I made a thing. It's a small part of a larger thing - updated

 Working on My Last Tesla Coil™ (AKA The Heirloom Coil) again, slowly.  One of many fiddly little things which need to be done on it is the connections to the primary coil.  Tesla coil enthusiasts have come up with half a hundred ways to make those connections, and nearly all of them suck. 

 I already went to extraordinary lengths to make it easier to bring good, short connections to the primary, which would not stick up above the primary (thus providing fewer targets for secondary strikes).  I've never seen any other coil builder do this, so naturally I've got to be speshul...

 So for some months I have been pondering some sort of connector / clip / interface gizmo which:

• could be easily installed in any of the radial slots
• provides a solid but easily changed connection to the primary from below
• provides a solid connection to the tank circuit cable and takes the weight of same off the primary conductor itself

I now have a conceptual design, I'm ordering materials - mostly phenolic rod and tube - to make the plunger assembly and the support, uh, thingummy / puck / module / unit / doohickey, which can be seen in sketch at the top of this image:

The wide part will sit on top of the deck to support the weight of the cable, the narrower part below that rides in the slot as a guide and mechanical support for the plunger. The hole through the center will actually be much larger than the one shown on the sketch, as originally I was thinking the plunger would be a brass bolt, but I have made it much larger.

 The hardware in the center of the photo is the prototype clip I made, which will go on the end of the plunger, and makes the connection to the copper ribbon of the primary coil.  I'm quite proud of that little thing.  It's nothing more than a .75" wide piece of copper strip, formed to fit in and around one of those little black steel "bull dog" paper clips, like this one:

 The spring steel of the binder clip maintains firm clamping pressure while the copper maintains a nice low-impedance path for the high RF circulating currents.

 Making that little bastard took nearly two hours, and I need two of them.  I suspect I won't need more than an hour to do the second one, now that I know how to do it.  The clips will be soldered to the ends of brass plungers made of .75" tubing, which in turn will slide inside phenolic tubes having closed ends with only a slot in the top, allowing the insulating tube to be fitted over the primary winding, like this:

 This somewhat over-engineered design is intended to cut down on corona and arcing between the primary connectors and adjacent windings.  I've never seen any other coil builder do this, so naturally I've got to be speshul... 

(wait, where have I heard that before?)

 I'm hoping to give this coil to a friend and brother, if he wants it. (he doesn't know that yet, and I haven't asked, and until just recently, he would have had no place to store or operate it - but he just moved into a bigger place with loads of room).  But he loves Tesla coils, he loves this kind of craftsmanship and attention to detail, and I suspect when he sees the finished primary taps he will lose his mind and that makes me happy.  I like it when grown men giggle.

 The thing is... I've built literally dozens of Tesla coils, ranging from a minimalist thing made with two transistors, two resistors, and a flyback from a TV all the way up to twelve kilowatt monsters that made arcs ten feet long.  When I decided to get out of the coiling hobby, I told myself I would build one more "really nice one" (just very general thinking at that point) either for myself, or if I needed the dough, it had to be something nice enough, and reliable enough, to sell for good money.

 So this thing is supposed to be one of a kind - something special.  I don't call it "museum quality" because what "museum quality" really means is that something rugged enough to stand up to being handled and operated either by the public or by unpaid volunteers, and built to last - but that does not mean it is the best of its breed.

 I mean for this coil to be as good as it can get without going to a solid state modern design - I'm a romantic at heart and I like spark gaps, although the consequence is that this coil isn't as efficient, and it will never sing music. 

 Call me a purist or a snob, but I don't really need my Tesla coil to make music.  They make terrible  loudspeakers anyhow, and I'm a music and hi-fi lover too.  Lots of big, long arcs and throat-searing quantities of ozone are music enough to this old-fashioned mad scientist's ears.

Tesla coil completion & "first light" tests

During the last week, I finished 'My Last Tesla Coil', at least insofar as it can be operated and is not an ugly mess to wire up. I was moved to push hard and get it working before Saturday March 23 (six days from today) which is the scheduled date for two grand opening parties for two businesses run by friends of mine.

The owner of The Concoctory wants the coil to be present and running as an attention-getter near the store.

