WEBVTT NOTE Cores, Inductors, and Piezoelectricity 1 00:00:00.000 --> 00:00:02.040 What a core does. 2 00:00:02.040 --> 00:00:09.064 Question. What core material property determines the inductance of an inductor? 3 00:00:09.964 --> 00:00:13.204 Answer. Permeability. 4 00:00:13.804 --> 00:00:26.971 Permeability is how readily a material carries magnetic flux. A high-permeability core concentrates the field, so the same winding has far more inductance. 5 00:00:26.971 --> 00:00:33.936 Question. How do ferrite and powdered iron compare for use in an inductor core? 6 00:00:34.836 --> 00:00:42.756 Answer. Ferrite cores generally require fewer turns to produce a given inductance value. 7 00:00:43.356 --> 00:00:51.965 Ferrite has much higher permeability, so fewer turns. Powdered iron is lower permeability and: 8 00:00:51.965 --> 00:01:00.165 Question. Which of the following materials has the highest temperature stability of its magnetic characteristics? 9 00:01:01.065 --> 00:01:04.105 Answer. Powdered iron. 10 00:01:04.705 --> 00:01:22.885 So the choice is a genuine trade: ferrite for compactness, powdered iron for stability. A tuned circuit that must not drift uses powdered iron; a broadband transformer or common-mode choke uses ferrite. 11 00:01:22.885 --> 00:01:26.305 And the counterintuitive third option: 12 00:01:26.305 --> 00:01:33.291 Question. Which type of core material decreases inductance when inserted into a coil? 13 00:01:34.191 --> 00:01:36.711 Answer. Brass. 14 00:01:37.311 --> 00:01:51.744 Brass is not magnetic, so it does not concentrate flux, but it conducts, so the coil's field induces eddy currents in it that oppose the field. The net effect reduces inductance. 15 00:01:51.744 --> 00:02:04.231 That is how a brass tuning slug works, and why turning a slug inward can move frequency in either direction depending on what the slug is made of. Losses. 16 00:02:04.231 --> 00:02:11.991 Question. Why are cores of inductors and transformers sometimes constructed of thin layers? 17 00:02:12.891 --> 00:02:18.211 Answer. To reduce power loss from eddy currents in the core. 18 00:02:18.811 --> 00:02:31.671 A solid conductive core has currents induced in it, and those currents heat it. Slicing the core into insulated laminations breaks the paths those currents would take. 19 00:02:31.671 --> 00:02:44.531 It is why mains transformer cores are stacks of thin sheets, and why RF cores are ferrite or powdered iron, non-conductive materials, where the problem does not arise. 20 00:02:44.531 --> 00:02:48.959 Question. What causes inductor saturation? 21 00:02:49.859 --> 00:02:54.899 Answer. Operation at excessive magnetic flux. 22 00:02:55.499 --> 00:03:03.761 Push enough current through and the core's magnetic domains are all aligned; it cannot carry more flux. 23 00:03:03.761 --> 00:03:14.429 Inductance collapses toward the air-core value, the current rises sharply, and in a switching supply that is how the transistors die. 24 00:03:14.429 --> 00:03:25.899 Saturation is a current limit, not a voltage one, and it is why inductors carry a saturation-current rating separate from their heating rating. 25 00:03:25.899 --> 00:03:34.686 Question. What is a primary advantage of using a toroidal core instead of a solenoidal core in an inductor? 26 00:03:35.586 --> 00:03:43.866 Answer. Toroidal cores confine most of the magnetic field within the core material. 27 00:03:44.466 --> 00:03:51.397 Self-shielding, as the General track noted, no coupling to neighbours in either direction. 28 00:03:51.397 --> 00:04:04.335 Question. What devices are commonly used as VHF and UHF parasitic suppressors at the input and output terminals of a transistor HF amplifier? 29 00:04:05.235 --> 00:04:07.925 Answer. Ferrite beads. 30 00:04:08.525 --> 00:04:28.685 A bead adds loss specifically at VHF while leaving HF nearly untouched, which is exactly what kills the parasitic oscillation the amplifiers lesson described without affecting the wanted signal. Piezoelectricity. 31 00:04:28.685 --> 00:04:32.367 Question. What is piezoelectricity? 32 00:04:33.267 --> 00:04:42.507 Answer. A characteristic of materials that generate a voltage when stressed and that flex when a voltage is applied. 33 00:04:43.107 --> 00:04:48.756 Question. Which of the following is an aspect of the piezoelectric effect? 34 00:04:49.656 --> 00:04:56.856 Answer. Mechanical deformation of material due to the application of a voltage. 35 00:04:57.456 --> 00:05:03.095 Both directions. Squeeze it and get a voltage; apply a voltage and it deforms. 36 00:05:03.095 --> 00:05:17.336 A quartz crystal in an oscillator does both continuously, which is why it resonates mechanically at a frequency set by its physical dimensions, and why it is so much more stable than an LC circuit. 37 00:05:17.336 --> 00:05:22.402 Question. What is the equivalent circuit of a quartz crystal? 38 00:05:23.302 --> 00:05:32.635 Answer. Series RLC in parallel with a shunt C representing electrode and stray capacitance. 39 00:05:33.235 --> 00:05:45.469 Worth understanding rather than memorising, because it explains the crystal's two resonances. The series RLC is the mechanical resonance, very high Q, very low R. 40 00:05:45.469 --> 00:05:50.830 The shunt C is the electrical capacitance of the electrodes and holder. 41 00:05:50.830 --> 00:06:05.556 Together they give a series resonance (where the RLC branch is at minimum impedance) and, a little higher, a parallel resonance (where the RLC branch's net inductance resonates with the shunt C). 42 00:06:05.556 --> 00:06:18.318 Those two are only a few kilohertz apart, and which one an oscillator uses is why the load capacitance specification from the oscillators lesson matters. Check yourself. 43 00:06:18.318 --> 00:06:32.893 You need an inductor whose value must not drift with temperature. Ferrite or powdered iron? Your switching supply's inductor gets hot and the current waveform spikes sharply. What is happening? 44 00:06:32.893 --> 00:06:38.179 Why does a brass slug reduce inductance when an iron one increases it? 45 00:06:44.179 --> 00:07:02.018 Powdered iron, highest temperature stability of permeability. Ferrite would need fewer turns but drift more. Saturation. The core cannot carry more flux, inductance collapses, and current rises steeply. 46 00:07:02.018 --> 00:07:15.619 Brass is non-magnetic but conductive: the coil's field induces opposing eddy currents in it, reducing inductance. Iron concentrates flux and increases it.