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Registered Member #8120
Joined: Thu Nov 15 2012, 06:06PM
Location: Moscow, Russia
Posts: 94
A spark gap tesla coils that fits on your palm can produce some impressive sparks, while running off 4 AA batteries.
I was wondering if a DRSSTC of the same size can do better. The electronics are somewhat bulkier, but main problem is that i had to go up to 40V bus voltage before i got any sparks at all. Too much for batteries, and if i was to stick a step-up converter in there, then i might as well stick in a 10KV transformer and a spark gap. The DRSSTC topology just does not seem to work with low bus voltage.
So, are there any ways of making it run off low voltage? Sure, you can charge a capacitor with a step-up converter, and run bursts off that, but i was hoping for something more elegant.
One idea i had is to make two "stages" - the primary rings up a small secondary, with HV capacitor and second primary in series, and the second primary acts on the actual secondary. The inductance of second primary should be negligible, but it would have the voltage and current across it on the order of what a spark gap one can do. Does that make any sense at all? Would it work, or is it still limited by the same primary_voltage*primary_current power limit that won't be too high with low bus voltage?
What i was thinking about is - DRSSTC idea itself basically combined the step-up transformer with tesla coil, allowing the coils to run directly off lower voltages, so maybe there is a way of generalizing it for N stages?
Have a look at Steve Conners design rules. , particularly at:
Primary impedance = sqrt(Lpri/Cpri) = 10 to 20 times (4/pi)*(Vbus/Iocd)
For a very low Vbus this implies a very low primary impedance. This might be difficult to get, especially if the operating frequency is high, since that means a very low primary inductance, e.g. a quarter turn or something like that. What is your secondary resonance f?
Also you need large input currents to get any power at all into the coil at these low bus voltages, i.e. you need to buffer the batteries with some large caps. A third problem is the voltage drop along the transistors. That might eat up most of the power.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
What Uspring said If you want to build a tiny DRSSTC, the best way is to use a DC-DC converter to charge an electrolytic cap to 300-400V from the battery, and then run an IGBT bridge off this.
High voltage electrolytics have a higher energy density than low voltage ones, and stray inductances make it impossible to generate extreme power levels at low voltages and high frequencies.
A N-stage coil would be a "N+1RSSTC" with N+1 resonant frequencies. There was talk of a triple resonant SSTC a while back based on the spark-gap magnifier, but we concluded that the conventional dual resonant coil works better. The more resonant modes the system has, the easier it is for energy to get trapped in unwanted ones.
Edit: This is just my opinion. I would love to see someone surprise me with the new lithium polymer batteries for RC models. Steve Ward's backpack Tesla cannon uses these batteries, but with an intermediate DC-DC and a bank of 400V electrolytics for energy storage.
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