Ring ferrites
(589)Ring ferrites belong to the family of ferrite filters, also known as ferrite cores. These filters come in a variety of sizes and shapes, such as cylinders with a hole along their axis of rotation, or the previously mentioned rings. They are often found at the ends of various types of cables and wires, e.g. computer mice, switching power supplies, or USB cables, sometimes even on both sides, near the plugs, and thus close to the input and output ‘to’ and ‘from’ a particular device.
How do ring ferrites work?
Ferrite filters, in the simplest of terms, serve to supress, in both directions, high-frequency interference. Their operation does not differ much from commonly used chokes or inductors. In their applications, ferrites are characterised primarily by impedance, but indirectly they can introduce a kind of capacitance and also resistance into the circuit. Applied to both DC and AC conductors, they act as a low-pass filters, suppressing interference at higher frequencies, and in fact dissipating their energy by converting it into thermal energy, which is then given off to the environment. Interference can be the result of e.g. electromagnetic influence of another device or electronic circuit located near the output signal. Such a situation is typical e.g. for switching-mode power supplies, due to the specific nature of their operation and switching currents at high frequency. This interference is referred to as electromagnetic interference, the so-called EMI or RFI (radio-frequency interference) noise. A current flowing through a conductor, according to Ampère's law, creates a magnetic field around it. In the case of current interference, this field is not constant and is subject to fluctuations, which then interact with the ferrite filter. This, in turn, suppresses the electrical signal interference that arises in the conductor.
In simple terms, as the interference frequency increases, the impedance of the conductor on which the ferrite core is embedded increases. However, this is only half true. Ferrites have frequency characteristics that show for which oscillating voltage frequency values they have the highest impedance. After passing a certain frequency value, however, their impedance begins to decrease again. When selecting a suitable ring ferrite, it is extremely important to choose one whose impedance value is the highest for the interference frequencies of our system. To find them, and at the same time to choose a suitable filter, experiments and tests can be carried out, e.g. by using frequency analysis, otherwise known as spectrum analysis.
Choosing ring ferrites – what to look for?
When designing an electronic circuit in which we want to use a ring ferrite, its heating must be taken into account. It is therefore necessary to provide adequate passive or active cooling using forced air circulation, e.g. by a fan. High current values and large interference at high frequencies will not only cause a significant amount of heat to be emitted, but may also lead to the saturation of the ferrite core, i.e. a drop in its inductance by as much as 90%.
Ring ferrites application
Ferrite cores are a simple and inexpensive way to apply low-pass filtering to an already existing cable infrastructure, for example by creating a choke, or in other words, an inductor. In the case of ring ferrites it is sufficient to wrap a conductor several (typically 5 to 7) times through the centre around the ring to provide adequate filtering against high-frequency interference.
Ring ferrites can be found in outdoor cabling, but ferrite filters are just as often found inside various types of equipment, e.g. in the form of ferrite beads or chokes, as filters for incoming power, but also to filter signals directly on PCBs, near transistors, microprocessors and other integrated circuits, as well as connectors.
The application of ferrite filters is also useful in the case of devices that may emit radio frequency energy, because in such a case the cable may behave like an antenna, so the signal transmitted through will be distorted. Such a phenomenon may lead to unpredictable behaviour of powered devices. The same may occur for signal cables, which can cause distortions in communication between integrated circuits.
When selecting a ring ferrite, you should pay attention to several factors, depending on the specific application, currents flowing through the conductor, as well as the space available, e.g. in the housing of a given device. The dimensions of the filter can be taken into account, including the internal opening, which should allow for winding a wire of a given diameter through its centre several times, the material of the core, which will translate into its frequency characteristics, and the inductance factor. Before selecting a ferrite core, it is also worth reading its detailed technical documentation provided by the manufacturer, which will contain all the most important information, along with the aforementioned characteristics.
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