First build need help / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

First build need help

Move Thread

LAN_403

Ham549

Fri May 09 2014, 08:02PM

Registered Member #2403 Joined: Tue Sept 29 2009, 12:15AM
Location:
Posts: 4

After wanting to do so for a long time I have decided to build a Tesla coil and have started gathering the parts. Now the current parts I am wondering about is the capacitor bank I have read one tutorial that said you needed 40 942C20P15K-F capacitor and at over $5.00 a pop that seems very pricey. I have also seen on ebay this

"20kV 0.01uF High Voltage Metallized Polypropylene Film Capacitor" two of these in series I would think could handle outputs of up to 16KV and is a lot cheaper than 40 of the other kind.

Graham Armitage

Fri May 09 2014, 08:30PM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

Hi and welcome. if you are just experimenting and learning about what makes a TC tick, I would not drop a ton of money onto an experimental project if you cannot afford it. I started off with my Teslas by building glass and aluminum foil caps. Some of my best performing spark gap coils used home-made polyethylene and foil rolled caps immersed in oil. I think the problem with your suggested caps is that in series you are looking at 5nF which could be kinda low. Depends on what size coil you are trying to build, is it a SG coil and if so powered by what transformer - you want to match the transformer output impedance to the capacitor impedance for max power transfer. lots of good info around on doing these calculations.

Ham549

Fri May 09 2014, 09:46PM

Registered Member #2403 Joined: Tue Sept 29 2009, 12:15AM
Location:
Posts: 4

I have a neon sign transformer that is probably around 9kV, I was thinking of spark gap but may want to switch to something more efficient. I am also keeping an eye out for a 16kV transformer for future use.

Graham Armitage

Sun May 11 2014, 01:14PM

Registered Member #6038 Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248

I have had great success with an air-quenched static spark gap on NSTs. have blown up some NSTs using rotary gaps, so avoid them. be sure to put a safety gap across the NST - voltage rises in the primary circuit can easily fry your transformer.

Eleccentric

Sun May 11 2014, 08:29PM

Registered Member #33460 Joined: Tue Aug 27 2013, 06:23PM
Location: Seattle
Posts: 46

It is a good idea to start with good old salt water capacitors at first, so you can spend your time and money optimizing the rest of the system. Once your spark gap and coil are working well, you can drop in a better capacitor without changing anything else.

Start off with 4 or 5 wine bottles, each will have about 1nF, or 8-10 beer bottles, which will each have about 500pF, if memory serves. It has been many years since I was building my first Tesla coils.

GrantX

Sat May 17 2014, 06:30AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Ham549 wrote ...

After wanting to do so for a long time I have decided to build a Tesla coil and have started gathering the parts. Now the current parts I am wondering about is the capacitor bank I have read one tutorial that said you needed 40 942C20P15K-F capacitor and at over $5.00 a pop that seems very pricey. I have also seen on ebay this

"20kV 0.01uF High Voltage Metallized Polypropylene Film Capacitor" two of these in series I would think could handle outputs of up to 16KV and is a lot cheaper than 40 of the other kind.

On Ebay there are cheap Chinese polypropylene capacitors that come up when you search "snubber capacitor", or "high frequency pulse capacitor" or a mix of those terms. Here's an example (although these ones are pricier than what I used): Link to Ebay. As you can see, they claim to be rated at 9A RMS and 130A peak pulse, and a max dV/dt of 130V/us (honestly not sure how good that dV/dt is in relation to Tesla coil duty).

I don't think they have anywhere near the same quality as the CDE caps, but they still work very well, and since they cost a bit less than the CDE 942's you can build the MMC with higher safety margins to compensate for the slightly weaker caps. I've been using a string of 12 of them for about a year now in a tiny 120VA SGTC without any noticeable degradation.

Ham549

Tue May 20 2014, 02:12AM

Registered Member #2403 Joined: Tue Sept 29 2009, 12:15AM
Location:
Posts: 4

Okay I seem to be reading people saying the capacitor builds up a charge to jump the gap but this goes agenest what I know about electricity. What I think is happening is the circuit is already at a set voltage that is enough to jump the spark gap on it's own, however since there is a capacitor there this provides an easer path for the power to flow so it doesn't jump the gap. However once the capacitor is fully charged the power has no where to go but the gap so it jumps the gap which at the same time discharges the capacitor starting the cycle over again. If what I am saying is correct if the capacitor was removed then the power would just continuously ark over the spark gap. Am I right?

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob