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I've recently built a relatively big Tesla coil. It is a larger version of the Slayer exciter but with 2 big MOSFETs. The same circuit, with a smaller coil produced small arcs without a top load, and bigger arcs with a top load. My problem is that, with a larger secondary coil, the circuit produces large arcs without a top load, but when I add a top load, it gets overloaded and the arcs get very small. Can I solve this problem by making the secondary coil shorter? It is quite long.

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    \$\begingroup\$ Sounds like an impedance matching issue. How do you power it? What’s your primary and secondary inductance and what’s your secondary capacitance? \$\endgroup\$
    – winny
    Commented May 20, 2022 at 17:05
  • \$\begingroup\$ I power it using DC from a full bridge rectifier and a 555 timer interrupter. I can't really measure the capacitance and inductance. I don't have the tools. I can only roughly calculate it. \$\endgroup\$ Commented May 21, 2022 at 8:23
  • \$\begingroup\$ What’s your calculated values? Schematic of your 555 based power stage? \$\endgroup\$
    – winny
    Commented May 21, 2022 at 17:22
  • \$\begingroup\$ My calculated inductance is 355uH. \$\endgroup\$ Commented May 25, 2022 at 13:32
  • \$\begingroup\$ Primary inductance? What about secondary inductance (assuming transformer type) and capacitance? \$\endgroup\$
    – winny
    Commented May 25, 2022 at 13:44

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This is an answer in the making, please don't accept it yet.

This is not a discussion forum, and there is the risk of this question being closed as unclear, but let us cut you some slack and attempt to help you ask a specific question what is destroying your MOSFETs.

There are many many ways to make a Tesla coil, transformer based as yours, primaryless half-bridge, resonant tank, distributed resonant tank and so on. You need to provide a schematic of yours.

Here is an attempt: enter image description here

Please install LTspice and create a schematic of yours. The coupling factor between the two coils, needs to be estimated or measured. You'll need to add your input voltage, exact make and model of MOSFET and so on.

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  • \$\begingroup\$ My top load is a disks, also my MOSFETS are not dying. It's just that with a top load the sparks almost disappear. It works without a top load but the sparks are not very large. \$\endgroup\$ Commented May 28, 2022 at 13:33
  • \$\begingroup\$ I added a picture in the original question. \$\endgroup\$ Commented May 28, 2022 at 13:39
  • \$\begingroup\$ I am using a IRFP260N \$\endgroup\$ Commented May 28, 2022 at 14:14
  • \$\begingroup\$ Do you think a shorter coil would solve this issue? \$\endgroup\$ Commented May 28, 2022 at 14:15
  • \$\begingroup\$ Hello! I just wanted to let you know that I solved the issue. The coil was too long and the circuit couldn't drive it. You helped me a lot! THANK YOU VERY MUCH! \$\endgroup\$ Commented May 28, 2022 at 16:15

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