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I’m building a coil-gun and I just got a 400 V 1.3 mF electrolytic capacitor which will discharge a large current. However, my coil is made of 1 mm^2 copper enamel wire which has an resistance of only 0.6 Ω and I am worried that if I constantly discharge my capacitor with only 0.6 Ω, it will soon break. Should I add a resistor or is it fine?

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    \$\begingroup\$ What does the datasheet say? If noting useful, what does the manufacturer say? \$\endgroup\$
    – winny
    Aug 8, 2023 at 9:19
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    \$\begingroup\$ What is the inductance of your coil, and what would be the current if you discharge your capacitor into the coil without adding any extra resistance? \$\endgroup\$ Aug 8, 2023 at 10:04

2 Answers 2

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Find out what the maximum peak discharge current is for the specific capacitor you are using and calculate the required resistance to stay below that current using R = V/I.

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A too-fast discharge will indeed damage a capacitor, but a not-fast-enough one will impact your functionality. (I am assuming that you want to have a short but powerful current burst through your coil)

Capacitors, like all components, can not withstand too high current (or discharge rate, both are the same in a capacitor). How high is too high exactly should either be directly referenced in the datasheet, or could be calculated.

Your main worry is that your capacitor has an internal resistance, causing it to heat up when current runs through it. The faster the discharge happens, the hotter it gets, and the less time that heat has to be dissipated.

Given your capacitor value, internal resistance, and coil inductance, you should be able to compute how much heat accumulates in your capacitor per cycle, from there, you can find out if your system is sound, how many cycles you can fire, and how long you should let it cool between cycles.

To minimize that internal resistance, there are two main axis :

  • Pick a capacitor technology with a low ESR (equivalent serie resistance, most electrolytic capacitors have a relatively high ESR)
  • Instead of a single capacitor, use an array of capacitors in parralel. that topology puts their ESR in parallel as well, creating less heat, and sharing it between multiple components.
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