Help me understand inductance. - Page 1
Forum section: electronics / beginners

author: hamster_nz
author_rank: Super Contributor
timestamp: October 16, 2018, 09:32:39 pm
content: I'm feel quite happy about capacitance, resistance, BJTs, MOSFETs and so on, but one thing I struggle with is inductance. I just don't 'grok' it. SMPS sort of make sense, and snubber/flyback diodes seem to be the wrong way around based on intuition. It is just me?I know the math (more or less), and I know that it could be approximately summed up as:QuoteSo is that pretty much it for inductors, ignoring coupled magnetic fields?Maybe the important bit that I am missing that the current from the collapsing field is in theoppositedirection to the current that created it? It is not trying to 'consume' the energy stored by sustaining the flow of current (like a capacitor does), but it tries to 'return' the energy stored in the magnetic field, so the current is reversed.As you can see, I just don't grok it. Help! Hopefully discussing it will make give me a lightbulb moment...

author: chris_leyson
author_rank: Super Contributor
timestamp: October 16, 2018, 09:59:49 pm
content: It's not like that with inductors. If you remove the voltage source that is causing the current to flow in the inductor then the magnetic field will try to keep the current flowing in the same direction and the voltage will reverse in order to try to keep the current flowing. You can instantaneously change the voltage across an inductor but not the current. Opposite for a capacitor, you can instantaneously change the currrent flowing in or out of a capacitor but not the voltage. In an inductor the energy stored in the magnetic field tries to keep the current constant, whereas in a capacitor the energy stored in the electric field tries to keep the voltage constant.

author: hamster_nz
author_rank: Super Contributor
timestamp: October 17, 2018, 01:55:09 am
content: Quote from: chris_leyson on October 16, 2018, 09:59:49 pmSo this is the bit that troubles me. How would the "voltage reverse in order to try to keep the current flowing". In that statement is the cause and effect the wrong way round?Would this be better said as "the voltage at the terminals reverses BECAUSE the energy in the magnetic field keeps the current flowing".That seem to work while looking at a boost converter's topology. The switch goes off, the end of the inductor goes + enough for current to flow through the diode and into the filter cap. For them the reverse voltage spike is a feature.Do buck converters use the inductance differently? They seem to use the inductor's ability 'moderate' current flow (prevent quick changes in current) to produce the low voltage on the output, and the voltage spike from switching is steered to ground by the diode - for them the spike it is a 'problem' (energy lost in the diode) and not a feature?

author: Wimberleytech
author_rank: Super Contributor
timestamp: October 17, 2018, 02:04:17 am
content: Maybe just focusing on the math will help.V = L (di/dt)When a source V is applied across an inductor, the current increases, so di/dt is positive.When the source V is removed, there is no source for additional current, so the current decreases, thus di/dt is negative.Because di/dt is negative, the voltage across the inductor switches polarity.

author: rfengg
author_rank: Regular Contributor
timestamp: October 17, 2018, 03:21:51 am
content: Quote from: hamster_nz on October 16, 2018, 09:32:39 pm

author: bsfeechannel
author_rank: Super Contributor
timestamp: October 17, 2018, 04:54:21 am
content: Quote from: hamster_nz on October 17, 2018, 01:55:09 amVoltages can induce currents as currents can induce voltages. They are the two sides of the same phenomenon.Since inductors tend to maintain their currents, when you switch from a voltage source to a load, the voltage has to be inverted so as to maintain the current.You find an analogous behavior with a capacitor. In order to maintain its voltage, it inverts its current when you switch from a source to a load.

author: IanB
author_rank: Super Contributor
timestamp: October 17, 2018, 05:08:56 am
content: Quote from: hamster_nz on October 16, 2018, 09:32:39 pmMaybe it is easier to think about what inductors do, rather than how they do it.Functionally, a capacitor resists any change in voltage. In a very similar way, an inductor resists any change in current.There's a good analogy with hydraulics here. Once water is flowing in a pipe, it resists any change in the flow. If you try to stop the flow, high pressures can result. This is just like the high voltages that result if you try to stop the current flow through an inductor.In the video below you can see the hydraulic analog of a switch-mode boost converter. Once the water is flowing through the downhill pipe it doesn't like to stop. By suddenly stopping the flow, enough pressure is generated to boost the water way uphill, far higher than it started.https://youtu.be/_wG7o8ic_OQ

author: BravoV
author_rank: Super Contributor
timestamp: October 17, 2018, 05:49:38 am
content: I always thought inductor analogy is like this hydraulic wheel below, when the wheel gained the rotating momentum caused by the initial current, and if the current stopped or even decreased, the rotating momentum will keep trying to maintain/move the current (liquid), hence the polarity reversal (pressure reversal) at the input & output pipes.Cmiiw.

