Re: Quenching Theory Question (fwd)

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Original poster: List moderator <mod1@xxxxxxxxxx>



---------- Forwarded message ----------
Date: Sat, 19 May 2007 07:48:40 -0500
From: Bert Hickman <bert.hickman@xxxxxxxxxx>
To: Tesla list <tesla@xxxxxxxxxx>
Subject: Re: Quenching Theory Question (fwd)

Tesla list wrote:
> Original poster: List moderator <mod1@xxxxxxxxxx>
> 
> 
> 
> ---------- Forwarded message ----------
> Date: Fri, 18 May 2007 21:04:36 -0400
> From: "Breneman, Chris" <brenemanc@xxxxxxxxxxxxxx>
> To: Tesla list <tesla@xxxxxxxxxx>
> Subject: RE: Quenching Theory Question (fwd)
> 
> This information was really helpful also, but really what I'm getting at
> is somewhat more specific, relating to capacitor charging through a gap.  
> Here's the overall picture: I was thinking of building a DC coil, and was
> looking up various ways to do so.  The two ways that I found were to use a
> charging reactor (which would have to be large and have to support a high
> voltage) or to use a charging resistor (dissipating significant power).  
>>From what I can see, these are necessary to prevent shorting out the power
> supply (presumably containing parallel capacitors) when the gap fires.  

Yes. However, using a charging reactor in series with a de-queueing 
diode results in the tank cap being charged to about 2X the DC supply 
voltage - see the following for an excellent explanation:

http://www.richieburnett.co.uk/dcreschg.html#resonant

I
> thought that there might be a better way to do this than these methods, by
> physically breaking the connection with the power supply when the gap
> fired.  The only way I could think of doing this was to use a modified
> ARSG, with three contacts.  One contact would be connected to one lead of
> the tank capacitor, and it would always be within gap firing distance of
> the rotor, which would alternate sparks between two different sets of
> contacts, one set attached to the power supply and one set attached to the
> primary.  The other power supply lead, tank cap lead, and primary lead
> would be connected together. 

This should work. However, there are some distinct disadvantages with 
the scheme

You may wish to consider a slightly different approach taken by another 
list member. He used a slightly different switching configuration that 
disruptively charged the tank cap from a low Z DC supply (with a large 
DC cap), through the TC primary. On every other presentation, the gap s 
reversed the polarity of the power applied to the tank cap (through the 
TC primary). The result is that on each gap firing, the tank cap sees an 
applied potential change that is 2X the DC supply voltage. During each 
charging "bang", the tank circuit oscillates to drive the secondary 
(just as in a regular spark gap coil). In this system, the DC supply 
also becomes part of the RF path during each high current 
charging/reverse charging cycle.

The advantages of this system are that each bang is coincident with 
reversing the polarity of the charge on the tank cap, the TC primary now 
limits the peak surge current, there's no need for a large charging 
inductor or limiting resistors, and the tank cap effectively operates at 
2X the supply voltage (4X the bang size) as in a resonant charging 
system. It's simple, and it works. From the tank circuit's perspective, 
disruptively charging the tank cap (from a very low impedance DC source) 
works in the same manner as disruptively discharging it. As long as the 
DC storage cap is much larger (say at least 10-20X) than the tank cap, 
the tank cap will charge to 90 - 95% of the nominal DC supply voltage.

> In addition to solving the shorted power
> supply problem in DC coils, this design would also have an advantage over
> traditional gap AC coils, because there would be no loss due to the power
> supply being shorted at peak voltage when the gap fired.  

Correct. However, if I understand correctly, your proposed switching 
scheme would connect the DC supply cap directly across the tank cap 
through one spark gap with no current limiting to recharge it. The 
circuit I described above always conducts charging surges through the TC 
primary for less electrode wear and easier tank cap and DC storage cap 
life and MUCH less gap wear.

This would be
> particularly advantageous for power supplies employing voltage doublers
> because the capacitors in the doubler wouldn't completely discharge.  It
> would also be an improvement over DC coils with a charging reactor because
> the break rate could be varied continuously without changing the charging
> reactor.  I have actually ordered parts to build such a gap, and have been
> working on several equations to describe gap operation. I can't think of
> any problems with this design, except for the issue with the capacitor
> charging through a spark gap.  This is where my question was going.  
> Because the DC power supply would have a capacitor in parallel with the
> power supply of a significantly higher capacitance than the tank
> capacitor, when the gap fired to charge the tank capacitor, the initial
> current would be high.  

Yes - and a spark gap will really not limit this current. The peak 
charging current may reach tens of thousands of amperes or more - not 
good for the gap or the caps.

My concern is that the tank capacitor might not
> charge to the same voltage as it normally would with a charging resistor
> or reactor.  

It won't charge to the same value as with a charging inductor since the 
resonant charging leaves the tank cap at about 2X the supply voltage. It 
will charge to about 90 - 95% of the supply voltage (depending on the 
relative capacities of the supply and tank caps).

So, what are the issues with charging the tank capacitor
> through a gap? 

Excessive charging current - with only the wiring, gap, and cap 
inductances to limit peak current, you have a potential capacitor 
destroying charging system.
.
> Also, as soon as the parts for this gap arrive, I'll
> construct it, test it, and let you all know how it works.  I'm just very
> curious about the aforementioned issues with charging a tank capacitor
> through a gap.  I've derived several equations regarding the functionality
> of the gap, bang size, power throughput, etc., but none of them take into
> account the capacitor charging through the gap.

Good luck and please keep us posted on your progress.

> 
> Thanks a lot, Chris
> 
> 

Bert
<snip>
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