Down the street and around the corner is a great little café called Cafe Crescendo, who are also opening that day. I gather that between them, they are trying to get the other local businesses to join in a sort of block party to draw attention further south to all of the existing and new businesses which are opening in that (newly renovated) south Broadway neighborhood.

Clever, fun, smart people are behind both operations and I hope I can add some sizzle and excitement to their opening day.

Yesterday, I set up the coil in the parking lot of Denhac, Denver's very own local hackerspace, and fired it up. After several extra trips to various places for forgotten parts and to put gas in the generator, and then perhaps fifteen minutes of the usual first-time tuning of the primary tap, adjusting the safety gaps in the power panel, and adjusting the main spark gap (an air-blast or "sucker" gap, the first of at least three gaps to be built for this coil) I obtained some reasonable results; approximately 36" streamers from the toroid.

I was expecting and hoping for closer to 60" streamers, however two problems rapidly became apparent during yesterday's tests:

1) the alleged "3,000 watt" generator (the only one readily available at no cost to me) is either failing to deliver its full rated power; or perhaps it is getting some RFI or spikes from the coil, causing its regulator to reduce the RPM; or -- and this seems the most likely cause to me -- the reactive load is causing a low power factor, resulting in more current being drawn from the generator than necessary and less "real power" being delivered to the load.

2) the spark gap was not quenching reliably (or at all). I don't have E/H field antennas for an oscilloscope which would confirm that theory, but I have enough experience with Tesla coils that I know a badly operating spark gap when I see and hear one. This gap (described here) uses an air blast to encourage the arc to extinguish (quenching the primary circuit) immediately after it has established. This is the desired "opening switch" operation of a spark gap, and it is much harder to achieve than the initial "closing switch" operation which precedes it in each firing cycle.

[aside: "opening switch" is a term of pulsed power jargon which refers
to switches -- there are a variety of methods -- which are designed to

interrupt current flowing in a circuit as quickly as possible. Opening

switches are frequently used to transfer energy between stages in a
pulsed power system.

A brief pulse of current is made to flow from a charged energy store
having a relatively long time constant into a following stage having a
shorter time constant, by way of passing electric current through an
already-closed opening switch. When most of the energy has been
transferred into the next stage, the switch is opened, causing the current
to cease very abruptly. This "traps"the transferred energy in the next
stage, whether the energy is being storedin an electric field (capacitance)
or a magnetic field (inductance).

When the energy is again transferred out of this second, "faster" stage
into either another power amplification stage or the load, the smaller
reactance of that stage causes the pulse to be delivered in a shorter time.
Because the same amount of energy is now being transferred in a shorter
period of time, the peak power increases proportionately, obeying the laws
of conservation of energy.]

While I have experimented with air blast gaps in the past, this gap was my first experience with the "sucker" design, which pulls air into holes in the electrodes, creating an axial shearing flow across the faces of the gap. While I was tuning up the coil, I noticed that the gap operation was not being affected much by the amount of air flow.

The gap did not work well at the original electrode spacing. With a conventional gap, if you want to increase the voltage at which the gap fires, you open the gap spacing. In a sucker gap, once the gap spacing exceeds the cross sectional area of the gas ports in the electrodes, the air speed across the faces drops dramatically, which spoils the quenching action.

The solution would seem to be opening the holes in the electrodes a bit (which I did after yesterday's test run) and reducing the gap distance, relying on the air flow to quench the gap rather than a long arc distance, which merely increases resistance in the gap and thus wastes power badly.

Some finessing may be required on one or both electrode faces as well.

The coil tuned up (the system resonates around 105 KHz) at 9 turns on the primary. I was shooting for 10 turns and there are 14 turns available, so that was agreeable. It would not hurt to have a slightly lower capacitance in the primary tank capacitor, which would allow (require) a few more turns on the primary. Counter-intuitively (because turns ratio in a Tesla coil doesn't matter as much as one might expect) increasing inductance in the primary tank circuit acts to reduce the peak current through the gap. Because gap losses increase as the square of the current, a small reduction in primary circulating current results in a large reduction in lost power. This improvement in efficiency completely swamps the small change in output which might otherwise result from reducing turns ratio.

