cores RoHS compliant.
Side-by-Side Comparison
| Property | Ferrite | Silicon-Iron (Microsil) | Permalloy (1J85) |
|---|---|---|---|
| Permeability | 2,000 to 15,000 | 30,000 to 50,000 | 100,000+ |
| Saturation Flux Density | 0.3 to 0.5 T | 1.6 to 2.0 T | 0.7 to 0.8 T |
| Optimal Frequency Range | 10 kHz to 5 MHz+ | 50 Hz to 10 kHz | DC to 100 kHz |
| Core Loss at High Frequency | Very Low | High | Moderate |
| Core Loss at Low Frequency | Low | Low | Very Low |
| Relative Cost | Low | Medium | High |
| Best Applications | SMPS, EMI Filtering, RF | Power Transformers, Line-Frequency Current Sensing | Precision Current Sensing, Shielding |
| Construction | Pressed & sintered ceramic | Tape wound (0.007" lamination) | Tape wound, annealed |
Visual Comparison
Selection Guide
Start with your application requirements. The operating frequency and the required current capacity will narrow the decision in most cases.
Choose Ferrite. Ferrite's ceramic structure provides the high resistivity needed to suppress eddy currents at switching frequencies. MnZn ferrite covers 10 kHz to 2 MHz. NiZn ferrite extends to 10 MHz and beyond. Saturation is lower (0.3 to 0.5 T), so verify that your peak current stays within the core's flux capacity.
Choose Silicon-Iron (Microsil). With saturation flux density of 1.6 to 2.0 Tesla, silicon-iron handles the highest current loads of any core material we offer. The 3% silicon addition reduces loss at mains frequency. Grain orientation maximizes permeability along the flux path. Ideal for power transformers, line-frequency filter chokes, and energy metering CTs.
Choose Permalloy (1J85). Permeability above 100,000 provides near-ideal magnetic coupling between primary and secondary windings. This translates directly to measurement accuracy. For current transformers measuring building power, industrial equipment loads, or utility metering, 1J85 delivers the linearity and sensitivity the application demands.
This is the transition zone where both ferrite and silicon-iron can work. If high saturation matters (high current, large energy storage), lean toward silicon-iron. If low core loss matters (high efficiency, low thermal budget), lean toward ferrite. Send us your spec and we can model both options.
Choose Permalloy (1J85). High permeability materials are the most effective at diverting external magnetic fields away from sensitive components. Mu-metal (another name for this alloy family) is the standard material for magnetic shielding in medical imaging, scientific instruments, and defense electronics.
Core Finishing
Bare metal cores require insulation between the core surface and the wire winding. Coatings also protect the core from environmental damage during handling and service life.
Applied at 0.015" (0.381 mm) thickness. Provides electrical insulation, mechanical protection, and moisture resistance. The coating is applied uniformly and cured at temperature to create a hard, durable shell. Standard for our silicon-iron and 1J85 cores used in current sensing applications.
Electrostatically applied dry powder, cured at high temperature. Creates a thicker, more rugged finish suitable for cores that will be handled during assembly or exposed to industrial environments. Used on larger tape-wound toroidal cores where additional mechanical protection is needed during winding operations.
Mylar Tape Wrapping
In addition to core coatings, we wrap cores in Yellow Mylar tape (3M Type 74) when the specification calls for it. Mylar provides an additional insulation layer between the core surface and the wire winding, and is standard practice for toroidal inductors in production applications.
Gallery
Send us your specifications and operating parameters. We will recommend the core material, geometry, and coating that fits your application.
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