Electronics > Beginners
Help me understand inductance.
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hamster_nz:
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:


--- Quote ---When current to flows it induces a magnetic field, and that stores energy. Once the the source of current flow removes, the energy stored in the collapsing field induces current and voltage in the conductor. The ratio of current to voltage depends on the remaining impedance within the circuit - so a field collapsing into an open circuit can create relatively high voltages, and into a short circuit can create very high currents.

--- End quote ---

So 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 the opposite direction 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...
chris_leyson:
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.
hamster_nz:

--- Quote from: chris_leyson on October 16, 2018, 09:59:49 pm ---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.

--- End quote ---

So 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?


Wimberleytech:
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.
rfengg:

--- Quote from: hamster_nz on October 16, 2018, 09:32:39 pm ---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?
Maybe the important bit that I am missing that the current from the collapsing field is in the opposite direction to the current that created it?

--- Quote ---
The snubber/flyback diodes across a large inductor similar to the energising coil of a relay prevents breakdown of the active device which drives it. When a relay is switched off, the inductor /coil winding produces a e.m.f in reverse in  order to keep the current across it the same direction prior to the event of the switchoff happening. (V=L di(/dt)) .
Since the current is interrupted abruptly , the V induced is large which may exceed the breakdown voltage of the device (for, e.g. the VCE of a BJT).
Since the voltage produced  is reversed to keep current flowing the same way , the diode across the relay windings, clamp this to roughly 0.7V which prevents the active device from breaking down.
If you sketch out the schematic , this should be obvious.
Better still, think of this in another way..........the diode is put the "wrong way around" as you state........in this "wrong way" around, think of how the voltage needs to be polarised to make the diode conducting.......and then try to infer how this voltage is being produced and how it protects the active device.

Also, the current from the collapsing field is not in the opposite direction to the field that created it........the voltage across the inductor reverses to keep the current in the same direction as it was, before it was interrupted .
--- End quote ---

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