Building a Tesla Coil: A Student Guide

[MacDaddio]

Hello everyone,

I am interested in building a Tesla coil for fun and education. I am an EE student in my junior year of college, so I have most of the basics down, but I'd like to learn a little more so that I do not set my house on fire. I've been going through the principals of Tesla coils, and I have made a few assumptions. Most of my information has been from the wiki page on Tesla coils.

The main idea behind the Tesla coil is to have a small-ish AC voltage source stepped up into the kV range in order to have dielectric breakdown of air. My first assumption is that the "spark gap" and tank circuit are used to minimize resistance to secondary coil. My second assumption is that the resonant frequency of the tank circuit must be much larger than the 60 Hz main. This is because the tank circuit has to see the input voltage as practically DC in order to charge the capacitor.

Please let me know if I'm wrong, as I'm very interested in this project. If anyone has a good source of information on Tesla coils, please link it! Most of the search results I get on Google are videos showing how cool they are. Which isn't very helpful in understanding them.

 

[zoki85]

My first assumption is that the "spark gap" and tank circuit are used to minimize resistance to secondary coil.

Spark gap is used to perform role of a switch. Capacitor is used to store energy before the "switch" gets closed. Than, the tank circuit (primary LC circuit) starts to discharge at high frequency. Secondary circuit should resonate at same frequency to pick up that energy via magnetic coupling.

My second assumption is that the resonant frequency of the tank circuit must be much larger than the 60 Hz main. This is because the tank circuit has to see the input voltage as practically DC in order to charge the capacitor.

This assumption is correct but for different reason. If the tank circuit frequency is too low than it starts to mess up with charging cycle of tank capacitor, and spark gap having problem to open.

 

[MacDaddio]

I understand how the tank circuit works, I just don't understand its purpose entirely. In theory, you could omit the tank circuit and spark gap and just have one monstrous transformer right? I have attached an image of what I'm talking about; please, no compliments on the art skill.

 

[zoki85]

I understand how the tank circuit works, I just don't understand its purpose entirely. In theory, you could omit the tank circuit and spark gap and just have one monstrous transformer right? I have attached an image of what I'm talking about; please, no compliments on the art skill. View attachment 75902

Looks to me you don't understand. Classical transformer alone can't replace role of a primary tank circuit. Tesla coils are high frequency devices, 60 Hz is too low for them.

 

[MacDaddio]

So you are saying that the purpose of the tank circuit is to increase the operating frequency of the secondary coil?

Why do Tesla coils need to be run at high frequencies? And what frequencies are you talking about: kHz?

 

[zoki85]

No. The secondary coil has it's own resonant frequency f_s, just like every LC circuit. Problem is that air core secondary coil must be of colossal size in order to have low f_s. One can significantly lower f_s by connecting big enough capacitor to the secondary terminal. However, this will significantly lower output voltage too. Also, making secondary coil with too many turns , to significantly increase inductivity and lower f_s isn't good. Therefore, primary tank circuit must have high frequency f_p too to satisfy resonant condition f_p=f_s.

 

[MacDaddio]

Okay I appreciate the explanation. This helped me grasp the concept of the tank circuit in this design.

I will revive this thread if and when I get stuck. Farewell!