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Steve Conner

Sat Aug 02 2014, 09:27AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, I used several chokes in series just to reduce the voltage stress on each one. The chokes all survived the ordeal, 2 of them ended up in a valve hi-fi amp.

The impedance of the charging circuit is rather high due to the high voltage and low current, so you need more inductance for the charging choke than you think, but it's not really too critical. IIRC the resonant frequency between the charging choke and tank capacitor should be about one-half of the break rate you intend to run at.

If the inductance is too small, you'll get power arcing in the rotary gap because the capacitor will charge up before the electrodes have separated enough to hold off the voltage. I had some trouble with this, but only at lower gap RPM than I normally used. The effect was simply to blow the mains fuse.

As Hydron points out, it is probably more economical to use 2 MOTs driving a voltage doubler, than a single MOT driving a quadrupler as I did, but this was really more of a fun exercise to see how much power could be got from a single MOT.

The Tesla-2 was a great coil, it made some pretty big sparks and sounded like an air-raid siren. I only gave up on it because the rotary gap self-destructed (the motor bearings were shot and the electrodes melted) and the secondary got badly burnt.

Ash Small

Sat Aug 02 2014, 01:58PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Ok, I was going to just use one MOT for the same reason as you, Steve, but I think I'll go with the concensus and use two.

Presumably this means I can use the original microwave oven doubler circuits? Presumably I can wire them up for +/-5kV approx.?

I assume max break rate is a function of current/capacitance of tank capacitor?

(I think resonant frequency/2 < max break rate, because you only need one half cycle to charge the tank capacitor, although I did read something on Ritchie's site that implied that it's possible to run a faster break rate than this and still obtain full operating voltage, but I'd need to re-read it to fully understand all the implications)

I'm wondering if the reason the bearings failed in your RSG was due to EDM (electro-discharge machining), possibly indirectly due to induced currents?...but this is speculation. I'm considering maybe driving the RSG via a belt drive (I'd still need to mount it on a bearing), and using a variac to control RPM.

I only have one 2.5A variac, so this would mean using another method to reduce input power if I want to test the TC at reduced power.

Hydron

Sun Aug 03 2014, 06:54AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

The standard microwave doubler circuit is no good for tesla coils anyway (has extreme ripple on the output). Check out this wikipedia page:

, there are a few methods there (including the circuit Steve used for his quadrupler).

The "bridge" circuit is the standard (and most economical) option for voltage doubling without large ripple, but has the disadvantage in this situation of not having the output nicely centered around ground (it will float up and down by ~2kV AC). This is not ideal but may be acceptable given the simplicity and lowest parts count.

Your two options if you want a supply centered around ground are to use one MOT in the same configuration as Steve, or two MOTs, each with a "Greinacher" circuit (one of each polarity). This would look rather similar to Steve's circuit and require the same components, with the advantage being the greater potential power from two transformers rather than one.

If you had to go with Steve's circuit or the Greinacher circuit then you probably could re-use the microwave oven cap for the first stage (doing exactly the same job as it did in the oven), but the microwave oven diodes may not be able to handle the peak currents, and second stage "peak detect" portion would have to use larger capacitors (many series electrolytics with conservative rating and careful balancing, or a number of series/parallel matched microwave caps) and diodes (same considerations as with the electrolytic caps). A "bridge" circuit would require the same parts as the "peak detect" portion of the other options. Those with greater experience may be able to weigh in on the question of whether the microwave oven diodes are re-usable.

Steve Conner

Sun Aug 03 2014, 09:08AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Re the lack of a variac: I used a universal motor for my RSG, and a dimmer switch rated for inductive loads (low voltage halogen lights etc) to control the speed of it. This freed up the variac to control the input to the MOT.

Once I was confident with it, I ditched the variac and used a relay from a microwave oven, controlled by a big red button, to switch the MOT directly onto the mains.

Ash, I'm pretty sure I still have the voltage multiplier and de-Qing diode stack from Tesla-2 kicking around somewhere if you would like them. It should be easy to modify the multiplier to take input from two MOTs.

Hydron

Sun Aug 03 2014, 11:03AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

I'd take Steve up on his kind offer!

Should be extremely easy to use the remains of Tesla-2 - just split the connection between the first two capacitors, feed one from each MOT, the center connection remains as ground (connect to both cores). Or you can just use one MOT as Steve did, and graduate to two if you need more power.

The beauty of a DC coil is that theoretically a ballast and variac are unnecessary - the choke stops the gap from power arcing and the power level can be set by the break rate rather than the input voltage.

All that said, do what Steve suggested - you want a variac for initial powerup, and even your 2.5A one should be enough for limited runs at lower break rates, with the option to ditch it and connect straight to the mains once it's running well.

Ash Small

Sun Aug 03 2014, 02:18PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Hydron wrote ...

The standard microwave doubler circuit is no good for tesla coils anyway (has extreme ripple on the output). Check out this wikipedia page:

, there are a few methods there (including the circuit Steve used for his quadrupler).

