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first magnifier test
Posted by alex on 7/12/2005, 8:55 am
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Re: first magnifier test
Posted by Bert Hickman on 7/12/2005, 11:35 am, in reply to "first magnifier test" Hi Alex, Congratulations on first light - that's a very nice looking system! As you're discovering, insulation is one of the most challenging aspects of when trying to achieve tight magnifier driver coupling. Just about everyone who has built a magnifier has struggled with similar problems (usually L1:L2 flashovers). Sparking between the secondary and primary would be very hot, bright, and white. Since your sparks aren't bright, they do sound very much like racing sparks. As you are probably aware, the detailed mechanism underlying racing sparks is still not very well understood. In classical two-coil systems, racing sparks appear when the coupling is too high relative to the intrinsic dielectric strength of the secondary. The usual fixes are to reduce coupling or to reduce the topload breakout voltage (using a breakout point) to reduce overall voltage stress seen by the secondary. Changing the secondary's insulation system or increasing its length may also help, but these changes are not usually recommended since the other fixes are so much easier. Following are some questions, some thoughts, and some things you might try. 1. What type of wire/insulation are you using on L2 and what is the physical length of the closewound and spacewound sections of L2? 2. Is the space between L1 and L2 tightly filled by the rolled Mylar? BTW, PET is indeed the same thing as Mylar. 3. Is the Mylar tightly rolled on top of L2 or is there an air gap between? Some thoughts and some things you may wish to try: During normal operation, subsequent voltage reversals on the secondary can further increase E-field stresses between the secondary and these nearby charged regions to the point where a surface spark can develop along the surface of the dielectric. This is sometimes called a guided spark. For a given voltage stress guided sparks propagating along a dielectric surface can be 3-5X longer than the distance in free air... and sometimes considerably longer. The surface spark is merely discharging excess stranded charge between regions of the dielectric, or between the dielectric and the winding. However, although they are initially electrostatic discharges, they may initiate followthrough RF discharges between portions of the secondary winding. Since these are lower energy sparks, they are relatively dim but they can cause progressive damage to your secondary and Mylar insulation. If you can prevent L1:L2 flashovers, the natural E=field grading across the single layer L2 winding and the E-field shielding from L2's toroid may be sufficient to prevent overvolting of the coil without the need for supplemental insulation. I suspect the root of your particular problem is the close presence of the Mylar insulation layer over L2. Unfortunately, it may be difficult to eliminate the insulation layer with your existing driver design. Some things you might want to try: 2. Increase the gap between the primary and secondary and eliminate the Mylar layer. Elevate L1 versus L2 (if necessary) to increase coupling to the desired level 3. If you need to provide supplemental L1:L2 insulation, try using layers of LDPE film or thin LDPE sheet material. LDPE's lower dielectric constant (k ~ 2.3) should also help to reduce E-field stressses within any air gap. 4. If changing the insulation system still doesn't fix the problem your compact primary design may, in fact, be introducing excessive transient stresses on the lower portion of the secondary (particularly during ringup). Most other successful magnifier designs have used helical primaries which may tend to more evenly bathe a larger portion of the secondary with primary flux. There has recently been some discussion of this (only educated speculation at this time) on the Pupman Tesla List. There is no clear evidence of this as yet. However, it would seem to make intuitive sense... Good luck and best regards, Bert
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Re: first magnifier test
Posted by alex on 7/12/2005, 2:31 pm, in reply to "Re: first magnifier test"
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Re: first magnifier test
Posted by Bert Hickman on 7/12/2005, 3:49 pm, in reply to "Re: first magnifier test" Glad to hear that getting the system in tune helped resolve the problem. I had made the assumption that the system was already in tune and that you were still seeing these problems. Please let us know your results under full power. Best regards, Bert --Previous Message--
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Re: first magnifier test
Posted by alex on 7/12/2005, 2:02 pm, in reply to "Re: first magnifier test" --Previous Message--
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