author: bsfeechannel
author_rank: Super Contributor
timestamp: October 17, 2018, 06:21:52 am
content: I think that the problem lies in the fact that OP thinks that voltages are causes and currents are effects. How can a voltage be at the mercy of a current?

author: Mechatrommer
author_rank: Super Contributor
timestamp: October 17, 2018, 07:15:01 am
content: think of it when the power source is removed, inductor will become a "charger", not "source". in charging mode, where the current goes will become more positive and theoretical infinite of inversed potential in open space time, while capacitance similarly will become a "charger", but instead of voltage, current will change direction infinitely in theoretical shorted space time. it always amazed me how these two work in pair complementing each other nicely, its one living proof that the law of this universe is not "random" as many claimed how it was created. in fact "random" is just another translation for our lack of understanding of the fact everything in the universe is governed by laws in one way or another, lumped in the world of knowledge of "statistics and stochastics". so "something" must have "selected" this kind of non random law,hence my signature is somewhat delusive, for the purpose of it to being delusive and for the sake of harmony and peace in this little forumotoh we can keep capacitance practically indefinite in time in few cents of component, but not that easy at keeping the magnetic field around (by applyingpure shortonce its created).

author: hamster_nz
author_rank: Super Contributor
timestamp: October 17, 2018, 08:05:50 am
content: Quote from: bsfeechannel on October 17, 2018, 04:54:21 amYes! This! It (kind of) makes perfect sense. I'm going to have to ponder/think/sleep on this one, but I can sort of feel the understanding starting to form....

author: hugo
author_rank: Regular Contributor
timestamp: October 20, 2018, 12:56:34 am
content: Just go to:http://www.falstad.com/circuit/The "Circuits" menu contains a lot of sample circuits for you to try.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 20, 2018, 02:01:00 am
content: If you (think you?) understand capacitors:Just swap V and I.It's that simple.To really cook your brain -- and drive home your knowledge of circuits! -- demonstrate the equivalence of parallel-series circuits where V and I are swapped, L and C, and series and parallel. The node voltage equations of one will be exactly the current mesh equations of the other (with V replaced with I, of course).(Uh, you will have to be far enough into intro EE to know what those things are, and the basics of how to do them. I don't know how "beginner" that is in this case...)Tim

author: kosine
author_rank: Regular Contributor
timestamp: October 20, 2018, 11:41:07 am
content: If you read the following carefully (maybe twice!), the fundamentals should hopefully make more sense:1. Electrons have momentum:Think of inductance as length (L).Say a short wire and a long wire both carry 1A; Both have the same current (I), BUT the longer wire contains more electrons in motion.It therefore has more electrical momentum. (Compare this with pipes carrying water.)Electrical momentum is termed "magnetic flux" and is given the Greek symbol phi.Since longer wires contain more electrons, the total momentum (phi) increases with length.But a greater current also means more electrons, so momentum (phi) also increases with current.Taken together, momentum (flux, phi) is proportional to L, and also to I.So phi = LI. (The units are chosen such that the proportionality constant is 1.)2. Momentum accumulates over time:Now, to get a current flowing at all, we need to apply an electromotive force. This is the voltage (V).Increasing this applied force/voltage will increase the momentum, but so will applying it for a longer period of time (t) because momentum accumulates.Therefore we can also deduce that phi is proportional to V, and also to t.So phi = Vt = LIThis is the textbook inductor equation, and is analogous to classical momentum:Momentum = Ft = dm/t (since F=ma and a=d/t^2. Think of "mass per second" as the current.)Inductors (as a component) are essentially a very long wire wound into a coil to make them more compact. (The coiling also increases the effective length due to magnetic effects, but that gets a lot more complex to explain.) Inductors allow you to build up momentum in the circuit, thereby storing the electrical equivalent of kinetic energy. If you suddenly break a circuit with lots of momentum in it, all that energy has to go somewhere. It'll either be converted into potential energy (i.e., a voltage), which is how a SMPS works*, or you can safely dissipate the energy as heat by adding a diode. That allows current to circulate back through the inductor until all the stored energy is used up.* If you break the circuit on the high voltage side of an inductor, all the electrons flow away from the break resulting in a negative voltage spike. Break the circuit on the low side, and all the electrons pile up and cause a high voltage spike. How large these voltages get depends on many things, including the length of the coil. At it's simplest, that's its inductance (L).