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Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Marco,

At 01:36 AM 4/21/2005, you wrote:
Hello Solid State guys,

Please excuse my naive question, but what is the difference between OLTC
(Off Line TC?) and DRSSTC (Double Resonance Solid State TC?)?

Aren't they both running off-line, with an half or full-bridge drive,
IGBTs or FETs, feeding directly the primary?

The OLTC charges a big capacitor directly off AC line voltage. For 120VAC input, that is 170V By pulsing at 120Hz with a line inductor, you can resonant that to ~340V. Many use a big DC power supplies to charge the primary cap too. But the voltage is low so the IGBTs can handle it.

The key to the OLTC is that the primary cap is charged to a set voltage at say 300V. Then it is discharged though a very low inductance primary (often less than 1uH!) to make enormous primary currents. Even with just one turn on the primary, the coupling is still very good. So rather than using high voltage to force big primary currents, the OLTC uses low voltage but into a very low inductance to produce similar high primary currents. The current is usually higher but does not last as long. The OLTC uses big IGBTS or ten small ones (me) to switch the high current. Reverse diodes complete the circuit during the other half of the cycle.

The DRSSTC's primary, secondary, and primary cap are all very normal. But the gap and charging circuit, are replaced by a high current H-bridge that feeds the primary inductor and capacitor with a +- 300V square wave at the Fo frequency at say 500 amps. That rings the primary voltages up to almost normal say 15000 Volt levels. Even though the primary inductor and cap voltages are high, the low impedance ground based driver voltages stay at about +-340V so it is just almost normal IGBT switching electronics.

Drive timing comes from current, voltage or whatever feedback, with
mixed-logic or microcontroller aid, right?

We let the H-bridge timing be run off the primary current. A natural LC "tuning fork". Basically just a circuit to detect which direction the primary current is going and pump more current into it. It automatically runs at the primary systems Fo frequency. The bridge switches to the opposite direction whenever the primary current naturally reverses. There are also systems that use the secondary voltage picked up by an antenna to do basically the same thing. But primary current timing seems to be gaining favor.

So besides the flexibility of drive timing, what are the main
differences (circuital and performance) between them?

The old "gap losses" are gone!! The drive weighs 20 pounds. The whole coil system weighs 35 pounds!! It seems about 50% more efficient. It can run continuously "forever". There are no moving parts or parts that wear out. Drive timing can eliminate the need for a variac.

The OLTC is a bit more reliable and forgiving, but the DRSSTC easily out performs it. I think OLTCs are at 6 foot and the DRSSTC are at 13 foot streamers!! These are not the old wimpy solid state machines at all!!! The DRSSTC can hold its own against any spark gap coil and the OLTC is frighteningly fierce!!

The OLTC has fairly high losses in it's high current low voltage primary system and it's power is fixed for each bang. Thus, it's performance suffers. The DRSSTC's primary system has lower loss due the high voltage and higher impedances and can continually supply energy during the bang. Thus, it runs VERY well!!!

Unlike the OLTC, DRSSTC's are well documented :o))


No two people seem to have made very similar DRSSTCs so far! There is a lot yet to be figured out!! The DRSSTC's firing timing can be controlled from uS to uS for super high control. The possibilities there are yet to be dreamed of!!

If your want to build one. The OLTC is technically simpler, but the DRSSTC is far better if you can do the more complex electronics. In your case, that would be no problem.



I'd really like to see a quick recap about these two solutions.