Building a solid state tesla coil

Step 10: The half bridge.

Here is the muscle of this coil, the half bridge. what this is are two mosfets that alternate switching on and off to produce alternating current.  They do this at a high voltage, mainly so we can pump power trough the primary. this causes a magnetic field to be formed that excites the secondary coil (resonator) and the resonant rise builds up the high voltage. Once it is high enough, it breaks out into air.
PARTS:
*************************************************
IRFP260N mosfet (X2) mounted to heatsink
200v 0.68 µF capacitor (X 2)
5Ω 2W resistors
*************************************************

Comments

[NCC-1701]

I have a couple questions. First, why did you change the dielectric material of C7 and C8 to metalized polyester when the original design calls for polypropylene? Second, why did you change the capacitance of C12 to 680uf instead of leaving it at 420uf ? Do these changes improve the operation of the coil in any way?

[sciencetor2]

well, i assume he could not find a 420uf cap, i couldn't, i don't know about the C7 and C8, coilers know that mettalized polyester is no good for tesla coils, at least in a spark gap one, only polypropylene works.

[NCC-1701]

after searching through dozens of Steve Ward's projects, I finally found a picture that shows the full model number of the capacitors he used in this project. Here is a site that sells them. http://www.alliedelec.com/search/productdetail.aspx?sku=70103269

[sciencetor2]

if you could find the 420uf filter capacitor that would be cool, i cant find anywhere that sells it

[CamDAX]

C7 and C8 both polypropylene or did yo Are u substitute?

[carnotricecooker]

Um, any reason the bridge rectifier on D9 wasn't connected to the variac too instead of straight to the 120V AC outlet? I'm also curious as to how the capacitor on C6 should be modified to improve gate drive as stated in the schematic.

[spark light]

D9 is supposed to be connected to the variac. It is not shown in the schematic because the original designer decided not to use one. However, I highly recommend it because it can prevent some nasty situations. To improve gate drive, you could increase the value of the capacitor in order to make more current flow. Something like 0.47uF is a.good option.

[pjpajamas]

So I hooked everything up, but I'm not getting any arcs. Shortly afterwards, my 5ohm resisters cracked and started to smoke. I replaced them with two 10ohm resistors in parallel. This time, I'm getting a lot of heat of R4. Perhaps I wired the GDT incorrectly?

[spark light]

that doesn't sound right... what limiting capacitor are you using? Also, double check that you didn't wire the resistors and get to the drain and source.

[pjpajamas]

Limiting capacitor? Are you referring to C6? I used the .1uf capacitor. I'll give the .47uf capacitor a shot. Also, can you clarify what you mean by "double check that you didn't wire the resistors and get to the drain and source"? I don't quite understand what you are trying to say.

[pjpajamas]

I just noticed that it was only R4 getting hot. Maybe my perception of GDT opposite is incorrect? I have three color wires, one green, one red, one black. I have both ends of the red wires in the gate driver. The black wire on the left of the toroid is going to R3 and the black wire on the right of the toroid is going to the section between Q1 and Q2, just like in the diagram. For the green wire, instead I have the wire coming from the right side of the toroid going into R4 and the left side on that bottom path as shown in the diagram. Does this sound right?

[spark light]

sorry, i mistyped. Yes, that sounds correct. are your gate drivers getting warm?

[pjpajamas]

It doesn't seem like they are getting warm. There is also a high pitched noise coming from the circuit.

[spark light]

do you have an oscilloscope or volt meter?

[pjpajamas]

I got rid of the high pitch noise. Not exactly sure how, but it's gone now. I'm going to test my interrupter with an oscilloscope tomorrow at my school's lab. What do you want me to measure with the voltmeter?

[pjpajamas]

I just discovered that I wired the C7 and C8 oddly and fixed that. Now both R3 and R4 get really hot if I leave it on for about a minute at about 24 volts on my variac, I am also pretty sure the high pitch sound is coming from my voltage supply, but everything seems in order there. What should I have my potentiometers set to?

[spark light]

To the resistors get hot without the bridge power connected? test the resistance of the drain to source of both mosfets in circuit. (test both polarities)

[pjpajamas]

I figured out what I had wrong. I wired the MOSFETs incorrectly. I got the coil to arc for a few seconds, but then one of my 12v regulators blew out. Probably because it's from Radioshack. I bought another, but it seems like my gate driver UCCs blew out as well. I no longer get 12v out on my voltmeter when I have the gate driver wired to the 12v out.

[blablabla123]

Would it be suitabe to use IRFP264's?

[spark light]

you should be fine using that mosfet. there may be a little more heating, but as long as you don't try overvoltaging, all should be well.