Induction coil - let's clear this up

[nix85]

In first video below they say as primary field collapses, it induces backward current in the primary, flowing against the battery current.

Yet in the second video we can clearly see that after primary is turned off, there is no current through it and it has voltage spike around 50V in same direction as battery voltage, NOT in reverse direction.

https://www.youtube.com/watch?v=lWyn_eV-DzM

https://youtu.be/xXteh1Ch738?t=834

Seems first video is wrong, what you think.

[nix85]

We all know inductor will try to maintain the current in the same direction after supply is cut off. http://www.learnabout-electronics.org/ac_theory/dc_ccts44.php

[kajunbee]

If the current were in opposite direction then a flyback diode would be pointless. The polarity changes but current is driven in same direction. The inductor now acts as source instead of a impedance. At least that's the way I understand it.

[kajunbee]

I had not watch the video and did not realize the inductor was ignition coil. But I still believe he is showing current charging condenser in wrong direction. It appears to me that when field collapses it will charge condenser to higher voltage, but current will flow in same direction. The charge doesn't flow across the switch contacts, but instead by displacement current through condenser. Once it charges to this higher voltage it will then want to discharge. At this point is when current will flow opposite direction as shown in video. There will be a ringing oscillation setup with the series LC circuit. How does that sound to you?

[TACHY0N]

As i said in the first post, first video shows it wrong, current never reverses.
As i have written in the second post "inductor will try to maintain the current in the same direction after supply is cut off.". Function of flyback diode is simple and obvious, from perspective of diode voltage polarity changes when supply voltage is cut off cause now the coil becomes the source. All that is basics and perfectly assumed.
I have since then explained many times how induction coil aka "spark coil" aka Ruhmkorff coil works and how cap limits the peak voltage and turns the singular transient into RLC oscillation. Clearly, cap is there to protect the switch from backEMF. I noticed on wikipedia page that there is a false claim that capacitor increases the voltage of the induction coil while it is exactly the opposite. It's been discussed around here on Figuera thread i think that when field is collapsing bigger the resistance higher the spike and faster it burns out, and just the opposite if collapsing field voltage is discharged into a cap which acts like a very low resistance at first and thus reduces the spike. One member correctly compared this to stopping a flywheel suddenly and thus generating higher friction/temperature very fast compared to slowing it down slowly and generating less friction/heat for a longer time. Diagram of a typical ignition coil attached below.

BTW no one has yet explained why they connect the primary and secondary, it seems totally useless and counterproductive, potentially damaging the primary circuit with very high voltage, but there must be some reason. Few times i googled it i did not find a reason for this. If someone knows, feel free to share.