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Registered Member #2416
Joined: Sun Oct 04 2009, 04:23AM
Location: Oceanside, CA
Posts: 91
Well I've found a supplier of the CDEs for 3 bucks a pop, but there is a minimum order of 50. : /
I suppose I could buy the lot and sell the renaming and make a little profit. Anyways I'm definitely going to completely remake my capacitor bank. I really do want to use the CDEs but... we'll see. That will have to take a back seat for now.
Anyways, conveniently, the non LTR capacitor bank for my 12/60 Tesla coil is exactly equivalent to a LTR bank for my 9/30 Tesla coil, if it worked for a 12/60, it'll surly work for a 9/30. I've replaced the mylar film bank on my mini tesla coil. That bank also worked quite well, despite what I've read here. But I was having insulation problems. I packed it a little too tight and because of it I had to close the gap on my spark gap a little. At best I could get 10in streamers. As usually I got a bit ahead of myself with the layout of the caps.
But you see, I've never blown a cap with either of my coils, I've never damaged an NST either. Neither of the banks were 2.5 times the RMS voltage of the NSTs or LTR. I guess that's why I'm a little reluctant to think that CDEs are the only way to go. But I acknowledge you guy's superior knowledge and I'll take your word for it.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Tony -
Some people have never experienced a failed NST or a tank capacitor in Tesla coil service; however, many, many coilers have. To get a better understanding of the HV, HF transients present in then tank circuit of a spark-gap Tesla coil, read some of Terry Fritz' white papers that led to the development of the "Terry filter".
Having a cap with a "DC" rating of 2.5X the NST RMS output voltage is recommended as a very conservative MMC design. MMCs have been built and run with the DC voltage rating equal to the peak voltage rating of the NST (1.414 X RMS voltage), but the probability of catastrophic failure is much higher than with a more conservative design.
Regardless, non-GFI NSTs are getting harder to find, and more expensive, especially the 60ma and 120ma models. Anhything you can do to minimize the possiblity of transformer death is probably worthwhile.
The C-D 942C series caps are well proven in Tesla coil service, and have become the default "standard" for MMC use over the past 6 or 8 years. Their survival is attributed to:
1. Their very high peak current rating, due to the use of metal foil as the end electrodes; 2. Their "self-healing" "hybrid" design, where solid metal end foils are combined with a "floating" centeral electrode of metallized polypropylene film. If a dielectric puncture occurs due to overvoltage (that would be fatal with other cap designs), a tiny circle of aluminum is vaporized from the metallized film plate, surrounding the puncture. This is how the "self-healing" function works. A capacitor can experience thousands of dielectric punctures, and the only effect is a slight decrease in capacitance value due to the loss of plate area.
So, yes, there really is something special about the C-D 942C series caps that makes them especially suitable for TC use.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
hi Xplorer, don't know if you have found the caps you wanted or not but you may consider using a cap like this for your SGTC.
you may have to reduce the inductance of your primary or put a big fat top load to reduce your secondary frequency. I have seen a picture using this capacitor and 4 MOTS that had 5 ft sparks. the advantage of this cap is that has the voltage rateing to survive your NST's and it is low inductance. In the end you want a cap that has a low ESR and hi DI/DT like the CDE caps. Just remember to use a inductor on each terminal with a safty gap to protect your transformers. the shape of the inductor I would use is a home made inductor of 26awg on a 1" diamiter PVC for 6 to 8" in length. remember more power = bigger spark
Registered Member #2416
Joined: Sun Oct 04 2009, 04:23AM
Location: Oceanside, CA
Posts: 91
Herr Zapp wrote ...
Tony -
Some people have never experienced a failed NST or a tank capacitor in Tesla coil service; however, many, many coilers have. To get a better understanding of the HV, HF transients present in then tank circuit of a spark-gap Tesla coil, read some of Terry Fritz' white papers that led to the development of the "Terry filter".
Having a cap with a "DC" rating of 2.5X the NST RMS output voltage is recommended as a very conservative MMC design. MMCs have been built and run with the DC voltage rating equal to the peak voltage rating of the NST (1.414 X RMS voltage), but the probability of catastrophic failure is much higher than with a more conservative design.
Regardless, non-GFI NSTs are getting harder to find, and more expensive, especially the 60ma and 120ma models. Anhything you can do to minimize the possiblity of transformer death is probably worthwhile.
The C-D 942C series caps are well proven in Tesla coil service, and have become the default "standard" for MMC use over the past 6 or 8 years. Their survival is attributed to:
1. Their very high peak current rating, due to the use of metal foil as the end electrodes; 2. Their "self-healing" "hybrid" design, where solid metal end foils are combined with a "floating" centeral electrode of metallized polypropylene film. If a dielectric puncture occurs due to overvoltage (that would be fatal with other cap designs), a tiny circle of aluminum is vaporized from the metallized film plate, surrounding the puncture. This is how the "self-healing" function works. A capacitor can experience thousands of dielectric punctures, and the only effect is a slight decrease in capacitance value due to the loss of plate area.
So, yes, there really is something special about the C-D 942C series caps that makes them especially suitable for TC use.
Herr Zapp
Thanks for the explanation Zapp, I'm learning more and more everyday. I really appreciate it.
teravolt wrote ...
hi Xplorer, don't know if you have found the caps you wanted or not but you may consider using a cap like this for your SGTC.
you may have to reduce the inductance of your primary or put a big fat top load to reduce your secondary frequency. I have seen a picture using this capacitor and 4 MOTS that had 5 ft sparks. the advantage of this cap is that has the voltage rateing to survive your NST's and it is low inductance. In the end you want a cap that has a low ESR and hi DI/DT like the CDE caps. Just remember to use a inductor on each terminal with a safty gap to protect your transformers. the shape of the inductor I would use is a home made inductor of 26awg on a 1" diamiter PVC for 6 to 8" in length. remember more power = bigger spark
I'll bookmark that capacitor. But wouldn't that be too large? The math says I need .02uf for a LTR capacitor bank. I guess my question is how important is it that the capacitor values are spot on?
Registered Member #2416
Joined: Sun Oct 04 2009, 04:23AM
Location: Oceanside, CA
Posts: 91
Well Murphy's Law strikes again. I was just saying that I've never blown a capacitor and two of those Philips caps blew on me. I was able to get about 12in streamers for like 20 seconds before they popped, and my gap wasn't even set as wide as it could go. :/
I've ordered 11 CDEs for it from EVR. Now that I'll have a decent capacitor I'll be able to widen the SG and get some decent sized arcs.
Registered Member #480
Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644
Xplorer -
Can you post a good close-up photo of the failed caps? Did they fail at the end connections, where the leads are attached to the ends of the capacitor "roll"? Or did they fail in the center, with the lead attachments intact?
Failures at the ends of the roll, where the leads attach, is frequently an indication of overcurrent.
Failures at locations other than the ends may be an indication of dielectric punctures caused by overvoltage, or breakdown caused by dielectric overheating.
What is the part number of your Phillips capacitors? Do they use polyester or polypropylene dielectric?
Registered Member #2416
Joined: Sun Oct 04 2009, 04:23AM
Location: Oceanside, CA
Posts: 91
Sure.
They popped on the side. It began to bulge and then some burnt plastic and smoke came out. They simply weren't able to take the voltage. Before I widened the gap they were fine.
I got the idea to use them from Herb's Bonsai TC.
I don't think they are the same, but they're similar.
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New Tesla Coil - Capacitor advice
