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Re: 2 simple questions



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At 08:33 AM 3/10/97 -0600, you wrote:
>1. how do you decide what diameter tubing to use for primaries - I am building 
>a small coil, and I bought a box of 3/8 tubing, but several persons 
>recommended 1/4.  I won't be messing with the tubing for a couple of weeks so 
>there is plenty of time to return it and get the smaller (cheaper) stuff.

William,

For a small coil, a primary hasn't got to be too thick..  The recommendation
to use 1/4" tubing is because is it cheaper and easier to bend into shape.
However, there is no reason why the 3/8" won't work.

>2. A more technical question.  I don't understand the need for the HV primary 
>voltage.  As best I can figure out, what drives the output is primary current, 
>not voltage - after all, there is no way you will have 15KV across a copper 
>buss bar - what you are doing, it seems to me, is creating a big pulse that 
>then resonates back and forth through a tank circuit made up of the primary 
>and the capacitor (whilst the spark gap conducts), and the primary only 
>(higher freq osc) when the spark gap is not conducting.
>So, why can't I just charge up a farad or so of "computer type" electrolytics 
>to 100VDC and then dump the charge through a 1,000 amp SCR into the primary 
>directly????  Has anyone tried this??  I would imagine having a LC circuit of 
>the tubing for the L, and a suitable AC capacitor, as a tank circuit.  Then, 
>the SCR and the capacitor bank would share a common ground with the tank 
>circuit.  The SCR would fire, dumping charge, and then the reverse current 
>would shut off the SCR.  The pulse would resonate in the tank for a while, 
>then you would fire the SCR again.   A crude schematic is represented below:

What you're describing is a form of solid-state Tesla coil.  Yes, they do
work, but they produce less power, and their outputs are usually electrical
brushes, not the impressive sparks and streamers of a regular Tesla coil.
Most people will agree that there is no way to replicate the output of a
real Tesla coil without a spark gap (and high voltage, which the spark gap
requires to operate properly).  Though there is this report from Mark Barton:

--begin snip--
-I have posted info on this system before, but will tell you it briefly. 
-A friend and I have built a VFET H-bridge driver that operates at 10KW 
-CW and about 40KW in burst mode (carrier bursts at about 100Hz rep 
-rate).  The carrier frequency is 80KHz.  The effects are startling.  CW 
-operation yields a 4 foot SHEET of electrical flame that sprays from 
-the discharge electrode with hisses and crackles.  Burst mode gives the 
-familiar tendrils of a classic TC along with that rrraaaaaaattt sound 
-we all love.
-
-Power supply: rectified and power factor corrected 240VAC yielding 
-400VDC.  Large bank of electrolytics to support burst mode operation.
-
-I believe the FETs are IRF540 type, 4 per leg.
--end snip--

This suggests that you CAN in fact produce an output like a real Tesla coil
if you simply modulate the output, simulating the interruptive action of a
spark gap; but this is the *only* report I've heard to this effect.

One other reason why high-voltage is important: resistance.  When a spark
gap discharges the capacitor in a regular Tesla coil, due to the
high-voltage it can produce a pulse of up to several thousand amps.

However, at low voltages, you're really limiting the peak amperage.  I don't
know what the on-resistance of a SCR is, but let's assume 10 ohms.  At 100v,
that's only a maximum of 10A it can conduct.  In addition, at the high
frequencies that a TC operates at, all conductors only conduct on the
outermost few mils of the material (the skin effect), which increases the
resistance of all conductors several orders of magnitude over the normal DC
resistance.

Hope this helps,
Chris C.






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