# AC current on DC capacitor in a Tesla coil

I'm following online guides to build a basic spark gap Tesla coil. Almost all of them have an AC current transformer like an NST, but the capacitors they specify appear to be DC (or rather they don't mention if the capacitors are AC or DC but almost all that one can buy are DC.)

Is it possible to use AC current to charge a DC capacitor?

Don't we need a rectifier?

Spark gap Tesla coil example:

• what's a "DC capacitor"? Commented Apr 20, 2023 at 13:04
• these are rated for a DC bias, not a DC (direct current). By the very definition of what a capacitor is, no direct current passes through it. Commented Apr 20, 2023 at 13:09
• … that's not a "DC bias capacitor"; no such thing exists.. It's a capacitor. There's nothing special about it aside from the maximum DC offset being higher. Commented Apr 20, 2023 at 13:12
• there. is. no. such. thing. and what do you mean with "AC current " (alternating current current… double wording, by the way) "charges a capacitor"? By what AC is, half of the time it charges, half of the time it discharges. Else it wouldn't be "AC". Commented Apr 20, 2023 at 13:18
• I'm really not convinced you understand what a current is, and what a capacitor is, and how these relate. You definitely don't understand what a "charge" is and what AC is. Someone addressing all your misconception would have to write a longish introduction into these basic topics. I'm afraid that's a bit broad. Commented Apr 20, 2023 at 13:23

If you isolate one half-cycle of AC, that's DC, at least for the time durations relevant in charging C1 and discharging it through L1. A few ms later, the next half cycle comes along. The DC is now the other way round!

Most capacitors are specified to be able to take a DC voltage, whether they are 'DC' capacitors, polarised ones like electrolytics (which cannot be used in this application), or 'AC' capacitors, non-polarised ones. For a Tesla coil, you will want a very good quality capacitors, Cornell Dubilier 942s are the cap of choice for coilers.

That diagram you show is rather unusual for a Tesla coil. The spark gap is usually in parallel with the output of the transformer, exchange the positions of C1 and the spark gap. The reason for this is that when the spark gap fires, it goes short circuit, and protects the transformer from inadvertent high voltages from the coils which might break the transformer insulation over. But won't that short-circuit the transformer? Also not shown on your diagram is the usual primary ballast, or the leakage inductance in a suitable (Neon sign) transformer, that limits the current.

Capacitors rated for DC-only are constructed in such a way that they may not last long with frequent reversal i.e. AC.

It's not a consideration that comes up very often, but it's also the reason why the biggest pulse type capacitors (say Maxwell brand, the kind you move with a forklift!) are rated for rapid discharge (unipolar), or reversal (inductive load + ringing), and may provide derating or lifetime curves for such service.

Also in part, why AC ratings are often unexpectedly low, e.g. a 250VAC capacitor handles 350V peak, but might be rated 630VDC or more -- and not because of transients (non X/Y rated).

So the choice of an AC rated capacitor (or series stack) is recommended here.

More to the point, the capacitor in the schematic is also the pulse-discharge capacitor, which definitely will see reversal as well, and at quite high frequency at that (100s kHz to MHz), so needs to be rated for that most of all. Typically one would choose "snubber" or "high pulse" type film capacitors here. Or C0G ceramic, but those are probably even more expensive in these values.

At best, I suspect the suggestion of "DC" capacitors is only for cost. If you don't care that the capacitors heat up in operation, and fail rather quickly, that's perfectly fine. Some hours or days of lifetime would be utterly useless for a commercial application, but for a Tesla coil, it might experience that much total runtime in its entire existence.

My Mum worked for a vegetable pre-pack plant who used shed loads of cling film which came with thick and long cardboard tubes so, knowing nothing, I went and wound a Tesla secondary on one. Then nothing happened.

I'm not sure I understand it or perhaps the connections and reason for the spark gap. It appears to be two resonant circuits. Your Mains Transformer, with Leakage Inductance plus the Primary Capacitance and your Tesla Transformer with its Parasitic Capacitance and Leakage Inductance.

If you drive an LC tank at its Resonant Frequency the voltage in the tank, assuming the drive exceeds the losses, then the voltage across the tank builds up. Take a guess that if you manage to tune your Mains Transformer Leakage/Load Capacitance Resonant Frequency to your Tesla Transformer Leakage/Parasitic Capacitance Resonant Frequency you get a multiplication of the effect.

Take another guess that the nothing happens as these voltages build up until the spark gap breaks over. That will happen when the Load Capacitance hits peak voltage dumping serious current into your Tesla Transformers Primary and things go Sparky Sparky.

You need to rate your Primary Capacitor for the AC voltage it will see at resonance and at the point the spark gap breaks down and the resonant current it will see. Basically, someone else can tell me I am wrong, a DC, generally electrolytic, will not cut it even if you cunningly connect two of them in anti-series.

Then you need to find the right capacitor and you will also find no meaningful data unless you look for who bought up Siemens or Philips AN Other where they will give you lifetime, for applied voltage and frequency, and pulse rating graphs.

You also have to, once again my guess, match the Primary and Tesla Tank resonant frequencies to get a result or you could...

Go mad like these Mad People and implement a half bridge LC resonant mode converter to remove your Primary Transformer and drive your Tesla Transformer directly. I know not a lot about these things but it seems more likely that you might want to consider an LLC resonant mode converter with, adjustable, primary side current limiting.

That is a thought or possible fantasy but if the Mad Men, unfair comment, got theirs to work, it looks like the feedback method used, and you are Hard Enough and use your Search Engine Fu skills to find out about LLC converters you might also believe there is a Ghost of a Chance.

• Not thinking things through but your Tesla Transformer has a Primary Inductance, one part of an LLC converter, and a Leakage Inductance, the other part of an LLC converter, along with a Parasitic Capacitance, another part of an LLC converter. It depends on how things might transform. In particular the capacitance. If you can work out how to use it then FEMM femm.info/wiki/HomePage might be able to model the transformer to get values. Then you take a guess at the reflected capacitance and LTSpice it. Just floating lazy ideas.
– Per
Commented Apr 21, 2023 at 17:45
• Lloyd Dixon says ti.com/lit/ml/slup105/slup105.pdf on the subject of SEPIC converters. So, being stupid, you arrange things in your Tesla Transformer so the Primary is outside of the Secondary placing the leakage inductance on the Primary side. Which sounds like wot a Tesla Transformer is. Meaty big coil outside the base of a Many Winding coil. Then reduce it to Mad Ladz pancake version but make them concentric rather than stacked, easier to model in FEMM. Don't forget the insulation.
– Per
Commented Apr 21, 2023 at 18:16
• Then I go a bit vicarious, In order to avoid stuffing your primary side switches you need a decent amount of primary side inductance so you don't want one of those Tesla type primaries. You want a proper Tight and Long Solenoid, equations available. Then you slide it over your Secondary Tight and Long Solenoid, even easier in FEMM, with the appropriate insulation, so the leakage is in the primary. Plus you rate your secondary based on enamelled copper wire insulation rating according to grade for something like 500V per linear turn.
– Per
Commented Apr 21, 2023 at 18:44