[Prev][Next][Index][Thread]

Re: Simple DC starter stuff



Tesla List wrote:
> 
> >From gchristo-at-clt.mindspring-dot-comFri Nov 15 22:41:15 1996
> Date: Fri, 15 Nov 1996 14:12:00 +0000
> From: Chris Gardner <gchristo-at-clt.mindspring-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Simple DC starter stuff
> 
> Im really new at this stuff, woefully ignorant of the technical level
> that you guys pursue, but very curious......   (like george)
> 
> Anyway, I built a coil in highschool that my grandfather helped me
> design. The primary parts were as follows;
>         6v lantern battery
>         Plate glass and Aluminum foil Capacitor
>         6v ignition coil (mabye 12)  Auto
>         6v buzzer
>         simple spark gap
> 
>  And the coil wrapped around 2.5 inch pvc,secondary wound of 30 guage
> silver plated vinyl insulated wire, with the primary, 12 guage single
> strand, wrapped right around the secondary on the same form. (from all
> my reading and reasearch lately Ive begun to gather that was not optimal
> (overcoupled?))
> The buzzer was rigged to provide intermitant contact at the buzzing
> frequency, (sort of like a rotary?) but I think that it was before the
> ignition coil in the circuit, actually Im sure it was.
> This would make more sense anyway.
> 
> It was alot of fun, and the spark from the small ball bearing on the
> top, approx 3/4 of an inch, was fairly harmless.
> 
> All I have left of this is the orriginal coil. I would really like to
> re-assemble this design, my grandfather passed away last year, he helped
> me build this off the top of his head, and I think I am only now really
> starting to appreciate how much he knew.
> 
> Any help which you gentlemen could offer would be greatly appreciated.

Chris,

Welcome back to coiling! The very first coil I built was much like
yours, almost 40 years ago :^( , only with a cardboard former. I don't
know if they'd even invented PVC way back then... :^).  It turns out
that you have tremendous latitude in re-building your coil. Its only
when want to push higher power levels, voltages, or efficiencies that
the "design rules" start to become more restrictive.

Lets cover the basic areas, and throw in minimal design considerations.
The major areas areas and cover them in a little more detail:

 1. HV Supply
 2. Tank Capacitor
 3. Primary Coil
 4. Secondary Coil (Resonator)
 5. Discharger
 6. Spark gap

1. HV Supply:
Assuming you wanted a coil similar to the original, you have a couple of
choices. You could use a standard 12V ignition coil,  or you could
purchase a rebuilt Model-T spark coil (which already has the built-in
interrupter and capacitor) at an antique auto swap meet, antique auto
supply house (check some of the Auto Magazines), or even an antique
store. With this type of coil, all you need to do is power it up and
you've got HV!

If you use a modern ignition coil, you'll have to use some type of
external interrupter, and you'll also need a 0.1 uF 600 WVDC (or higher)
Mylar capacitor. You could drive the coil from a solid-state
oscillator or even through a capacitor and an SCR light dimmer, but I'm
assuming you want a design close to the original. A small 12 Volt DC
SPDT 5A or 10A relay can be wired to make a buzzer (better approach), or
you may be able to find an electromechanical-type buzzer. The problem
with using a buzzer is that the buzzer's coil will be in series with the
ignition coil's primary, severely limiting the output of the ignition
coil. The contacts on the buzzer will not withstand the full load of the
ignition coil like the relay can.  

1A. Relay method:
Connect the relay coil and primary of the ignition coil in parallel.
Connect one end of the coil pair to the one NC contact on the relay.
Power will be applied to the remaining NC contact and to the other coil
end. When power is applied, the relay will open, breaking the NC
contact. The relay re-closes and the cycle begins anew. The 0.1uF
capacitor should now be connected across the NC contacts on the relay.
This will improve the power of the spark coming off the secondary of the
ignition coil. 

1B. Buzzer method - Locate a 12 Volt electromechanical buzzer, and
simply
connect it in series with the ignition coil primary. This should provide
you with a low-power HV output, but will have significantly poorer
performance than the previous alternatives. 

2. Tank Cap:
Inexpensive double-strength window glass and aluminum foil plates will
work fine for a low-power Tesla Coil (TC). You can always build better
plastic film type caps if you really "get the bug". The foil plates
should be at least 1" smaller on all edges than the glass. The plates
can be made
from heavy-duty aluminum foil, pieces of aluminum flashing, or even
aluminum foil duct-tape (with adhesive backing) used for
heating-ventilation work. The capacitance per plate can be estimated by
multiplying 13.5 pF per Square inch of usable plate area. For example,
an 8" x 9" piece of glass with foil plate area of 42 square inches (6" x
7") would have about 560pF of
capacitance. You can easily add more plates or increase the plate size
as
required by hooking individual caps in parallel. Your coil will require
a tank cap of about about 1200-1500pF.

