understanding inductor behavior in switching threw simulation question

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yefj

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Hello,From the theory of inductors we know that in a case ofcurrent jump it has to keep the current continues but the voltage is has to change .
However as you can see in the simulation below when source current changes then the current threw the coil changes also.
So what does it mean when they say that inductor inforces a continuety in current?
How can I see it in the simulation below?
Ltspice file is attached.
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You are driving inductor with an ideal current source (infinite compliance), respectively the inductor is hindered from keeping current continuous. That's an unreal test setup because a real current source has limited voltage capability.
 

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how you recommend to improve the sitution and simulating a change in current then?
Thanks.
 

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Hello,I have used a voltage source as shown below ,So although I have a pulse on the input , the voltage on the output is almost zero,but the current is continues.
How can I see the V=L*di/dT in action here?
I have a current rise of 1mA inductance of 1uH T=1nsec but Vout=629mV however according to calculation Vout needs to be 1V.
Two questions:
1.why My vout doesnt fit the calculation?
2.Why do I get these small spike in the voltage on the inductor?
Thanks.


 

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The inductor does several complicated things. To grasp the relationship between voltage and current...
It may be easier to imagine the inductor operating in propulsive mode. That is, at a moment when the flux field has built and current is flowing.

Quickly attach an ohmic resistance across the ends but no voltage source is attached. Call the time t=0. The inductor generates voltage, sending current through the resistance around a loop.

Now the above setup serves as a tutorial to see how inductive reactance works. Time constant is LR. Convention agrees that the characteristic behavior lasts for 5 time constants. Ampere level immediately is at a peak, then declines on a familiar curve. Volt level likewise immediately starts at a peak, then declines on a familiar curve.
 

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yefj said:
1.why My vout doesnt fit the calculation?
2.Why do I get these small spike in the voltage on the inductor?
Click to expand...
1. Current waveform isn't shown. If you look at it, you'll see that V= L di/dt is still fulfilled. When voltage ramp finishes after 1 ns, current hasn't yet reached final value, current rise time is larger than 1 ns, respectively L di/dt smaller than 1V.
2. "Small spike" is L di/dt.
--- Updated ---

See all-in-one:

 
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Hello ,So how can we see from the plots the property that inductor inforces continuety in the currnt threw it?
Thanks.
 

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Hello, from the manual below ,where can I see in simulation the the continuety of current in inductor?
Thanks.
"If you don’t have a source of infinite voltage, the current through an inductor will not change instantaneously. That is the principle of continuity for an inductor."

Continuity of capacitive voltage and inductive current

We explain two principles of continuity. One for capacitors and one for inductors.
spinningnumbers.org
 

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Post #1 is accurate with a current ramp of 100 mA in 1ns thru 1 uH yields 100 V

Just do not try a current source step pulse with Tr=0 except to test the insulation strength of your simulator (hah)
Your calc. be using old or wrong data.




It's always a good thing to calibrate your mental calculator and your handy simulator. At least once you need a correction somewhere.

Here I use an ideal VCCC with 1V in and 100 mA out with an ideal R, L and R shunt to limit voltage with a step current with 0 ns risetime.

The L/R = 1e-6/1e6 risetime is 1ps and the voltage has decayed 63% and current risen the same.

 

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yefj said:
Hello, from the manual below ,where can I see in simulation the the continuety of current in inductor?

"If you don’t have a source of infinite voltage
...
Click to expand...
The words in the textbook step into a realm of fiction...
Although underlying is a nearly unbelievable characteristic of inductors. Namely as a component that crosses over from magnetism to electricity and back. We make use of them, and apply formulas, yet we little understand how they do what they do.

Attempting to convey a feeling of what we can't see in the inductor, I devised my home-brew animated simulator to portray:
* current bundles
* flux field
* voltage.

Even with that we need something more in a simulator before it gives a feel for what's happening.

<iframe width="967" height="480" src=" " title="Inductor behavior with DC (Animated)" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture; web-share" referrerpolicy="strict-origin-when-cross-origin" allowfullscreen></iframe>

 
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Hello, I know that capacitor stores potential difference and inductor stores magnetic energy why is presented in current.
Still I can understand your words.
You said that "The words in the textbook step into a realm of fiction..."
So what is not fiction? what is reality?
Thanks.
 

