MMC where to buy

[billybob]

Does anyone know of a good website where I can buy some MMCs for my arising tesla coil?

 

[DerStrom8]

The beauty of MMCs are that they're made up of a number of small, common capacitors available from various sources. There is not a particular supplier for "MMCs".

Try searching for the 942C series capacitors from Cornell Dubilier. They are one of the favorites among the Tesla Coiling community, particularly the 0.15uF 2kVDC variant (500VAC). You can buy these from regular distributors such as Digikey, Mouser, Verical, TTI, etc., as well as places like ebay.

942C20P15K-F | Octopart

Find 942C20P15K-F on Octopart: the fastest source for datasheets, pricing, specs and availability.

octopart.com

 

[billybob]

Capacitor K41-1A PIO 4kV 4000V 0.1uF Military Tesla Coil Soviet Ussr TESTED NEW | eBay

Sealed housing. Insulation resistance at Сr <0.25 μF >20000 MOhm. and ripple current circuits and in pulse. Weight ~330g.

www.ebay.com

just found these. Would 4 or five of these in parallel work?

 

[DerStrom8]

Those caps are only rated for 4kV and 0.1uF capacitance. Did you ever calculate what capacitance your bank requires?

If so, use the MMC calculator here to determine how many caps in series/parallel you'll require to meet your capacitance and voltage requirements:

https://deepfriedneon.com/tesla_frame6.html

 

[billybob]

Those caps are only rated for 4kV and 0.1uF capacitance. Did you ever calculate what capacitance your bank requires?

If so, use the MMC calculator here to determine how many caps in series/parallel you'll require to meet your capacitance and voltage requirements:

https://deepfriedneon.com/tesla_frame6.html

I thought that larger capacitance would be ok, since the caps don't charge to that point anyway and just discharges through the spark gap. What am I missing?

 

[DerStrom8]

I thought that larger capacitance would be ok, since the caps don't charge to that point anyway and just discharges through the spark gap. What am I missing?

Impedance matching to the transformer, and most importantly the resonant frequency of the primary circuit. Higher capacitance will dramatically decrease the resonant frequency and will take your coil out of tune. You can only take it so far out of tune before no tapping of the primary will be able to overcome the effect.

There are lots of "moving parts" at play that all need to be taken into account when designing a Tesla coil. Lots of variables that need to play nicely with each other in order to get the coil to operate properly. This is one thing that many people (myself included) tend not to realize when building their first TC.

 

[billybob]

Impedance matching to the transformer, and most importantly the resonant frequency of the primary circuit. Higher capacitance will dramatically decrease the resonant frequency and will take your coil out of tune. You can only take it so far out of tune before no tapping of the primary will be able to overcome the effect.

There are lots of "moving parts" at play that all need to be taken into account when designing a Tesla coil. Lots of variables that need to play nicely with each other in order to get the coil to operate properly. This is one thing that many people (myself included) tend not to realize when building their first TC.

Ah ok, how much room do I have then? can I increase the capacitance by a factor of 0.1 uf or not even that?

 

[DerStrom8]

You'll have to do the math for calculating the resonant frequency. The formula is:

where f is the resonant frequency in Hertz, L is the inductance of your coil in Henrys, and C is the capacitance of your MMC in Farads. How much of an effect changing the capacitance has depends on the inductance of your primary coil. And remember, the Fres of your primary circuit needs to match as closely as possible to that of your secondary circuit in order for the TC to be in tune.

 

[billybob]

If the calculator suggests 0.0353 would 0.0375 be ok for the MC bank or does the rating need to be closer for resonance?

 

[DerStrom8]

I would suspect the difference of 2.2nH will be acceptable. Tapping the primary a bit shorter should be able to account for any variance in frequency caused by that additional 2.2nH.

Where did you get 0.0353uF though? Are you still planning on the 9/60 NST? The capacitance value to match the impedance would be 17.6nF, not 35.3nF.

 

[billybob]

I would suspect the difference of 2.2nH will be acceptable. Tapping the primary a bit shorter should be able to account for any variance in frequency caused by that additional 2.2nH.

Where did you get 0.0353uF though? Are you still planning on the 9/60 NST? The capacitance value to match the impedance would be 17.6nF, not 35.3nF.

Yes, I was also seeing how it would look if I phased another nst. But the price the MMcs are looking I don‘t think that’ll happen

 

[billybob]

If I do end up adding a transformer to it, would I be able to just add a few more MMCs until the resonant matches or is this a one and done thing?

 

[DerStrom8]

I'm not sure I understand the question? You can add or remove capacitors as needed. Just remember that caps in series divide the capacitance but add the voltage, caps in parallel add up the capacitance but voltage rating stays the same. Your voltage rating of your MMC should be a MINIMUM of twice the transformer voltage (18kV in your case), preferably 3 times.

 

[shortbus=]

Have you looked at what these guys have to sell? They usually have the best selection and prices.

Surplus & Sales of Nebraska

Where the hard to find electronic parts are found and onhand. Great selection and prices on motors, transformers, capacitors, resistors, switches, power supplies, cases, and more for the hobbyist, makers, builders, scientists, ham radio enthusiast. New old stock and manufacturer overstock.

www.surplussales.com

 

[billybob]

Have you looked at what these guys have to sell? They usually have the best selection and prices.

Surplus & Sales of Nebraska

Where the hard to find electronic parts are found and onhand. Great selection and prices on motors, transformers, capacitors, resistors, switches, power supplies, cases, and more for the hobbyist, makers, builders, scientists, ham radio enthusiast. New old stock and manufacturer overstock.

www.surplussales.com

Holy cow! These guys do have a lot! Awesome!

 

[billybob]

Do I go by the Ac voltage rating of the capacitors Im looking at the 942C series and according to the deepfriedneon calculator I would need 324 capacitors! What am I doing wrong?

 

[billybob]

If I calculate by the Dc voltage rating I get, 16k volts at 0.0375uf. I need 0.0353uf for resonant frequency but I think the difference is nothing the tap cant fix. Yet this is DC I don't have a rectifier.

 

[DerStrom8]

Do I go by the Ac voltage rating of the capacitors Im looking at the 942C series and according to the deepfriedneon calculator I would need 324 capacitors! What am I doing wrong?

That is often the case - it sometimes takes too many of a certain type of capacitor for it to be practical, so you will need to continue your search for acceptable caps.

If I calculate by the Dc voltage rating I get, 16k volts at 0.0375uf. I need 0.0353uf for resonant frequency but I think the difference is nothing the tap cant fix. Yet this is DC I don't have a rectifier.

ALWAYS go by the AC rating. Even in "DC Tesla Coils", the resonant circuit is, by definition, an AC circuit.

Again, are you sure you need 35.3nF? I thought you had decided to go with the one transformer?

 

[billybob]

That is often the case - it sometimes takes too many of a certain type of capacitor for it to be practical, so you will need to continue your search for acceptable caps.

ALWAYS go by the AC rating. Even in "DC Tesla Coils", the resonant circuit is, by definition, an AC circuit.

Again, are you sure you need 35.3nF? I thought you had decided to go with the one transformer?

That's kinda what I thought. The thing that gets me is I've seen people on youtube and such making a tesla coil without a seemingly large cap bank from MMCs.

 

[DerStrom8]

That's kinda what I thought. The thing that gets me is I've seen people on youtube and such making a tesla coil without a seemingly large cap bank from MMCs.

That's usually because they find just the right capacitor to make up their resonant tank cap which is convenient. This is definitely the most difficult part for me when I'm building a Tesla coil - finding a resonant capacitor. It's still difficult for me to this day.