So, before next weekend, I've got to find a slightly beefier generator. It's probably that some PFC capacitors on the line side of the HV transformer will reduce the amount of current being drawn from the 120V supply, but that's unlikely to happen in the next six days.

I also need a better-working spark gap. I have already modified the existing spark gap, but it will take a little experimentation before I can confirm whether the changes make enough of a difference.

It might actually be possible to build a Richard Quick -style multi-gap before next weekend, especially since I started work on one over a year ago; I have all the electrodes fabricated (but not yet drilled) and I probably have a suitable muffin fan in stock...

MLTC progress

I completed the HV section of the power cabinet for 'My Last Tesla Coil' last night. This morning I powered up the whole cabinet and verified that:

• no flashovers or evident leaks, no visible or audible corona at full volts

• full volts at both HV connector ports - no corona or leakage from HV rear panel connectors

• no breakdown problems with the RG-213 cable I'm using for HV leads between the power cabinet
and the coil

• the Terry Fritz filter section _appears_ be okay

In short, I wasn't really expecting everything to work fine the first time. There is no front panel on the power cabinet yet, so there is no way to control power to the HV transformer, but the coil is a LOT closer to First Light and tune-up testing than ever before.

There is an event in Denver at which this coil is wanted for a Grand Opening near the end of March. We'll see.

Tesla coil and "why so many projects?" also, "why don't you ever finish anything?"

1. Tesla coil
I am making headway on the Tesla coil, and have redoubled my efforts (in between trying to do real work for real money, about which more down below) because a friend is opening a very cool new art-nerd / techie arts & crafts type store in Denver, and very much wants to have an impressive Tesla coil at her grand opening in March, which is, um, roughly a month away. Oi. We'll see. I'm trying.

2. "Why do you have so many (unfinished) projects?"
I suffer from ADD - Attention Deficit Disorder, and have my entire life. It's been marginally worse since I had a protracted fever a few years ago. I now take Ritalin, and it has changed -- or I should say 'is changing' -- my life. Sadly, that's 45 years too late, but better late than never.

In the past, whenever I hit a snag in a project that I couldn't get around, I tended to find other things to occupy my hands while thinking about how I could get back to work on the other project, with the least added effort and expense. Quite often it's lack of an expensive part of material that holds me up. Whatever the reason that one, or two, or three projects are on hold, I need something to occupy my time.

Another reason for having multiple projects is simply that some of them are inter-related. I need a good working vacuum system (among other things) for several other, larger, experiments I'd like to do. The micro-Marx pulser is needed to trigger the experimental gas switch I've completed, but have not yet tested.

And while I have had a life-long tendency to take too long to finish projects, I am in fact finishing projects now that I am on the Ritalin. I have more perseverance and much more focus than ever I ever enjoyed before.

Unfortunately, having my job yanked out from under me due to a reorganization left me unable to pay for materials and parts. I also need to spend time looking for work AND trying to identify and reach my own customers for work that I can do with my own resources.

And finally, health problems of one sort or another continue to interfere with my ability to get things done.

All that said, more progress has been made on my various "mad science" / research / hobby projects in the last year than in the previous ten.

I've also been posting videos occasionally to my YouTube channel, which is HERE.

short update on Tesla coil

Haven't had much time or money for my science projects lately, but I am making plodding progress on the Tesla coil. The only remaining work is on the power panel (which, unusually, includes the HV transformer and its protection devices) and the enclosure for the primary cap.

Two additional spark gap designs are planned, but only one of the three has been finished.

"First Light" (ie, the first power-on tests and tuning) should occur some time before Christmas.

Here's a short video showing some parts I finished recently:

http://www.youtube.com/watch?v=MNoBmwaZFaQ

new video - rebuilding variable transformers

I use variable transformers* a fair amount, most often in crude high voltage power supplies, such as Tesla coil power supplies. They are expensive to buy new, so I am always on the lookout for good deals on used units. Often, an old variac will have defects from abuse and will need a bit of TLC before they can be put back into service.

