How to make a slide choke ballast

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Gregory Peters by way of Terry Fritz <twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>

Josh, list,

If you want to make a slide choke ballast. I've had good success with 
the following:


1. Wrap a coupla hundred turns around some PVC or thinwalled 
cardboard tube. Use the thickest wire you can afford that will fit in 
the space and give enough turns. I used 6" diameter PVC, but 4" would 
probably be a better compromise between efficiency and length of wire 
needed.

2. Go to your local welding supply and ask for some regular arc 
welding (steel) rods. Ask for some which are water damaged, or have 
damaged packaging, etc. I got 50kg (100 pounds) for about $20. Get 
the thinnest diameter possible.


3. Soak the rods in water overnight. This quite nicely loosens the 
flux. For the best core density, you will need to remove the last bit 
of flux, which is hair thin and sticks to the metal. This job was 
made easier by simply placing a handful of rods in a bucket and 
hitting them hard with a high pressure hose. This method is 
recommended, even if you need to hire a water blaster. It takes a 
long time to wipe several hundered rods by hand. The water blaster 
will easily leave the rods shiny and bare.

4. Either coat the rods with an insulator and wrap them together in a 
bundle, or better still, immerse them in a cylinder of epoxy (or 
similar) to make a solid, efficient, noise free core.

5. Slide the core in and out of the PVC to vary the current.

Notes:

a) The bigger the core, the less it will heat up. It is also probably 
more efficient. However, you will need more wire to get enough turns 
on it which may produce too much unwanted resistance. As these cores 
are open ended, saturation is almost impossible. You can probably get 
away with a 3-4" core for ~10kVA. Make sure the rods are thin and 
insulated!!! My 6" core shows absolutely NO SIGN of heating 
whatsoever. A smaller core may heat up more and may melt the former 
or core insulation however. If the rods are thin enough and well 
insulated, eddy currents may be minimal enough to keep the core cool, 
even on smaller cores. I don't know, as I haven't made one this small.

b) Efficiency can be improved by using silicon steel transformer 
laminations rather than welding rods. However the core may need to be 
bigger as you are unlikely to get as high a core density as you do 
with welding rods. I'm currently using laminations in my core. I got 
them for free at my local trnasformer winding shop.

c) The better efficiency in (b) above may not be desirable for tesla 
coil use. Some loss in the inductor is good for filtering out peaks 
and spikes.


Hope this info is useful,

Greg.