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In really early days of electronic school, the teacher used to say something about not unplug power too quickly at an inductor or capacitor and we were used to slowly turn the voltage generator from a signal generator down to zero. Something about the transients, something about the charge stored...

I'm now interested in working with a power converter, but what was said many years ago still lingers with me but I can't remember exactly what was said at that time.

Can someone please remind me what is the rule when it comes to safely handling inductors and capacitors in a (basic) circuit?

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3 Answers 3

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Thou shall NOT open-circuit a charged inductor.

Thou shall NOT short-circuit a charged capacitor.

If you think about it from their fundamental equations:

\$V = L\dfrac{di}{dt}\$ - a sudden change in current (i.e. forced open circuit) will result in infinite voltage.

\$I = C\dfrac{dv}{dt}\$ - sudden change in voltage (i.e. short circuit) will result in an infinite current.

It's obviously not infinite in practice (due to strays and the ability to change the voltage/current fast enough) BUT it is significant enough to damage electronics...

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I'd also add: - Thou shall NOT reverse polarizing an electrolytic capacitor, because it may explode –  Mario Vernari Aug 30 at 15:03
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That thy days may be long on the face of the land. | Nor shalt thou exceed the maximum charging voltage of Lithium Ion batteries, and their closer kin, lest 'vent with' flame shalt consume thy battery, and even thy laptop, and verily it may come to pass, even thy mode of transportation, terrestrial or aerial, such as it may be. | Thou shalt not apply voltages to Tantalum capacitors which are even but a skerrick above their rated values, if energy of significance is readily to hand, nor reverse polarity even a jot, and thou shalt be better blessed if thou dost ban them from your halls and .... –  Russell McMahon Aug 30 at 15:35
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.... dwelling places and even thy places of commerce, that they fail not importuneously, as is their wont. | In the pursuit of not invoking di/dt as aforementioned, thou shalt equip diode or zeners or resistors or other forms of snubbing devices aroundaboutst thy inductors if it seem good to thee to attempt to interrupt the current flow there, lest the voltage very great doth in its anger leap up and consume thy switch and thy semiconductor and whatsoever else thou may have connected to it conductively. | Thou shalt not exceed Vgs max, nor Vdsmax, except but thou usest avalanche rated .... –  Russell McMahon Aug 30 at 15:41
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.... devices in manners designed for same. | Thou shouldst ponder the arcane aspects of SOA and Pd and absmax and Rthja & Rjc and Rdson and Vgsth and the many others of their ilk early in thy youth that thou doth not accidentally transgress the laws which Murphy has set as a snare and a trap for you for the releasing of arcane smoke and the like. Thou .... –  Russell McMahon Aug 30 at 15:45
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Skip a bit, brother... –  Doug McClean Aug 30 at 19:34

Inductors store flux when current flows through them. When the inductor is de-energized, the flux turns back into current. When this current attempts to pass through a very high resistance it results in a very high voltage, because Ohm's Law. Damage and/or injury can result. This is why we use flyback diodes on inductive circuits.

Capacitors can store their charge for a long time, even when the power is disconnected. This is why we discharge capacitors manually before servicing high-voltage equipment. Since the dielectric can also absorb some of the charge and retain it when the capacitor has been discharged, we must make sure to discharge it multiple times in order to make certain that the capacitor is empty.

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Maybe it might be helpful to clarify "When the inductor is de-energized, the flux turns back into current. When this current attempts to pass through a very high resistance it results in a very high voltage ..." One point is, the inductor is going to maintain the current part of I=V/R as constant, so to maintain V/R at the constant, V must become huge when R becomes huge. The other point is one needs to know which part of Ohm's law will be maintained as a constant; with a battery voltage will not change significantly when a switch is opened because it is a voltage source. –  gbulmer Aug 30 at 15:47
    
A key point about capacitors is that disconnecting them while they're charged isn't dangerous in and of itself, but reconnecting them while they're still charged can sometimes be very bad. A good way to avoid reconnecting a capacitor when it's charged is to give it a chance to discharge before disconnecting it. –  supercat Aug 31 at 2:12

So you know it has something to do with transients, right? Let's make a thought experiment from this. Say that you have an inductor, it was connected to a power source for a very long time. Say the power source delivers a 1A current. Then because of its properties (an inductor is little more than a short circuit when it comes to steady state) the voltage across it will be 0V.

Now imagine that you remove the power source and change it for a 0 ohm resistor. What would happen? Right after removing source, the current through the inductor is still 1A and is now forced through the 0 ohm resistor, resulting in a V = I × R = 1A × 0Ω = 0V. So far so good, nothing changed.

Now imagine that you changed the resistor for a 10Ω part, what would happen right after removing the power source? The inductor will now force its current through a 10Ω resistor: V = I × R = 1A × 10Ω = 10V.

Now it is easy to imagine what happens if that resistor gets larger and larger: 100Ω results in 100V, 1kΩ in 1kV, 1MΩ in 1MV, and so on. A resistance nearing infinity will imply an (theoretical) infinite voltage and that is where physics really gets interesting.

Of course there is only a finite amount of energy stored in the inductor and therefore the high voltage will not exist for very long, only a brief moment after removing the power source.

A similar thought experiment can be done with a capacitor. A capacitor is little more than two plates that do not touch, so a very high resistance and in steady state it is charged with a voltage and no current can flow. Similar to the inductor we can again connect parallel resistor, but now you start with a very high value and work back to 0 for a short circuit and calculate the respective current right at the moment after the voltage source was removed.

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