How to ground a Tesla coil?

[Laci]

Many times when my coil was arcing to the base of it(which was connected to ground)transistors died.Up until today the base was connected to mains ground,which was connected to the neutral to the full bridge rectifier to the transistors,so any ungrounded high voltage could easily go to the transistors,causing them to break,probably short internally.I was working today to solve this problem with no results.I connected the base of the coil to a long wire and its other end through the window to a 4m long steel rail,which is completely fixed vertically in the soil to keep it stable at around 7m distance from the river.The weird thing was that the multimeter showed 0.000ohm resistance(no resistance) from the new grounding to the house grounding,which is at around 6m away from the new grounding.The first transistor died with this setup so it is obviously wrong,probably the same thing as connecting the base of the coil to the mains ground.I tried other things,like disconnecting the house grounding or running the coil from another wall socket with different grounding but the base(with the new grounding) is always shorted to every ground.How can I ground my coil to be able to arc to grounded objects for longer arcs and not killing the transistors?

[profdc9]

The base of the secondary should be grounded to the strike ring and a ground stake.   I use a copper pipe jammed in the ground as a ground stake.  If you do not have a ground stake, for example, if you are inside, you can use a counterpoise.  A counterpoise is basically a large capacitive object that can provide charge to the secondary coil.  It can be as simple as a large sheet (0.25 m^2 area at least) of aluminum foil placed on the floor underneath the coil attached by a wire to the bottom of the secondary.    Make sure the wires from the secondary coil and the strike ring / counterpoise / ground stake do not pass several cm from anything else, as the secondary coil might collect charge from those as well!  I prefer to use aluminum roof flashing as it doesn't crinkle readily unlike aluminum foil, but it doesn't matter.  A piece of sheet metal works just as well.

DO NOT ATTACH your RF ground (secondary coil) to your mains neutral or earth ground!  The mains ground is a poor ground for that and you will potentially get high voltage into other things.  Throwing a sheet of metal on the floor can work fairly well, especially on concrete which is a relatively conductive material.

A long wire has a lot of inductance and will not allow the high frequency RF to pass, even if it reads a low resistance at DC.  This is why you need the counterpoise, because it is connected with a short wire to something on the floor under the coil.

Dan

[Laci]

Thank you for the reply,Dan!
The aluminium foil solution is a great idea,but I have no idea how well it would work on the second floor,or is this grounding independent from the house grounding?

[profdc9]

What we call a "ground" is really just a way to provide a source of electric charge.  The earth ground, for example, is used because the earth is very big, somewhat conductive, and basically can provide a nearly unlimited amount of charge without changing its voltage.  In other words, the Earth has a very large capacitance.

It would work fine as a RF ground for your coil if your coil was close enough to it that you could connect it with a short, low-inductance cable.  However, since you are on the second floor, a long wire is required, this wire has a lot of inductance, and this inductance prevents the rapid rise in a current that is required for your coil to produce a streamer.

The ground wires in your house could potentially do this, but they are long and also have relatively high inductance and so are not suitable, as well as being connected to other valuable equipment you might have.

A large sheet on the second floor is separate from the earth and house grounding.  It is a separate capacitor plate, which while not as big as the Earth, can provide some charge while not rising in voltage too much.

Dan

[Mads Barnkob]

I will soon release my chapter on grounding for Tesla coils, a part of the DRSSTC design guide, which takes a lot of these questions and problems into consideration.

Until it is finished, follow the good advises here and while you try to make a artificial ground plane/counterpoise, try to match your topload capacitance with 10 times as much in your ground plane, you properly can not do this, as it always turns out to be pretty big when it is the capacitance of a plate