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I though that an inductor, when it doesn't have current flowing through it anymore, would quickly discharge in the opposite polarity. More precisely, I was expecting one quick peak in a negative voltage, and then the voltage going back to its previous value once the inductor is discharged.

The kickback, however, seems to behave more like a slowly fading sine wave on my oscilloscope, going in multiple times in high ranges of negative / positive values before fading away. (see picture)

  1. Why are there are multiple peaks?
  2. Why are there are peaks of positive voltage as well?

Am I missing something?

Oscilloscope readings of a 5 volts circuit being opened Oscilloscope readings of a 5 volts circuit being opened

EDIT:
Picture of a 1.5 volts circuit using battery (without breadboard) enter image description here

Oscilloscope readings when the 1.5 volts circuit is closed enter image description here

Oscilloscope readings when the same 1.5 volts circuit is opened at probe 1x enter image description here

Edit : Oscilloscope readings when the same 1.5 volts circuit is opened at probe 10x enter image description here

Oscilloscope readings when the same 1.5 volts circuit is opened at probe 1x but with longer wires enter image description here

Edit : Oscilloscope readings of a circuit composed of a 3 volts battery , 2 short wires to the battery (~ 1 inch each), and the 1x probe connected directly to each of these wires

enter image description here

Charles-Ugo Brouillard
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  • How to you "open" the circuit? By doing manually, there is something called "bouncing". The connection between cable and battery is lost and reformed for a number of times in high frequency until it is finally broken. – ElectronicsStudent Jan 06 '23 at 03:33
  • Your scope should/could have a 1kHz test-Output. Use this signal to drive the inductor instead of the battery for a quick test. – ElectronicsStudent Jan 06 '23 at 03:35

1 Answers1

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Capacitance somewhere in your system has made a resonant LC circuit. The current through your inductor charges a capacitor, which then discharges through the inductor, creating a current through your inductor, which charges the capacitor...

For a more detailed explanation, see this answer from Andy aka.

Matt S
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  • mmmm that would make sense ! However, there is no capacitor on my breadboard. The only think can think of is : could the capacitors in the oscillator itself be the culprit ? – Charles-Ugo Brouillard Dec 27 '22 at 22:17
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    Breadboard? They're famous for having stray capacitances and inductances... https://electronics.stackexchange.com/questions/302850/what-order-of-magnitude-should-i-expect-the-parasitic-capacitance-to-be-for-a-so – Matt S Dec 27 '22 at 22:20
  • Well, that is unfortunate... i'll go ahead and confirm thats the problem – Charles-Ugo Brouillard Dec 28 '22 at 14:13
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    @Charles-UgoBrouillard All pairs of conductors are capacitors. Two adjacent rows in a breadboard are a really _small_ capacitor (something like a picofarad, I dunno), but they're a capacitor all the same. – Cassie Swett Dec 28 '22 at 20:18
  • I confirm that the breadboard is NOT the problem. The behavior is repeated using the following circuit : 1.5 volt battery + => wire (positive probe on that wire) => 680 uH inductor => 220 Ohm resistor => wire (ground of probe on that wire) => negative side of the 1.5 volt battery. Oscillator is a Siglent SDS 1052DL+. Anyone have a clue ? – Charles-Ugo Brouillard Dec 29 '22 at 21:58
  • Interesting results. Did the frequency of oscillation change? Also, the Siglent is an oscilloscope, not an oscillator. Does your scope have a 1x probe or a 10x probe? – Matt S Dec 30 '22 at 02:53
  • Yes oscilloscope indeed, pardon my french. The probe is at 1x. Didnt see any significant change in the frequency, although it seems to be somewhat shorter (it seemed to oscillate longer before (we are talking nano seconds here)), but the duration of the oscillation is variable each time. – Charles-Ugo Brouillard Dec 30 '22 at 17:10
  • When you say "shorter", do you mean the oscillation settles faster, or its resonant peaks come closer together? Can you update your question with the new details -- ideally, a picture of your circuit as wired and the new oscilloscope reading? – Matt S Dec 30 '22 at 17:20
  • There you go, added the pictures of the 1.5 volts circuit described in my previous comments. – Charles-Ugo Brouillard Jan 05 '23 at 22:24
  • Based on your picture, and a [back of the envelope calculation](https://www.emisoftware.com/calculator/wire-pair-capacitance/), your wire pair is about 1 pf of capacitance. I'm also eyeballing a resonant frequency of about 1.5 MHz, which requires [about 16 pF of capacitance.](https://www.omnicalculator.com/physics/rlc-circuit) Does the oscillation change if you use shorter wires? You might also be seeing the effects of your probe, [see here.](https://electronics.stackexchange.com/questions/472572/understanding-capacitance-in-oscilloscope-probe-tip) – Matt S Jan 05 '23 at 22:41
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    @Charles-UgoBrouillard Inductors also have *self capacitance* which can be significant. Inductor data sheets usually report the resonant frequency of the inductor which is due to the inductance and self capacitance. Oscilloscope probes also have capacitance, around 13pF for a 10x probe and generally higher than 50pF for a 1x probe. – qrk Jan 06 '23 at 01:24
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    I can see all the effects mentioned in your post. But i would like to append another possible cause for the ringing. Assuming the connection battery -> wire -> DUT is broken manually, there can be much more severe effects due to bouncing. – ElectronicsStudent Jan 06 '23 at 03:36
  • I added 2 oscilloscope readings pictures in the original post , one with longer wires, and another one with the probe at 10x. (And indeed, the circuit is manually broken by hand, to be kept into consideration) – Charles-Ugo Brouillard Jan 06 '23 at 13:33
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    @Charles-UgoBrouillard Note that a 1x probe itself has quite a lot of capacitance. A 10x probe has much less (this is the main reason no one ever uses 1x probes), but still a fair few pF, enough to ring like that. – Hearth Jan 06 '23 at 13:42
  • Added the picture of the probe connected to 2 shorts wires directly to the battery without any inductor : it seems that there is indeed capacitance in the probe itself. Would it be safe to say that this is the main issue causing the oscillation when using the inductor ? – Charles-Ugo Brouillard Jan 06 '23 at 14:42
  • Definitely possible. Eyeballing your curve, the time constant looks like it could reasonably be 100 pf of capacitance (e.g. 15-pF probe capacitance + some scope capacitance, or maybe just your 1x probe) discharged through the scope's 1M input. – Matt S Jan 06 '23 at 15:11