Saturday, November 24, 2012

DRSSTC Δt2: Primary Construction

It's been a long, long time since I first started putting together my first Tesla coil, and now I'm finally back at it. It was supposed to be a summer project, but I guess I spent too much time designing it and got distracted when the time came to actually put it together. But I'm glad I got through the secondary winding while I still had momentum. Recently, I had a crazy dream where some MITERS people were testing coils and blew up a transformer outside, so I went to hide my secondary but couldn't find it in a pile of neglected, unwinding coils. Anyway, that inspired me to get back to work.

This update is mostly about the construction of the primary circuit, consisting of an flat spiral inductor and a series capacitor that form one half of the resonant power transfer. The circuit elements are easy enough to construct: the inductor is only 6-8 turns and the capacitor is an off-the-shelf component. But they are integrated into the base of the coil, so I had some mechanical work to do (for once...).

The spiral coil is made with 8AWG grounding wire (McMaster P/N 7512K641). 25 feet of it was just barely enough to make the complete spiral on a 20in diameter circular base of 0.25in-thick polycarbonate. I marked out the spiral by unwrapping a string from an appropriately-sized spool fixed to the center of the base. Then, I drilled two small holes on either side of the spiral marker line in 90º intervals. These holes served as small zip-tie mounting points to hold the grounding wire in shape around the spiral.

Originally, I had planned to have the spiral coil on top of the polycarbonate base, as in this rendering. However,  I settled on flipping the base upside down so that the coil is below it. This makes more sense from a keep-the-high-voltage-covered perspective, although only as a supplement to the higher-priority safety plan of "don't go near it at all ever". The performance hit from decreasing the coupling would matter, but I intentionally wound the primary closer to the bottom of its PVC pipe coil form to keep the designed distance between primary and secondary coils about the same.

I calculated six turn for the primary, but added an extra turn and a half for tuning, to adjust the primary's resonant frequency. Tuning requires a movable tap of some kind, which I decided to make from scratch since jumper cable clips seemed like a bad idea.

Since the primary circuit has a total resistance of only about 70mΩ, it was important not to add a significant amount in the form of contact resistance. (Especially considering that the peak current going through the connection could be several hundred amps and the RMS current as high as maybe 50A.) I opted for a homemade screw terminal made from a small aluminum block with a copper strip bent around one side. Most of the current probably goes through the aluminum, but the copper gives it a second path that doesn't involve steel cap screws. Attached to the spiral coil, it looks like this:

Did not notice that sign when I took the picture...
I haven't measured the contact resistance, but I imagine it will add a couple milliOhms at most. To adjust the tap position for tuning, the two cap screws are loosened and the block is moved around the coil to a new location. This is the node between the inductor and the capacitor in the primary circuit, and as such is the highest-voltage point on the primary, designed to hit about 3kV in operation. I used some high-voltage noodle wire (McMaster P/N 9620T22) to connect the clamp to the capacitor; probably overkill, but it looks cool too.

Speaking of capacitor, I put together a 3S3P pack of 100nF CDE 940C30P1K-F film caps to make the primary capacitor:

It's hard to see through the double layer of heat shrink, but each capacitor has a string of four 1/4-Watt resistors of 250kΩ each, for a total of 1MΩ. These act to balance the series capacitors and also to drain the bank quickly (RC = 100ms) when it's disconnected from power. I used four resistors in series per capacitor, and three capacitors in series for the bank, so each resistor only sees about 1/12th 3kV peak (250V peak).

One other small but annoying mechanical task was making something to hold the secondary coil in place. Luckily, I found a PVC fitting of the right diameter and managed to just barely blind-tap some 4-40 holes into the side of it to hold it to the polycarbonate base:

A wire passing up through the base connects the secondary coil to ground, through one of the 80/20 columns on the side of the driver box. I'll probably add some insulation to the screw head, since it sits right below the high voltage node on the outside of the primary coil. The secondary slips right onto the PVC fitting, and it almost looks like a functional Tesla coil:

I should be able to start testing the driver next. (It's build and mostly wired.) The first task will be identifying the resonant frequency of the primary, since my driver isn't self-resonant. I will probably run some low-voltage frequency sweeps and mark out some taps on the spiral for a range of resonant frequencies around 150kHz. Once I know what the tunable range is, I can pick a spot and try it out with the secondary at increasing levels of power and re-tune as necessary. (Since it's not self-resonant, I'll have two tuning "knobs": moving the tap and adjusting the drive frequency.)

So, some day soon there will be sparks! ...and eventually music. And if you're getting impatient (understandably, since my progress has been disappointingly slow), you can soon make your own with a kit from oneTesla, started by a few HV-competent MITERS members for all your musical Tesla coil needs.

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