The item in this video was one such. It had been left sitting in a single position for many years, while more current than it was rated for ran through it. This resulted in one area of the (brass) slip ring being badly eroded and pitted, the (copper-graphite) slip ring brush being badly pitted and eroded, and some of the winding contact areas being pitted.



The (graphite) winding brush was in good shape thankfully, as it would have been a bear to replace. This first video shows how I turned down the surface of the slip ring to remove the pitted areas. The slip ring is mounted on the underside of the rotating brush plate, so you can't get a good idea of its condition unless you take the variac apart. Before you buy an old, used variac, it would be a very good idea to examine its brushes, its windings, and its slip ring carefully. If you didn't do it before you bought it, you would be smart to do it afterward before you use it, to avoid unpleasant surprises. Even if you don't own a lathe, you can do a lot of good to the slip ring with steel wool or a Scotch-Brite pad. If you decide to clean up the contact area of the windings, beware that these are often plated with a thin layer of brass to increase wear resistance vs. bare copper. If at all possible, you want to leave as much of that brass plating on the windings as possible, to extent the life of the unit. Frankly, for occasional amateur / home use, you probably won't put enough wear on those windings for it to matter, but there's no sense wasting good workmanship if you can avoid it. Not all variacs have this feature, some just provide a bare copper surface.

_____
* commonly called "variacs" by old timers - Variac™ was a trademark of the now-defunct General Radio Corporation. Other common brand names include PowerStat™ from Superior Electric and Staco, both of which are still making and selling excellent products. Finally, there is an importer of Chinese variable transformers who is using -- legally or not -- the name "Variac" in their company and domain names. Whether or not that company bought the trademarked brand from Teradyne (the inheritors of the Variac brand) I neither know nor care, because I have examined the build quality, wire size, and so forth of their units, and they do not seem worth the pricetag. Caveat Emptor!

even yet more further additional progress on my "Last Tesla Coil", book two, part twelve, chapter 27...

You might think this is the nearly-finished primary of a Tesla coil. And you would be reasonable to think so. But you would be mistaken. This is a laceration engine.

Well, I couldn't wear gloves because I needed the dexterity and the clearance between turns. I tried. I tried rubber gloves, but they didn't protect, just as I'd expected.

After I get done putting Band-Aids™ on all my fingertips, I'll have one more go at straightening and evening-up the turns, leaving them a bit tighter (to keep them from shifting -- as much), which is to say less of a perfect spiral, and more of an octagon. Then I will cut the outer end to its final length and screw it down. Wee! All this should be done a few hours after this post goes up.

It will be badly tempting to lash up a primary circuit for it just so I can try it out, but although I have most of the parts, nothing is put together. Also, I don't have quite enough capacitance, methinks. Gotta go through the calcs one more time after I measure the as-built primary inductance and plug it into the formulae. Furthermore, the HV transformer for this coil is practically unreplaceable, and I'll be damned if I will blow it up, which means I really need to finish the "Terry Filter" for it before running it. *sigh* Did I ever mention this? This coil will be fed by DC, so no resonant charging. I wish to be very kind to my precious 120mA NST, and I am effectively isolating it from the primary circuit entirely.


I wish there was a way to make that primary stay this pretty pink color forever, but there isn't. If I spray it with acrylic now, the acrylic will get all over the finished wood and mess up its finish. It wasn't practical to spray it before installing it - I thought about it for all of ten seconds. There's over 90 feet of 1" copper strip here, so I'm not gonna be dabbing Futura on it with a cotton swab, or laying it out on the sidewalk to spray it.

I'll spray or coat (Futura floor wax also works great) all the brass. That'll have to be enough shiny. The copper will all be dull brown in ten years.

I'll take a "glamor shot" of the complete Tesla coil primary and secondary -- minus one minor dodge I have to add to the strike rail -- later tonight.

Okay, it's later:


Inquiring minds will want to know what else needs to be finished before it runs. Here's a laundry list:

* primary capacitor - this will be an MMC of course, the most reliable sort there is. I have most of the caps for this, bought as a "finished" (bleh) MMC, but I am not comfortable with the number of caps in each string (determines the maximum safe operating voltage of the complete array) so I will need to add a cap or two to each string. Since that lowers the capacitance of each string I'll need to buy enough additional caps to add another string or two as well. The cap will be built with brass "strapping" terminals mounted on the exterior so that any number of strings may be placed in or out of circuit.

