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I created a boost converter as given here:

Boost converter transistor voltage weird behavior

enter image description here

I do not know what happened but as I connected a smaller capacitor about 22 pF at the output in parallel with the 1 kOhm resistor the oscilloscope showed an inductive kickback of 100 volts every switching period (I even saw a spark).

The boost converter is powered by a 1 Volt battery. My question is are these sparks dangerous if I were to accidentally touch them and why did the smaller capacitor contribute to inductive kickback?

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  • \$\begingroup\$ Your link jumps me to the comments in that question and in the question there are multiple schematics. Please right-click on the schematic in question, copy link, and use the add Image button and paste the link into your post. \$\endgroup\$ Commented Oct 2 at 23:05
  • \$\begingroup\$ Please add a correct, up-to-date schematic. Preferably with no errors. Alternatively, you can use the schematic editor tab (i.e. CircuitLab) which is available in the toolbar of your edit window. Not only will that make you circuit clearer, it will also aid in diagnosing whatever problems you experience. \$\endgroup\$ Commented Oct 2 at 23:15
  • \$\begingroup\$ The transistor does not turn on at all. \$\endgroup\$ Commented Oct 3 at 0:40
  • \$\begingroup\$ I’m voting to close this question because the schematic does not correspond with the actual circuit which exhibits the symptoms described. The question will therefore have dubious value to a future reader. \$\endgroup\$ Commented Oct 3 at 15:22
  • \$\begingroup\$ This old circuit also exhibited the same symptoms. The new updates you made do not change the fact that connecting a smaller capacitor causes voltage spikes. Both cirucits cause voltage spikes at the output with small capacitor values. \$\endgroup\$ Commented Oct 3 at 21:07

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why did the smaller capacitor contribute to inductive kickback?

A boost converter works by storing energy in the inductor then releasing it to the capacitor so, as the capacitor becomes smaller the transferred energy into that capacitor produces a larger peak output voltage and, eventually, the voltage produced is so large it will damage the output transistor and possibly cause the diode to breakdown.

22 pF is a poor, poor choice. It's in the order of a million times smaller than what is normally used (say 22 μF). So, if the output voltage using a 22 μF was (say) 20 volts, with a 22 pF, the output would rise 1000 times higher and this will cause problems with the transistor and/or diode way before 20000 volts was reached.

I even saw a spark

It doesn't surprise me at all.

are these sparks dangerous if I were to accidentally touch them

They will sting a little but, for a healthy person they shouldn't be dangerous reading between the lines of your circuit.

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  • \$\begingroup\$ I thought the capacitor was there to store charge and smooth out ripple? Wouldn't a smaller capacitor just make the ripple less smooth and provide a greated resistance to the AC? I dont understand why the voltage peaks to be honest. By the way if these sparks are a couple of thousands of volts could this damage my oscilloscope as well? \$\endgroup\$ Commented Oct 3 at 9:54
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    \$\begingroup\$ The capacitor is transferred the energy of the inductor in each switching cycle and, it's that energy and the capacitor value that govern the output voltage entirely. This is why we use pwm to control the output voltage because in doing so we control the energy given to the capacitor. \$\endgroup\$ Commented Oct 3 at 10:03
  • \$\begingroup\$ Ok. One more question if I may: since these sparks are a few thousands of volts and Khz frequency can they cause bremsstrahlung radiation? \$\endgroup\$ Commented Oct 3 at 10:06
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    \$\begingroup\$ I have never heard of that. They will certainly cause RF emissions from the cables that carry the spark current. \$\endgroup\$ Commented Oct 3 at 10:10
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    \$\begingroup\$ A spark is a spark whatever the mechanism but, the level of spark voltage on your circuit would be nowhere near that from a Tesla coil. \$\endgroup\$ Commented Oct 3 at 10:13
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I do not know what happened but as I connected a smaller capacitor about 22 pF at the output in parallel with the 1 kOhm

1k load resistance at what output voltage and how much is the power level? If a boost converter is operated under no-load condition, the capacitor would continue to store energy and eventually blow-up. Further, the output capacitor value depends on the expected output voltage ripple and the switching frequency. You refer this application note.

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