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Registered Member #687
Joined: Mon May 07 2007, 02:21AM
Location:
Posts: 14
Hello, I am Ben. I have been building coils for several years. Though I would share with the group my most recent coil. It is a DC Resonant design powered by a single 1kW MOT and full wave voltage doubler. Spark gap is a asynchronous (naturally) rotary. I am most proud in that a majority of the components were salvaged from dead FM broadcast transmitter carcasses. An example of this is the large solid state rectifier tube replacements that were just begging to be used.
I intended this coil to be more of a furniture / conversation piece than anything, hence the turn-of-the-century quack medicine machine look.
Registered Member #687
Joined: Mon May 07 2007, 02:21AM
Location:
Posts: 14
Here is a couple more views of the "business end" of the machine. The close up is of the "de-q-ing" diode, with 8H charging reactor in the background, and to the side the filter network I am using. The diode appears to be a tube replacement assembly that Wilkenson Electronics sold to there customers to retrofit old transmitters.
The second view is an of the entire contents of the machine. You can see my original glass plate cap (0.01uF). I have since started using 2 0.05uF Dearborn plastic caps in series giving 0.025uF. This doubled the spark output, but the caps get somewhat hot. Probably cant handle the current.
This was my first DC coil, and I still have some things to finalize in the construction. One question I have is placement of the filter network in the circuit. I have it between the power supply and the charging reactor. Is this correct, or should I place it between the "de-Q-ing" diode and the tank? Other circuits I have seen have it placed where I have it, but it seems to me that the charging reactor needs some protection from RF. Thoughts?
Second question I have is regarding the "de-Q-ing" diode assembly I am using it is a string of probably 24 1000V 1A diodes in series with small ceramic disc caps in parallel with the diodes. I understand the purpose of the caps, but these also have small neon lamps in parallel with the diode as well. What is the purpose of this, other than looking neat when the current flows?
In case anyone was wondering, I am not a woodworker, I just happened upon a cheap old tube radio cabinet that let it self well to TC construction.
Registered Member #118
Joined: Fri Feb 10 2006, 05:35AM
Location: Woodridge, Illinois, USA
Posts: 72
The purpose of the neon lamps (and associated series resistors) is to show, by unlit lamps, if any of the rectifiers have short circuited so that they can be quickly identified and replaced. Suppficient margin is provided by the chain so that a shorted diode or two won't overvolt and cause cascade failures of the rest of the chain.
Registered Member #687
Joined: Mon May 07 2007, 02:21AM
Location:
Posts: 14
Thanks Bert, that makes sense. That would certainly help troubleshoot a failed transmitter.
Jim, I am still planning on some replacement for the capacitor, probably an MMC. I noticed recently that one of the surplus places has Maxwell 35kV 0.03uF pulse caps for a reasonable amount of money. I may have to get one of those.
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DC Resonant TC
