- Blog entry posted in 'Uncategorised', March 02, 2011.
While a Tesla coil is being operated, it often produces high-frequency voltage spikes on the NST side of the tank circuit, especially if an arc from the topload strikes the primary coil. These spikes can be extremely harmful to your transformer and to the mains supply, so it is very important to have some sort of low pass filter to suppress them.
One of the most common filters nowadays is the "Terry Filter," designed by Terry Fritz--an experienced Tesla coil builder. They use several high-voltage capacitors and resistors to shunt the voltage spikes to ground through an extra spark gap, and to decouple the NST from the rest of the circuit.
Unfortunately, all the parts that one would need to build this may get relatively expensive, and since this is a low-budget Tesla coil, the Terry Filter is not much of an option.
Another possibility would be to just have the extra spark gap, or safety gap. If your transformer is center-tapped, as many NSTs are, you would need to have a gap for each output of the transformer. To do this, you would connect each output to the outer contacts of the spark gap, and an inner contact connected to the transformer's ground. The gaps should be set just wide enough to prevent arcing in the safety gap during normal operation. This setup would look something like this:
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I will probably also include one of these on my Tesla coil, but you should not rely only on a safety gap to protect your transformer.
The last method I will be discussing is RF chokes. These, I will definitely be using in my design. An RF choke is a coil of wire wound around a ferrous core. The idea behind chokes is that they will allow low-frequency current (from the transformer) to pass through, but will stop the high-frequency voltage spikes (from the Tesla coil).
At the moment, I am planning to try using the primary winding of two microwave oven transformers as my chokes. I will connect each MOT in series with each output of my NST. These are my MOT chokes:
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Again, you can get these for free out of old microwave ovens that you can find at scrap yards and recycling centers, and again, be sure to safely discharge the high-voltage capacitor before removing the transformer.
The next part of my SGTC: The Primary Coil
ben7, March 18, 2011
cool :) I have yet to make a tesla coil, but I want to do it sometime soon. I read before about high freq. osc. killing high voltage windings in transformers. So I don't have an NST but I have a small transformer that puts out about 10kV :) I have homemade capacitor made from aluminum foil and a thick plastic drawer, on the output of the transformer-with the capacitor parallel to the transformer-the capacitor makes big, bright, and surprisingly loud arcs :D
DerStrom8, March 18, 2011
Hi, Ben7. I am happy to hear of your interest :) The transformer and the capacitor are probably the two most difficult parts of a Tesla coil to get. Once you have obtained them, and matched them correctly, you are in the home stretch, so to speak. The rest is quite easy. I wish you the best of luck on your own Tesla coil! :) Regards, Der Strom
ben7, March 18, 2011
[QUOTE=DerStrom8;bt401]Hi, Ben7. I am happy to hear of your interest :) The transformer and the capacitor are probably the two most difficult parts of a Tesla coil to get. Once you have obtained them, and matched them correctly, you are in the home stretch, so to speak. The rest is quite easy. I wish you the best of luck on your own Tesla coil! :) Regards, Der Strom[/QUOTE] The only problem I have is I don't know how many mA my transformer puts out :(
DerStrom8, March 18, 2011
[QUOTE=ben7;bt402]The only problem I have is I don't know how many mA my transformer puts out :([/QUOTE] Yes, that could be a problem. I don't know what type of transformer you have, but there are a few ways you can find the output current. One is if it has a VA (volt-amp) rating on it. To find current, simply divide the voltage by the number of volt-amps and you are left with the number of amps. Another way would be if the label has an output wattage. Wattage is the same thing as VA (P=IV, where P is watts, I is amps, and V is voltage). You may also be able to simply estimate the output current by the thickness of the arc. Though it may not be very accurate, it may give you a basic idea. Regards, Der Strom
DerStrom8, March 18, 2011
post removed