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yefj said:
So what is not fiction? what is reality?
Click to expand...
'Infinite voltage' is the term used by the author in your highlighted text. We don't have infinite voltage yet the author conveys something about the invincibility of inductors. Perhaps you've heard that advanced technology looks like magic to the uninitiated. Our world uses inductors to perform magic every day. Let's reverse the scenario of voltage and current. Aprupt stoppage of current in an inductor can generate voltage that soars and is liable to get out of hand if we're not careful.

* Example... I neglected to add a load to my experimental boost converter. In half a second of operation I watched output voltage climb to a level that would have blown up the output capacitor. I had to act quickly to cut power.

* Example... a lineman used an ordinary VOM to take an ohm reading of a huge transformer or inductor. After sending 1.5V through windings he pulled away the probes. Stopping the few milliAmps caused an immediate voltage surge which jumped as an arc and electrocuted the lineman.
 

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Hello Bradtherad,Yes its V_l=LdI/dT a sudden end of current will create voltage spike.
so Why they sat that inductor keeps the curent continues?
The continuety thing is confuses.
where can we see this continuety?
Thanks.
 
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Let's take a charging inductor for an example.
Let the final inductor current be Io.
iL = Io*(1-e^(-R*t/L)) = Io - Io*e^(-R*t/L)
diL/dt = 0 - Io*e^(-R*t/L)*-R/L = Io*R*e^(-R*t/L)/L
VL = L*diL/dt = L*Io*R*e^(-R*t/L)/L = Io*R*e^(-R*t/L) = (Io*R)*e^(-R*t/L)

You can verify this with simulation by looking up the inductor voltage and current at time any time, t.

You can approximate this by using piecewise linear approximation so you can use VL = L *delta_iL/delta_t.
--- Updated ---

yefj said:
Hello Bradtherad,Yes its V_l=LdI/dT a sudden end of current will create voltage spike.
so Why they sat that inductor keeps the curent continues?
The continuety thing is confuses.
where can we see this continuety?
Thanks.
Click to expand...
Well, when the inductor is charged up, a magnetic field is developed around it at a certain orientation. Breaking the path of current will not stop current from flowing. The the magnetic field will collapse of course and if the current path was broken without a new path for the current to flow, then that inductor will break the barrier of the insulation in its path, and you can see this in the form of a spark. Actually current still does flow and the amount of current that flows depends on the insulation and the amount of energy that was stored in the inductor.
 
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Inductors are like momentum for electric current - when you try to stop the current, it pushes back with a "flyback" force. In the real world (outside of quantum physics and superconductors), all inductors have some resistance and resist changes in current. This means you can't instantly switch current on or off with an inductor.
Switches aren't as simple as just "on" or "off" either. Every real component has some resistance (R), inductance (L), and capacitance (C):
  • Capacitors have a little inductance from their shape.
  • Wires have all 3 (R, L, C) as shown by the Telegrapher's equations. Coiling a wire increases inductance, and adding magnetic material boosts it more (but has limits).
  • Switches have capacitance when open, based on their size and current rating. This capacitance changes depending on the switch type.
  • Air switches create an arc after about 1 microsecond while the capacitance decreases as the gap spreads.
  • Diodes and PN junctions are all Varicaps, but just have poor tolerances.
  • Yet all switches have capacitance from their geometry which increases with size. This is true for all types including diode switches.
  • Electronic switches need a brief "dead time" (around 1 microsecond) to let stored energy fade.
  • All caps have a self-resonance frequency (SRF) due to the series inductance of the conductor.
  • The time it takes to switch current depends on the inductance and resistance (L/R = switching time). Adding capacitance can slow this down further.
  • The flyback voltage from an inductor is limited by things like semiconductor ratings, capacitance, leakage, or special circuits (snubbers).

  • Current can't instantly stop in inductors - it has to gradually decay to a steady state, like momentum slowing down to a constant.
 
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And to add to this, the duration that the voltage spike is the time it takes for the current to decay from its initial value (at the instant where the switch was opened) to it's final value (which can be down to 0 A).
--- Updated ---

I should have added here that the voltage spike we observe is as a result of the current (although l, decaying) continuing to flow through our R which has suddenly changed from Rinitial to Rinitial+Rinsulation.
 
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