OLTC
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OLTC (Off line Tesla Coil) Off line Tesla coil. Runs direct from the supply voltage and has no hv step up transformer. The
Primary coil 2 coils of 2
turns of 5mm copper tubeing . The Secondary 2100 turns 0.125mm enameled copper wire wound on a glass storagejar 4" in diameter. Drive 2 x IGBT's Output 12" Sparks from about 200W in.at 2Khz. BPS 100hz - 2khz Circuit Diagram (not complete and subject to change) The Second working OLTC in the UK see http://www.scopeboy.com Steve Connors site for the first (and probably the third by now) OLTC primary Calculator Web tools to help design an OLTC |
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Half way through winding the
secondary |
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Close up of the primary coil, caps and IGBT's |
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It works, It needs some work, but it works. |
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Some more playing, re-built the driver to overdrive the IGBT's as per origonal OLTE spec. Adjusted the height of the secondary to change the coupling. This ment that I could tune the coils more accurately. Much better output 3-4" into air , 6" long sparks to earth. |
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I Then I
upped the BPS to around 1Khz, It ran fine for a while, until an arc
started between the dequeueing diode and the transorb across the charging inductor. Whatever happened I have an IGBT , transorb and a diode to replace, so time for a minor rebuild and the addition of a small fuse in the charging circuit after the capacitor to prevent it from happening again. The Failure was caused by a trapped wire in my control box adding 50hz to the gate drive for the IGBT's, Hopefully this won't happen again. |
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After Various component replacing & problems I got it running again.Two things became obvious, The turn off of the igbt's must happen accurately at zero voltage otherwise the losses are large and the coupling from primary to secondary is very important. To adjust the coupling I added various sized "chocks" under the secondary until I got maximum output, The "on time" of the igbt's also needed adjustment as the coupling changed. I changed the variable for the "on time" adjustment to a 10 turn pot, this allowed for much more accurate setting. After reaching about 6" sparks (with running sparks down the coil) I decided that I still hadn't got the tuning right. By adding another topload I proved that the primary cap (1.88uf made from 4 x 0.47uf's) was too large, so I removed a 0.47uf and replaced this with 4 x 0.1uf's, (better) and then 3 x 0.1uf's even better. 2 x 0.1uf's did not improve the output so I stuck at 3 giving me a primary cap of 1.71uf. The picture to the left is running at 2khz and the arc is at least 12" to air. The noise from the arc is deafening. (See move further down) |
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12" spark to air |
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12.5" arc to earth |
| For the technical, two
traces. Top trace is from a scope probe about 1M away from the coil. Not actually connected, just the probe. The second trace is the voltage across the primary capacitor, note the charging to around 600V and the 2.5 cycles of oscilation before the transistor is turned off. (Only shows 2 cycles due to the triggering delay of my scope) ( |
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Click picture for a movie of My OLTC "testing" a computer mother board. |
NEXT ---> The AD-OLTC