Often
times, a Tesla coil builder will need a ballast. The purpose of a
ballast is to limit current into the step up transformer, there are
three reasons for doing this; to keep your coil form flipping a
breaker, to keep your step up transformer from melting, and to limit
arc size to suit your location (sometimes 12 ft arcs are not suitable,
like in your bedroom). Thre are two types of ballast, inductive
and resistive. A resistive ballast limits the current by
resistance (duh) this is simply becaus of ohms law (I=V/R) if you
add more R, your I (current) goes down for a given V. This is all
fine and dandy but all that power is wasted as heat, and can never be
recovered. Then there is the inductive ballast, an inductive
ballast limits current by setting up a repulsive magnetic field in a
coil of some sort. This time there is no wasted energy, but when
the magnetic field collapses, there is a current surge (this is called
inductive kick). A savvy coiler can design his coil to recapture
this energy, so your transformer doesn't melt but you get a decent
amout of power.
Generally to make a resistive ballast, one uses somthing with a heater element (like a toaster, or two, or a space heater) and it is wired in series with the coils step up transformer primary. to make an incdutive ballast, there are many other options. Often times a microwave oven transformer with a shorted secondary is wired in series with the primary to limit current. sometimes a roll of wire, right off the shelf is used. On more controlled systems a variable ballast is used, this can be anything from a shorted arc welder or a variac wired in series. sometimes ballasts are made of PVC pipe stuffed with steel rods (like welding rods) and wrapped with wire, the rods can be moved to vary the inductance (don't use a soild piece of steel it will get hot and melt stuff as seen here). Here is a picture of the ballast used on my large coil (coming soon). Well current limited transformers (like NSTs and OBITs) do not need a ballast, MOTs are current limited but still need one as the leakage inductance is too low. Current limited transformers have metal "shunts" that limit current by passing energy back to the primary via an induced magnetic field called leakage inductance, I am not entirely sure of the mechanism of how this works, so look it up yourself.
Here is how not to make a ballast, a roll of speaker wire around a solid steel bolt.
Generally to make a resistive ballast, one uses somthing with a heater element (like a toaster, or two, or a space heater) and it is wired in series with the coils step up transformer primary. to make an incdutive ballast, there are many other options. Often times a microwave oven transformer with a shorted secondary is wired in series with the primary to limit current. sometimes a roll of wire, right off the shelf is used. On more controlled systems a variable ballast is used, this can be anything from a shorted arc welder or a variac wired in series. sometimes ballasts are made of PVC pipe stuffed with steel rods (like welding rods) and wrapped with wire, the rods can be moved to vary the inductance (don't use a soild piece of steel it will get hot and melt stuff as seen here). Here is a picture of the ballast used on my large coil (coming soon). Well current limited transformers (like NSTs and OBITs) do not need a ballast, MOTs are current limited but still need one as the leakage inductance is too low. Current limited transformers have metal "shunts" that limit current by passing energy back to the primary via an induced magnetic field called leakage inductance, I am not entirely sure of the mechanism of how this works, so look it up yourself.
Here is how not to make a ballast, a roll of speaker wire around a solid steel bolt.