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date	article	author	comment
22 jul 2021 20:53:18	Extreme low voltage oscillator	Guy	A small update on my side: I tested roughly the same circuit but replaced the transformer with two 0805 size inductors with values of 180uH, and only used one JFET with Vgs at cutoff about 3V. It works and oscillates at up to 1.5MHz with a 22pF gate capacitor and I can adjust the frequency by replacing the capacitor or inductor. However, the circuit draws about 5mA and I wanted to lower the current. For that I will need to source higher value inductors and try again. It's quite reasonable when you think of it, when the JFET is open there is a straight path to ground through the inductor equivalent to the 'N2' winding. Also, I found that increasing the 'N1' inductance reduces the minimum supply needed for oscillation but does not reduce the overall current draw of the circuit. This proves that you really don't have to have the coupling of the windings in the transformer, discrete inductors work well. I like this inspiring small circuit. Guy Shemesh - ePiccolo Engineering
11 jul 2021 19:26:36	Magnetic circuits	Freddy	Not only the amount of energy that can be stored changes by applying an air gap: Also the magnetic resistance of the magnetic circuit increases, and with that the inductance decreases. And the product of the number of turns and saturation current increases. There is a lot going on with an air gap!
11 jul 2021 15:34:16	Magnetic circuits	Guy	Alright Freddy I accept your explanation. So this basically shows the potential in the air gap! essentially you localize the flux with the core and then force it through the air gap, at least for that small volume you have a very high energy store. Very interesting
11 jul 2021 14:41:38	Magnetic circuits	Freddy	@Guy, you are right if the field strength H where constant in every section of the magnetic circuit, but it is not. On the other hand, the flux density B is constant with some minor variations. These variation are caused by irregularities in the cross sectional areas. Especially the cross section of the airgap is affected by this. But as long the air gap length is much smaller than the width it's safe to assume an equal flux densitie in the whole circuit.
11 jul 2021 14:12:18	Magnetic circuits	Guy	Freddy, you wrote that the energy stored per volume in a magnetic field is calculated as: w = 0.5 * B^2 / μ. However, B conceals another μ, you could equally have written w = 0.5 * μ * H^2. And now the opposite is true, the higher the μ, the higher the stored energy. I need to refresh my physics studies but as far as I remember, H is invariant when crossing material boundaries while B by its definition is affected.
11 jul 2021 13:24:24	Magnetic circuits	TalkingElectronics	You wrote: "When the core is saturated, it will not "accept" or "deal with" further increases in current." When I connect an inductor to a DC power supply, the current will increase with a steady pace until the core approaches the saturation region. From here the current increasement wil accelerate to an much higher dI/dt as if there was no core present. There is no point where the current will reach a steady value. You have not understood what I am saying. When you apply current at a low value, it turns into magnetic flux and when the current is turned off the magnetic flux collapses and returns voltage and current into the winding. This process is effectively available up to the point before the magnetic circuit is saturated. If you apply current past the point of saturation, only the magnetic flux up to the point of saturation is returnable. The last section I can't follow. Magnetic flux is not lost unless you have an magnetic leak. Now every magnetic circuit has magnetic leaks, but this is not what you referring to. If air gaps wasted energy, then the air in the gaps would be quite hot. This is not the case. The whole idea of an air gap is to "use up" the magnetic flux that would be generated by the DC component of the waveform so that only the amplitude of the waveform above the DC component is delivered to the magnetic circuit.
11 jul 2021 13:15:52	Magnetic circuits	Freddy	The energy stored per volume in a magnetic field is calculated as: w = 0.5 * B^2 / μ How lower the permeability μ, how higher the stored energy per volume. This is also shown in the equations in this article by replacing H of equ. 13 with equ. 2. And it is also stated in the wiki article about magnetic energy. You wrote: "When the core is saturated, it will not "accept" or "deal with" further increases in current." When I connect an inductor to a DC power supply, the current will increase with a steady pace until the core approaches the saturation region. From here the current increasement wil accelerate to an much higher dI/dt as if there was no core present. There is no point where the current will reach a steady value. The last section I can't follow. Magnetic flux is not lost unless you have an magnetic leak. Now every magnetc circuit has magnetic leaks, but this is not what you refering to. If air gaps wasted energy, then the air in the gaps would be quite hot. This is not the case.
10 jul 2021 22:35:48	Magnetic circuits	TalkingElectronics	You have the wrong idea about an air gap. The magnetic core will store 1,000 times more energy than the air gap. When current flows through the turns of a coil it produces magnetic flux. Suppose the current flowing is 1 amp. If you have a magnetic circuit without an air gap, it maybe the case where it only takes 100mA to fully saturate the core. When the core is saturated, it will not "accept" or "deal with" further increases in current. So we introduce an air gap and 90% of the magnetic flux from the first 100mA is LOST in the air gap and the magnetic circuit only sees 10%. This allows the next 100mA to deliver some of the flux to the magnetic circuit and this concept will continue to 100%. An AIR GAP does not STORE - it WASTES
10 jul 2021 22:17:41	Magnetic circuits	TalkingElectronics	My discussion of a class-A amplifier is just an example of a DC component of current flowing all the time and the current reduces and increases slightly to create the AC component. This can occur in many situations including the old-style DC power supplies using a 3-terminal regulator.
10 jul 2021 20:52:55	Magnetic circuits	Freddy	Are you trying to tell me that class-A amplifiers with output transformers are the main usage for transformers and inductors? Are these devices not a little bit outdated or at least a rarity? I thought that Switch Mode Power Supplies the mayor usages where for core containing transformers and inductors. If an air gap can store more energy than the core is to be fair a tricky one. It really depends on de the reference parameters. Fact is that an inductor with the same core and inductance can handle much more current and store much more energy if it has an air gap. Be aware that the number of turns between an inductor with and without an air gap are not the same to obtain the same inductance value.