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Splitting MMC cap on DRSSTC

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Mark Dinsmore

Mon Oct 02 2006, 11:21PM

Registered Member #484 Joined: Thu Jul 06 2006, 07:29PM
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Posts: 4

Hi,

I'm just about to start cleaning up my coil, as everything is working well now. I was thinking about splitting the MMC and putting each half between the primary coil and the drive electronics. This would have the advantage of DC and low frequency isolation of line voltages to the primary, reducing the shock hazard. The disadvantage would be that both ends of the primary would have significant rf voltage on them. Has anyone done this, or is there any reason why I should not try this?

Thanks

Mark

Terry Fritz

Tue Oct 03 2006, 01:15AM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
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Posts: 297

Hi,

It is unusal, but I can't think right off of any big problems with it. It's "sposta work fine"...

Cheers,

Terry

HV Enthusiast

Tue Oct 03 2006, 01:16AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Splitting the MMC would work, but there is no reason to really do this. During operation, you aren't going to be anywhere near the coil anyways, and you'll get hit first by the secondary when the bridge is operating. Also, assuming a shock from the primary (if its only RF voltage) is more safe than DC or the line voltage is a poor assumption.

Steve Ward

Tue Oct 03 2006, 01:50AM

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

Also, assuming a shock from the primary (if its only RF voltage) is more safe than DC or the line voltage is a poor assumption.

Yeah, but if he can still get rid of the hazzard of DC (which is pretty real, if say 1 device failed and your filter caps still had some energy) then why not? I dont think he was assuming an RF shock is "ok".

My first DRSSTC used a split tank cap, simply for wiring reasons. It works just fine, after all, its just a series circuit either way.

Sulaiman

Tue Oct 03 2006, 02:35AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

If each half-MMC has 'balancing' or discharging resistors I'd expect no problems,
NST SGTCs commonly use split capacitors for symetry about earth.

With no bleeding resistors I'd expect a dc buildup due to corona/ozone/ions
which could overvolt the capacitors.

Mark Dinsmore

Tue Oct 03 2006, 12:10PM

Registered Member #484 Joined: Thu Jul 06 2006, 07:29PM
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Posts: 4

Yes, Steve, I was thinking mostly of the energy stored in the filter caps. I have about 130 joules in mine at full charge, and I really don't want that potentially exposed on the primary windings.

I have balancing resistors on my caps already; it doesn't take much of an imbalance to pop the caps.

Thanks again for your replies,

Mark

HV Enthusiast

Tue Oct 03 2006, 12:36PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
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Posts: 3068

wrote ...

Yes, Steve, I was thinking mostly of the energy stored in the filter caps. I have about 130 joules in mine at full charge, and I really don't want that potentially exposed on the primary windings.

From a safety design standpoint, that is still not a good idea as you are relying on the capacitor to provide the mechanism for safety.

If you are truly interested in safety, then build a plexiglas shield over the primary windings. This would ensure no one makes contact with the windings. Also, a shorting switch on the capacitor bank is a must if you are interested in safety. This is basically a relay and a low value resistor so when the system is turned OFF or a fault condition occurs, the relay trips immediately shorting out the capacitor bank, and at the same time disconnecting the bank from the line. (Use a DPDT relay)

Dan

Steve Ward

Tue Oct 03 2006, 10:15PM

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

Yeah, Dan is right. There is no replacement for "ultimate" safety features. My coils all use a relay to drain the main filters through a few hundred ohms when power is pulled.

As to voltage balancing... the exact same problem still exists if the caps are wired up all on one side of the primary. Like i said, its a series circuit, why should the order matter? I dont use any bleeder caps usually, but its probably not a bad idea to include some.

Mark Dinsmore

Wed Oct 04 2006, 12:13AM

Registered Member #484 Joined: Thu Jul 06 2006, 07:29PM
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Posts: 4

I agree that the ultimate in safety would be to build the discharging and error sensing circuitry you described, but that adds a significant amount of complexity to the system. Having a few tenths of a microfarad rated at 3200V between both sides of the primary and the line/filter caps reduces the potential energy and current faults from hundreds of joules and tens of amperes to a few millijoules and milliamperes of 60Hz current, both quite safe, with zero additional complexity. I just thought splitting the caps would add one layer of safety without a lot of effort.

BTW Steve, do you combine inrush current limiting with your discharge relay? It seems like that could be easily accomplished, but I haven't had time to flesh out the circuit yet. It's a bit complicated when you have a separate, switched variac driving the filter caps, but there should be some topology that works at all settings of the variac.

Thanks

Mark

Steve Conner

Wed Oct 04 2006, 01:38PM

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

I happily design Tesla coils with exposed live parts on the assumption that nobody would be dumb enough to go near it while it's powered. I don't think splitting the tank cap would give any advantage in terms of protecting people from electric shock, but it might limit the fault current in the event of a primary-to-secondary flashover. My DRSSTC burnt a hole in its secondary every time it flashed over, and is dismantled just now waiting on a rewind. :P

"Safety" and "Tesla coil" don't belong in the same sentence IMO. If you're still hell-bent on trying to make something inherently dangerous "safe", here are a few pointers:

Safety mechanisms should be as simple as possible: the more complex they are, the more likely they are to go wrong, and not fulfil their intended function while giving you a false sense of security. That means:

Bleeder resistors instead of (or in addition to) dump relay circuits for discharging capacitors. if the dump relay sticks open or the dump resistor burns out, you still need a way of discharging the bank. I got into the habit of using bleeder resistors on everything, even the tank capacitor of a Tesla coil. Obviously, if you do use a dump relay, it should be normally closed, so a loss of power automatically causes a dump (rather than leaving the bank undumpable)

Multiple redundant mechanisms. For instance, if you have a huge capacitor bank that would blow you into the middle of next week when touched, it wouldn't go wrong to have a bleeder resistor, AND a dump relay, AND an analog meter to show the bank voltage, AND a big red neon connected across the bank. (and don't connect them all to the bank through the same piece of wire, in case it falls off) Often, these things would be useful for other purposes besides safety, so it's not such a hassle as it seems to implement them.

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