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Registered Member #242
Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210
Well last semester turned out to be the semester from hell, but atleast its over. I was too busy working on an inverted pendulum for a final project. here is a video of it finally working. Most of the time was spent trying to get the H-bridge circuit to work. here is the breadboard. and here's the breadboard after some mosfet failure with no current limiting.
Right before i quit to work on the pendulum, I was able to get some 1" sparks from it using a battery charger for my supply. I set the break rate to very low and all of the florecent lights in the garage would flicker.
I might try to get some sparks out of it before school starts again, but it might have to wait until spring time. I might try to incorporate it into a school project
Registered Member #242
Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210
I've been thinking about the work remaining and I have a few questions.
First off, I need to make a strike rail. My plan is to use some 1/4" copper tube and mount it acouple inches up on the wooden form of the primary. Do I need a second strike rail on the outer side of the primary?
The powersupply will consist of a basic voltage doubler and Variac. I'm still trying to locate a suitable inductor to use for the choke. Any suggestions on where to look?
I'm going to solve my pulse transformer problem by switching to fiber optic. I'd have to buy some shielded cable and whatnot for the pulse transformers anyways, so might as well go for the fiber optic.
Another concern that I have is the coating of polyurethane on the secondary. I'm thinking that I might not have put enough on. Anyone have a problem from not coating the secondary enough?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
First off, I need to make a strike rail. My plan is to use some 1/4" copper tube and mount it acouple inches up on the wooden form of the primary. Do I need a second strike rail on the outer side of the primary?
I would say that you will want more than 1 strike rail, in fact, you may want more than 2. The output sparks of the tesla coil tend to follow a general path, arcing outward from the toroid, then bending back in near the base of the coil (where the primary is). So, assuming that sparks are coming in horizontally at your primary, you need to be effective at catching them before they hit the primary. Id say put one strike ring about 1" higher than the primary, but add about 3" of radius. Then do another strike ring at say 2/3 the height of the primary, but make its radius maybe 5" greater than the primary. Then maybe finally, a strike ring mounted right to your table at an even greater radius. They dont have to be copper rings... any metal will be good enough (so i dont suggest spending all that money unless you want copper for some reason). Even really small wire will work fine here, minding structural integrity.
I'm still trying to locate a suitable inductor to use for the choke. Any suggestions on where to look?
Do you have any idea what inductance you need? Pspice would probably help you figure that out since it doesnt seem (at first thought) like a nice simple problem to optimize by hand. Im thinking you could likely modify a MOT in some manner, probably seperating the E and I sections with some sort of gap (to increase the current it can handle before saturation, and to reduce the inductance). I think that any inductor you would use will have an air gap for energy storage (someone correct me if im wrong) so most of what you might find will need modification of the core.
Anyone have a problem from not coating the secondary enough?
Normally this isnt a problem unless you have problems with sparking between primary and secondary, or the secondary arcing back onto itself. The thick layer of poly doesnt *prevent* this, but can sometimes save the secondary from much damage. My large DRSSTC has seen a fair share of really nasty arcs between the coils, with no visible damage to the winding. I think i used something like 6-8 coats of poly varnish (it was the whole 1 quart can). So i wouldnt say its a necessity, but it could prevent you needing to do some sanding and refinishing if something bad happens.
Registered Member #242
Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210
Do you have any idea what inductance you need? Pspice would probably help you figure that out since it doesnt seem (at first thought) like a nice simple problem to optimize by hand. Im thinking you could likely modify a MOT in some manner, probably seperating the E and I sections with some sort of gap (to increase the current it can handle before saturation, and to reduce the inductance). I think that any inductor you would use will have an air gap for energy storage (someone correct me if im wrong) so most of what you might find will need modification of the core.
hmm...I've kinda stayed away from Pspice. Not quite sure how to simulate it, and also not quite sure what I would be looking for in the output. I guess its about time to try to figure it out. So...anyone have some Pspice models that I can use? I have a couple MOTs lying around that I can 'modify'. Now do I need to rewind it? I'm thinking I can just remove the HV winding and use the primary winding as is. Its 18 gauge wire BTW.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
Jrz126 wrote ...
I'm still trying to locate a suitable inductor to use for the choke. Any suggestions on where to look?
I`ve used (more or less modified) mot`s for the OLTC`s and OL-BRISG`s that I have made, and have microsim models that will predict a good size. If you tell me the size of the primary cap, as well as the bus voltage, I`l be happy to run the numbers for you.
Registered Member #242
Joined: Thu Feb 23 2006, 11:37PM
Location: Erie PA
Posts: 210
Well I got the MOT apart. The angle grinder made quick work out of it. I also welded a bead across each side to prevent the laminations from coming apart. What frequency should I try to measure the L at?
My primary cap is 12.92uF, and it'll be running off ~600V. Maybe alittle less, my diodes are only rated for 1200V each.
Well I have to say that I am quite modivated to get this thing working now. I figured I'd be up to my waist in snow by now, so I really wouldnt be able to test it too much in the garage. But the weather is 50*F+ and sunny, so I might as well try it.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The inductor value isn't too critical. I just took an MOT, sawed the core apart, got rid of the HV winding, clamped the core back together with a 1/8" piece of plexi to set the airgap, and used that. Of course our MOTs are 240V and yours are 120, so you'd end up with one quarter of the inductance. Maybe you could fit two MOT primaries onto the same core, or use a couple of units in series? Or maybe it would work with 1/4 of the inductance, I don't know.
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OLTC project
