Are MMC's really all that great? / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

Are MMC's really all that great?

Move Thread

LAN_403

dan

Wed Aug 02 2006, 05:42AM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

I mean salt water capacitors aren't all that bad, are they? They are cheap/free and easy to construct. They are apparently a bit lossy and they are heavy. But I don't really think a MMC is much better when it comes to losses though.

I just had a quick thought about dissipation factor (DF) of a dielectric. Isn't the amount of energy lost through DF relative to the dielectrics surface area? So correct me if I'm wrong but a MMC is usually built with a bunch of poly caps. All together the poly caps would have a huge surface area. Glass since it has a much higher dielectric constant would only require a fraction of the surface area. So in the end wouldn't the losses be the same or at least fairly close? Plus once you factor in the bleeders/equalizing resistors it would seem to me that MMC's aren't all what they are cracked up to be.

Now this is just my two cents but I think the main reason why coilers are moving away from salt water capacitors because MMC's are lighter and don't leak salt water if they suffer dielectric break down. (Plus it's all about fitting in with the crowd

, "If everyone else has a MMC then I need one too" sort of thing). But I guess I shouldn't be saying anything because I can't afford $200-300+ on all the caps I would need to make a MMC for my coil. So I don't really know the difference.

I'm not trying to put anyone down or anything. Hey if I could afford to blow a couple hundred bucks on a MMC I would. It's just that I keep hearing all this praise that MMC's are far less lossy then bottle caps just because their dielectric has a lower DF but I never seen the maths comparing the two.

Steve Ward

Wed Aug 02 2006, 08:42AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Isn't the amount of energy lost through DF relative to the dielectrics surface area? So correct me if I'm wrong but a MMC is usually built with a bunch of poly caps. All together the poly caps would have a huge surface area. Glass since it has a much higher dielectric constant would only require a fraction of the surface area.

No, not surface area, but volume and type of material. Im not sure, but the DF for glass is far worse than PP. PP exhibits *excellent* low DF for these frequencies. You can liken DF to hysterises loss in magnetic materials. There is some loss of energy every time you charge and change the charge on the capacitor (just like a change in magnetic flux causes core losses). I dont know if that analogy helps at all, but its basically dependent on the material at hand, and the amount of "power" processed.

But I guess I shouldn't be saying anything because I can't afford $200-300+ on all the caps I would need to make a MMC for my coil. So I don't really know the difference.

Almost everyone i know who has used both salt-water caps and MMCs notice a remarkable increase in performance. MMCs offer lower ESR, which is the big thing to reducing the tank circuit losses. I have seen only a few coils that actually perform well with bottle caps, and that was on Greg Hunters site, but the amount of power he had to put in was a few times more than should have been required for the performance.

So in the end, i have to say that yes, MMCs are really all that great... and so are other real pulse capacitors, but most of them will cost much more than a MMC.

Steve Conner

Wed Aug 02 2006, 10:15AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Meh. MMCs really are great

They don't leak salt water for a start. And their plates are made out of metal, which owns salt water from an ESR point of view.

I don't think they're too expensive either. They maybe cost double what it would to buy an equivalent amount of bottles of beer

My first MMC cost about $80 to build and delivered 40" sparks. My next one was made of about 20 1uF 1000v caps (total cost about $80 again) and got me 80".

Then I went on to some 10kV 0.05uF commercial pulse caps that I bought used for $40 each, which was just slightly more expensive than the same number of joules in MMC form.

dan

Wed Aug 02 2006, 01:11PM

Registered Member #223 Joined: Mon Feb 20 2006, 06:42PM
Location: Ottawa, Ontario, Canada
Posts: 125

Well thanks for clearing that up. I know MMC's have their advantages over salt water capacitors. It's just seems that there is a lot of conflicting/missing information about DF in capacitors out there on other websites. [Plus I just got back from a night of drinking and was just starting to sober up when I thought of the above and posted it; So I was pro beer everything at that moment. In other words kiddies "Don't drink and post"

]

Well bottle caps can have at least one or both plates made of metal. Like mayo jars have a wide enough opening to get your hands in there to glue and smooth out the foil. This would reduce the ESR from a few ohms to almost nil. However using 11 carona beer bottles as my tank capacitor I noticed absolutely no heating even after a 30min run. So they can't be dissipating that much energy. Maybe a few watts spread out over the whole bank. But the same is probably true about the bleeders across all the capacitors in a MMC is it not? Without measuring the tempratures before and after there is no way to be sure how much energy is being lost. Maybe I should stick a glass thermomitor in one of the center bottles and see how much it goes up.

I know I have heard people having a 10% or so improvement after switching to a MMC. I have spent some time tuning my coil with salt water caps and it's getting very close to the max output for my power input.. (26'' with 360VA in, only 6'' to go!). Probably a MMC would put me all the way there. I know most people who use salt water capacitors don't have a means of measuring capacitance and only calculate it. (me) All bottles aren't created equal so the estimated capacitance can be off by a huge margin. So when you drop in a MMC which you know within a good tolerance what the capacitance is going to be thus making fine tuning a little easier.

The biggest culprit of losses is the spark gap it self. I mean the noise, heat, and light coming off of it represents huge losses. Way more then the tank cap which is probably why everyone is going solid state.

Coiling is just a hobby for me and I generally perfer to use what I have avalible to me. I don't really like ordering anything from the states because customs duties and taxes are insane! All the local electronics stores here have rather inflated prices execpt for a few items. Like 555's, 2n3055's, tl494's, lm741's, etc. So I end up pulling most of my electronic stuff out of the trash. Its amazing what the local neon sign shop throws away

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