Ferrite Core Transformer

[Marguerite Gilbeau]

Ifthe transformer was glued before you disassemble it, it should be broken right where it was glued, cleaned to bare ferrite and re-glued in the same way. It's likely to keep its performance close to what it was.

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). Moreover, 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 non-conductive. 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:

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 comprehensive range of 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, an extensive frequency range, but often with a very high loss factor (low Q).

It is essential to select the suitable 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)

There are two broad applications for ferrite cores that differ in size and frequency of operation: signal transformers, which are of small size and higher frequencies, and power transformers, which are of large size and lower frequencies. Cores can also be classified by shape, such as toroidal, shell, or cylindrical cores.

The ferrite cores used for signals have a range of applications from 1 kHz to many MHz, perhaps as much as 300 MHz, and have found their main application in electronics, such as in AM radios and RFID tags.

Ferrite rod aerials (or antennas) are a type of small magnetic loop (SML) antenna[3][4] ubiquitous in AM radio broadcast band transistor radios. However, they began to be used in vacuum tube ("valve") radios in the 1950s. They are also helpful in very low frequency (VLF) receivers,[5] and can sometimes give good results over most of the shortwave frequencies (assuming a suitable ferrite is used). They consist of a coil of wire wound around a ferrite rod core (usually several inches longer than the coil). This core effectively concentrates the magnetic field of the radio waves[6] to give a stronger signal than could be obtained by an air core loop antenna of comparable size, although still not as strong as the signal that could be obtained with a good outdoor wire aerial.

Other names include "loopstick antenna", "ferrod", and "ferrite-rod antenna". "Ferroceptor" [7] is an older alternative name for a ferrite rod aerial, mainly used by Philips where the ferrite core would be called a "Ferroxcube" rod (a brand name acquired by Yageo from Philips in the year 2000). The short terms "ferrite rod" or "loop-stick" sometimes refer to the coil-plus-ferrite combination that takes the place of both an external antenna and the radio's first tuned circuit or just the ferrite core itself (the cylindrical rod or flat ferrite slab).

The first one has an air core, as shown in the article by John, M0UKD. This transformer doesn't even work as a transformer until you add a capacitor in parallel to the load. The secondary winding of the transformer and the capacitor should form an LC-circuit resonant on the desired frequency. The transformer works very well, but the bandwidth is narrow.

For the second one, I used FT240-43 ferrite core, as shown in the article by Rudy, N6DOZ. This transformer is wideband and works on 3-30 MHz. Although when loaded to 2450 Ohm non-inductive load (metal-oxide or metal-film resistor) instead of 50 Ohm we see a slightly inductive load:

From what I know this is because the large secondary winding works as an inductance in series with the load. This inductance can be compensated by placing a capacitor in parallel to the primary winding of the transformer. Here is the effect of placing a 57 pF capacitor (47 pF 10 pF):

The part that I don't quite understand is why the transformers work so differently depending on the core material? Why the one that uses a ferrite core is so wideband while the first one doesn't even work as a transformer without an extra capacitor?

The Power Chart characterizes the power handling capacity of each ferrite core based upon the frequency of operation, the circuit topology, the flux level selected, and the amount of power required by the circuit. If these four specifics are known, the core can be selected from the Typical Power Handling Chart.

The power handling capacity of a transformer core can also be determined by its WaAc product, where Wa is the available core window area, and Ac is the effective core cross-sectional area. Using the equation shown below, calculate the WaAc product and then use the Area Product Distribution (WaAc) Chart to select the appropriate core.

Transformers are passive devices used to transfer electrical energy from one circuit to another. Their primary aim is to increase or decrease the voltage levels between the circuits. They have two major core constructions- ferrite core transformers and iron core transformers. Out of these, ferrite core transformers are the most popular ones and there are several reasons they deserve to be so.

Ferrite Core Transformer is non-conductive, ferromagnetic compound that has its winding made from ferrite cores. They are used for high-frequency applications because they carry low coercivity and offer low eddy current losses.

1. Manganese Zinc (MnZn): They have higher saturation levels and higher permeability than NiZn ferrites. These are suitable for applications that have an operating frequency of less than 5MHz. Also, their impedance makes them ideal for inductors up to 70 MHz.

2. Nickel Zinc (NiZn): They have higher resistivity compared to MnZn ferrites. They are used in electrical applications when the frequency ranges between 2 MHz to several hundred MHz and are suitable for inductors above 70 Mhz.

On the other hand, conventional iron core transformers have an iron core built-up with laminations to prevent losses due to eddy currents in them. It becomes impossible to make the laminations thinner to make them effective at higher frequencies.

Ferrite Core Transformers are efficient and deliver excellent performance in a variety of conditions. If you are planning to purchase the finest quality ferrite core transformers for your electrical applications, you can contact Alisha Coils & Transformers (A Division of Cosmo Ferrites Ltd). Also, if you need any guidance or consultation before buying these transformers, they are happy to help you find the right ones.

Has anyone else encountered a similar problem or can offer insights into what might be causing these issues in the KiCad simulation tool for both cases? I would greatly appreciate any guidance or suggestions you can provide.

If you have a transformer, both the primary and the secondary side will need a dc path to ground. Otherwise ngspice (and all other spices) cannot determine which is the eletric potential of this floating part.

I have now tried to combine both models of magnetic core and inductive coupling with all the additional circuitry into one subcircuit for KiCad. This is what it looks like now:

Inductor_simulation1647945 28.3 KB

After adding the voltage source and the resistor in series with the inductor, the circuit still gives me the same error as before when used with the voltage source. With the current source it works as before with the same result, no errors.

Either by platform usage or stray mod, you are now a basic user. You can edit the previous post with the file if you wish. Might be good for posterity as that link will probably disappear at some point.

@holger I would like to ask you if you can review ngspice merge request #15 now as kicad with ngspice is working better and better and so would really like kicad to replace the command line driven ngspice

For documentation purposes, I need to redraw badly drawn vintage schematics.

For the IF transformers, I can find symbols like TRANSF4 with iron core and ADT1-6T without any core, but instead of modifying these symbols, I wonder if a symbol is already existing for an IF transformer ?

After all, an IF transformer is a very frequent component in any vintage radio.

TRANSF4 comes closest, but I would need a dual dashed line to indicate a ferrite core, and an oblique line with a T-shaped end on it, diagonally over the symbol, to indicate the transformer needs to be tuned.

The schematics can include either transistors or tube. It is about heterodyne receivers with tunable IF transformers in one or more stages behind the mixer, for which I am looking for the correct symbol.

I recommend to just draw your own (or modify from an existing symbol). The first few symbols you make or modify take some time, but after a short learning curve you can do things like this in a few minutes, and that is quite possible quicker then searching for it or asking someone else.

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