In electronics, a ferrite core is a type of magnetic core made of ferrite on which the windings of electric transformers and other wound components such as inductors are formed. It is used for its properties of high magnetic permeability coupled with low electrical conductivity (which helps prevent eddy currents). Because of their comparatively low losses at high frequencies, they are extensively used in the cores of RF transformers and inductors in applications such as switched-mode power supplies, and ferrite loopstick antennas for AM radio receivers.
Ferrites are ceramic compounds of the transition metals with oxygen, which are ferrimagnetic but nonconductive. Ferrites that are used in transformer or electromagnetic cores contain iron oxides combined with nickel, zinc, and/or manganese compounds. They have a low coercivity and are called "soft ferrites" to distinguish them from "hard ferrites", which have a high coercivity and are used to make ferrite magnets. The low coercivity means the material's magnetization can easily reverse direction while dissipating very little energy (hysteresis losses), at the same time the material's high resistivity prevents eddy currents in the core, another source of energy loss. The most common soft ferrites are:
- Manganese-zinc ferrite (MnZn, with the formula MnaZn(1−a)Fe2O4). MnZn have higher permeability and saturation levels than NiZn.
- Nickel-zinc ferrite (NiZn, with the formula NiaZn(1−a)Fe2O4). NiZn ferrites exhibit higher resistivity than MnZn, and are therefore more suitable for frequencies above 1 MHz.
For applications below 5 MHz, MnZn ferrites are used; above that, NiZn is the usual choice. The exception is with common mode inductors, where the threshold of choice is at 70 MHz.[1]
As any given blend has a trade off of maximum usable frequency, versus a higher mu value, within each of these sub-groups manufacturers produce a wide range materials for different applications blended to give either a high initial (low frequency) inductance, or lower inductance and higher maximum frequency, or for interference suppression ferrites, a very wide frequency range, but often with a very high loss factor (low Q).
It is important to select the right material for the application, as the correct ferrite for a 100 kHz switching supply (high inductance, low loss, low frequency) is quite different from that for an RF transformer or ferrite rod antenna, (high frequency, low loss, but lower inductance), and different again from a suppression ferrite (high loss, broadband)
https://en.wikipedia.org/wiki/Ferrite_core
Magnetic-core memory was the predominant form of random-access computer memory for 20 years between about 1955 and 1975. Such memory is often just called core memory, or, informally, core.
Core memory uses toroids (rings) of a hard magnetic material (usually a semi-hard ferrite) as transformer cores, where each wire threaded through the core serves as a transformer winding. Three or four wires pass through each core. Magnetic hysteresis allows each of the cores to "remember", or store a state.
Each core stores one bit of information. A core can be magnetized in either the clockwise or counter-clockwise direction. The value of the bit stored in a core is zero or one according to the direction of that core's magnetization. Electric current pulses in some of the wires through a core allow the direction of the magnetization in that core to be set in either direction, thus storing a one or a zero. Another wire through each core, the sense wire, is used to detect whether the core changed state.
The process of reading the core causes the core to be reset to a zero, thus erasing it. This is called destructive readout. When not being read or written, the cores maintain the last value they had, even if the power is turned off. Therefore they are a type of non-volatile memory.
Using smaller cores and wires, the memory density of core slowly increased, and by the late 1960s a density of about 32 kilobits per cubic foot (about 0.9 kilobits per litre) was typical. However, reaching this density required extremely careful manufacture, which was almost always carried out by hand in spite of repeated major efforts to automate the process. The cost declined over this period from about $1 per bit to about 1 cent per bit. The introduction of the first semiconductor memory chips in the late 1960s, which initially created static random-access memory (SRAM), began to erode the market for core memory. The first successful dynamic random-access memory (DRAM), the Intel 1103, followed in 1970. Its availability in quantity at 1 cent per bit marked the beginning of the end for core memory.[1]
Improvements in semiconductor manufacturing led to rapid increases in storage capacity and decreases in price per kilobyte, while the costs and specs of core memory changed little. Core memory was driven from the market gradually between 1973 and 1978.
Depending on how it was wired, core memory could be exceptionally reliable. Read-only core rope memory, for example, was used on the mission-critical Apollo Guidance Computer essential to NASA's successful Moon landings.
Although core memory is obsolete, computer memory is still sometimes called "core" even though it is made of semiconductors, particularly by people who had worked with machines having actual core memory. The files that result from saving the entire contents of memory to disk for inspection, which is nowadays commonly performed automatically when a major error occurs in a computer program, are still called "core dumps".
https://en.wikipedia.org/wiki/Magnetic-core_memory
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