Yep, I was studying that page again last night, before I came up with this:

Which I posted in a thread in the HV section (thread here:

). I posted it in a separate thread in case it was a non-starter, but the consensus is that it should work pretty much 'as is'. (I am planning to use a string of electrolytics for the output stage, if I try this circuit)

Steve Conner wrote ...

Ash, I'm pretty sure I still have the voltage multiplier and de-Qing diode stack from Tesla-2 kicking around somewhere if you would like them. It should be easy to modify the multiplier to take input from two MOTs.

I'll PM you Steve, thanks. I want to try using the microwave oven parts, and see what happens, but I imagine modifying your old quadrupler will be an improvement. Will the de-Qing diode be up to the job, though? I think I'll have (potentially) ~twice the current your Tesla 2 had, obviously depending on tank capacitor and break rate, etc.

The next job will be tackling the maths regarding tank capacitor, Fres, break rate, secondary and topload, etc., and working out how much inductance I need for DC resonant charging. All this will require a bit more research......

Ash Small

Sun Aug 03 2014, 11:44PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Continuing on from my last post, Intuition suggests I go for a big toroid, maybe one metre diameter by 30cm high. If this turns out to be too big for breakout to occur, I then have two choices, either introduce a 'breakout point', or increase the capacitance of the primary tank capacitor, as, from what I've read, it is the ratio of these two capacitances that determines the voltage in the secondary tank circuit.

From what I've read, secondary coil height should equal toriod diameter, so secondary height then becomes one metre.

I want to keep copper losses to a minimum, so go for 1mm diameter wire, enamelled, I'll do some maths later on regarding skin depth, once frequency has been established.

I then need to establish diameter of secondary. This will determines Fres.

(These are just some thoughts I've had sitting here enjoying a bottle of wine on a Sunday evening. Any comments on the above 'thoughts' will be welcomed. I'll post again in the morning.)

EDIT: Maybe 30cm is a bit big, 20cm (8") or so may be a bit more aesthetic, although I seem to keep reading that people eiter try a bigger toroid and get better results, or add a second toroid and get better results. I assume most people use one of the simulations or whatever to decide toroid size, but then get better results when they add more capacitance. I've also not found much in the way of deciding toroid size on any of the sites I've tried. Ritchie's doesn't say much. Steve Connor's pretty much just says 'it worked better when we tried a bigger toroid', Terry's site is down, etc., although I have read that if the toroid is too big and no breakout occurs, 'add a breakout point, or add more primary tank capacitance.

I'll probably try one of the simulations, though, once I've got some initial numbers to input.

Hydron

Mon Aug 04 2014, 10:43AM

Registered Member #30656 Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208

For reference, my DRSSTC spits out ~2m streamers/arcs with a 160x700mm (dia.xheight) secondary and a 160x750mm (heightxdia.) toroid, and could probably be pushed a bit further if I play with the current limit.

You should be able to get at least close to that streamer length with a spark gap and a similar sized coil, but I suspect it would be pushing a two MOT supply to try a larger coil.

If anyone else has comments on this, please reply, I've never built a spark gap coil, though I did do all the research into it about a decade ago.

Steve Conner

Mon Aug 04 2014, 01:43PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

This is the biggest spark I got out of Tesla-2.

For reference the secondary is 20" tall. I used almost 3x the bang energy the coil was designed for, and ended up burning out both the spark gap electrodes and the secondary. The single MOT power supply was probably delivering something like 2.5kW.

Re toroid size: the classical guideline is that the minor diameter of the toroid should be equal to the diameter of the secondary, and the major diameter equal to the height of the secondary. You won't go far wrong following this. Most people (myself included

) allow themselves to be influenced by whatever parts they can find to make a topload.

Ash Small

Mon Aug 04 2014, 01:50PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

Hydron wrote ...

For reference, my DRSSTC spits out ~2m streamers/arcs with a 160x700mm (dia.xheight) secondary and a 160x750mm (heightxdia.) toroid, and could probably be pushed a bit further if I play with the current limit.

You should be able to get at least close to that streamer length with a spark gap and a similar sized coil, but I suspect it would be pushing a two MOT supply to try a larger coil.

If anyone else has comments on this, please reply, I've never built a spark gap coil, though I did do all the research into it about a decade ago.

I'd also be interested in comments from others regarding this subject.

The dimensions of your coil/topload sound about the minimum I'd want to try. (Intuition tells me I want a big toroid and a secondary with low copper losses)

I assume decreasing the break rate would compensate to some extent the effects of a large primary capacitor, although another option would be to just add more MOT's and doublers. Increasing primary tank cap also means I can use a smaller choke in the resonant charging circuit..

Any further comments will be appreciated.

EDIT: Missed your post as I was writing my reply, Steve. Thanks for the input. That photo is what I was referring to when I said you got better results by increasing the size of the topload

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