* spark gaps - I have one spark gap completed, but intend to build at least two more. The finished one is the "sucker gap" previously about posted here. I've got a disk, hub, and shaft done for an asynchronous rotary. I've got all the electrodes made, but not yet drilled, for a Richard Quick style gap. I've made nothing else nor thought about the housing for that. I'd kinda like to make a gap like one that Old Nik' was known to have used in his original NYC lab before it burned to the ground. It was just a bunch of brass balls in series. Not the best performing gap, but if memory serves, that was long before he'd invented several new kinds of fast-quenching gap switches.

* control panel - again, I've got most of the expensive parts for this, but there's a lot of finicky work to be done on said parts. I'll need to make new cards for the meters. I'll need to obtain a few specialty parts like a current transformer. The HV transformer is HEAVY so I'm considering building a cart for the control panel rack cabinet (which will be heavy enough without the transformer in it) and possibly putting the transformer in the cart/base. I might leave it in the panel cabinet and use the cart/base for storage of cables and parts. I haven't thought very much about that. And there's surely a ton of small parts I don't have, like controls. Much work to be done there.

* the damned primary tap connectors. These are gonna be finicky to make, but a joy to have and use versus any other primary tap method I've ever seen used by anyone, anywhere. So there. Okay look, that crazy rotating thing with the sliding brushes doesn't count - that's not "tapping". Yeah. So anyway, I think I've got the design worked out. I haven't started collecting various red metals (copper, brass, bronze) yet. And I have to make two of them. Bleh.

* cabling & miscellanea - the ground connections, a ground rod kit with straps, the umbilical between control panel and coil, to name a few examples.

So there is still a long, long way to go before we see first light on this beast.

PS: I noticed just now that pictures in some of my older posts are missing. I'll look into this in a bit. Probably nobody but me noticed anyway.

more progress on "My Last Tesla Coil"

Here is a sight a few of my friends thought they would never see. The primary deck and all of the primary supports are finally complete. It may not look like much, but it's been a great deal of hand-sawing and sanding to get to this point.

This was also the day I I cut the two pieces of the strike ring and installed them. Those bits - including the strike ring insulators - will come off before I varnish the primary supports.


I have a bit of sanding to do on one or two primary supports before I start varnishing, but they are essentially done and they are installed semi-permanently. I refrained from using glue on them in case one gets broken off at some date in the distant future. With only screws holding them on, it will at least be possible to repair them.

I should have the varnishing of the primary supports done in a week, at which point I will start installing the primary copper. I will probably give them either a clear coat (depending how it looks on a test) or a very light stain and urethane, because these supports are hard rock maple and have a beautiful grain.

Oh by the way, the ground connection to the strike ring is already made, at least as far as the primary deck ground connection that exists so far... can you guess where and how?

PS: oh, and yes, that strike ring constitutes a shorted turn, an undesirable thing. I remember thinking a long time ago that I should do something about that when I got around to cutting it to fit and installing it. And now that I've done that, I still can't decide what to do about the shorted turn issue, because everything I've thought of so far seems ugly. I will probably conceal a break within an insulated sleeve concealed inside a slightly oversized mounting tab.

Tesla coil primary deck progress

Due to being unemployed for three months and my arthritis acting up badly, I've been largely ignoring hobby projects lately. I've always got an excuse. But today I finished cutting and routing the slots in the primary deck.

The slots are for access to the primary windings from below. One of the things that has always bothered me about most Tesla coils (including mine) was that the adjustable primary tap connection was nearly always brought out from below the primary deck around the edge and over the top. This had two effects: it wasted inductance in the primary circuit (ie; stray inductance which does not contribute to the magnetic field of the primary) and it created a convenient high spot from which corona leaders could start - a place for the secondary streamers to strike.

I have a little cleaning up to do, then I'll touch up the varnish in the slots, do some more finishing work on the bottom, and then mount the primary supports, and finally re-coat the whole top deck.

More as it happens. Stay tuned...