Another option is to use a number of wine-cooler bottles partially
filled with saltwater, all sitting in a plastic tub which is also
partially filled with salt water. A bolt with an attached wire can be
immersed in each bottle, and connected to other similar bottled. Each
bottle will give you up to about 500pF of capacitance. Tuning is simple
- simply change the number of bottles or the water level in the tub.

3. Primary Coil:
Some provision needs to be made for tuning the coil. The primary tank
circuit and secondary natural resonant frequency (with top discharger
present) should be "close" to one another. Usually, this is done by
making the primary "tappable" by winding with bare copper wire or tubing
and tapping via a wire tied to an alligator clip. If you used a fixed
number of turns of insulated wire, you'll need to vary your primary
capacitance, and/or change your secondary's self-resonant frequency by
adding and changing the size and height of the discharge terminal versus
the top of the winding.  

The simplest construction for your coil is a vertical helical primary
wrapped around a series of slotted dowel rods (plastic if possible, or
heavily varnished wood) aranged in a circle, forming an overall primary
coil diameter of about 8". #10 or #12 AWG house wire will work fine for
a low power coil. Bare ground wire (Most HW stores carry 8 Gauge) is
even better since you can easily tap the coil primary to adjust tuning.
Use small tie-wraps to secure the wire to the dowel rods. Because of the
small coil secondary diameter, your coil will only require about a 10
turn primary with 3/8" spacing between adjacent turns, about 4" high.
This will give you about 21 uH of primary inductance (max), or about 2.1
uH/turn.

4. Secondary Coil: 
You didn't mention the length of your secondary coil. For a 2.5"
diameter coil used as a standard TC, the length of the winding should be
about 4-6X the diameter (max). The bottom of the coil needs to be tied
to ground. For a very low power coil, AC ground or a grounded water
pipe will do (but it will generate some interference on radios and
TV's). Some provision should also be made to change the relative height
of the secondary coil versus the primary so that the inductive coupling
can be adjusted. 

Because of the small secondary coil diameter, and the relatively thick
vinyl wire insulation, your coil's self-resonant frequency will be quite
high.
If you used #30 AWG hookup wire, the insulation thickness is about 10
mils, and if it was silver-plated wire-wrapping wire, about half this
thickness. The inside wire diameter is about 10 mils, so depending upon
the type of wire, you have about 33 turns/inch or 50 turns/inch assuming
you've close-wound the coil. Since I don't know the coil lengt or type
wire, I've generated a little table which estimates the natural
frequency of the secondary (with no discharge terminal). Pick the one
that comes closest to your coil size and wire type.

         (MHz)       (MHz) 
         Hookup      Wire-
(Inches)  Wire       Wrap  
Length     F0         F0   
------    ---        ----   
  9       2.11       1.41   
 10       1.96       1.31   
 11       1.84       1.23   
 12       1.73       1.15    
 13       1.64       1.09   
 14       1.55       1.04   
 15       1.48       0.99   

5. Discharger:
The small discharge terminal at the top doesn't provide any corona
protection for your top turns, and adds only minimal top capacitance. If
you add a larger diameter sphere or make a small toroid (you can
carefully wrap aluminum tape around a small styrofoam donut found in
craft stores. Cut a circular center plate out of aluminum flashing and
use the aluminum tape to attach it to the toroid. Once you do this, the
frequency of your coil will decline. 

For example, if you make a 6"x 1.5" toroid, it will add about 5 PF of
top-load capacitance, which will substantially lower the operating
frequency. You'll need to retune the coil by increasing the number of
turns tapped on the primary, and you may need to increase the tank
capacitance. If you had a 10" long coil, wound from #30 AWG wire-wrap
wire, the estimated operating frequency without a toroid would be about
1.31 MHz. Once you add the toroid, the new frequency would be about 900
kHz. The toroid will drop the frequency of your unloaded coil by 250-400
kHz. 

6. Spark Gap:
For your low power coil, a couple small pieces (1.5" long) of 3/4"
copper pipe can be used. These need to be secured so that they look
like: "0x0" where the gap (x) between can be adjusted. Don't open the
gap beyond about 3/8". The gap will be connected directly across the
output of your ignition coil. The tank cap and primary coil will be
connected in series and the combination of the two connected across the
spark gap. Use heavy gauge wire (10 AWG) to connect the cap and primary
to the spark gap, since some fairly heavy current will be flowing
through this portion.  

This has been a very abbreviated summary for rebuilding and boosting the
capability of your coil, and should help you get started. If you have
further questions, just ask! If you want to discuss this in greater
detail, I can be reached at 630-964-0512, or you can contact me directly
at my E-mail address. Good luck!!

Safe coilin' to ya!

-- Bert --