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Messages - T3sl4co1l

Pages: [1] 2 3
1
Solid state Tesla coils / Re: tesla coil varnish is dumb
« on: July 14, 2019, 09:59:40 PM »
Huh.  What's the point of varnishing the outside, if there's air between the wires, under the tape?

I have one experience with water-based urethane; I was thoroughly underwhelmed by its performance.  It cost much more, went on much thinner (it's mostly water, so the dried film is quite thin and many coats are needed), is softer (though still strong enough not to be peeled off by fingernails alone), and seems to turn a bit gooey/sticky with prolonged contact with oils.

There probably are better-performing mixes, and I happened to get the worst one out there.  Ironically, I feel like the stuff I had, wouldn't have given you trouble -- it doesn't seem brittle enough to crack.  But then again, vinyl tape is rich with plasticizers (basically, oils that turn the plastic gooey -- pure PVC is a brittle white solid -- see where this is going?), and maybe that's strained or weakened your coating, causing it to crack.

Varnish is kind of dumb anyway, since at the voltages we're talking here, you need whole cm's of thickness to effectively insulate something.  Modest layers can still provide some protection, such as corona suppression or protecting against smaller streamers; I don't have enough experience in this direction to know how significant this is, so I'll let others fill in.

Tim

2
If that's the traditional combination of materials, it probably won't be very conductive, and the coating is a flux/slag material that tends to bead up on the surface and drip off (presumably can be refreshed with the right goop).

SiC is a modest conductor when heated, making microwave heating possible, and maybe induction still (but maybe with higher voltages/frequencies than are available here).

SiC-graphite crucibles can practically be quenched in water; they're very tough, as ceramics go. :)

Tim

3
Yes, exactly.  There is a best-load matching condition.

Too low resistivity, and there's no voltage drop in the work, the magnetic field reflects nearly perfectly, and frequency simply rises, instead of DC current being drawn.

Too high resistivity, and there's no current flow, the magnetic field penetrates nearly perfectly, and little DC current is drawn.

We know that, somewhere inbetween, current consumption is higher.  We don't necessarily know if we're on the rising or falling slope of that curve, or optimally on the peak, but we know definitely that there must be a peak, somewhere between these extremes!

This has been today's shop application of calculus.  Cheers. ;)

Likely, it just so happens that a full heel of molten copper is below the peak, while a chunk of steel above curie temperature, is above the peak.  Graphite of this thickness is certainly above.

Different people have different setups, landing above or below as well.

The positioning of that peak, is driven by the number of turns of the work coil, how close it is to the work, the characteristics of the power supply, etc.  Frequency affects skin depth, which affects the equivalent load that a given chunk of material reflects.

If you're below the peak, I would suppose, try adding another turn or two; if below, remove a turn or two.

This type of supply also delivers the most power into the lowest Q factor, and the lowest impedances.  Q can be lowered by reducing the distance from coil to work.  (You need some space for insulation, and loads like copper won't give a terribly low Q anyway, even at point blank distance; there's only so much room to push in this direction.)  Impedance can be lowered by reducing inductance and increasing capacitance.

Tim

4
When the load resistance is low (e.g. cold copper), it tends to reflect rather than absorb magnetic field; this reduces power consumption compared to a better matched load.

Whether current draw increases or decreases with heel size and temperature, depends on which side of best match you're running at.  Which in this case I think depends on the crucible thickness, and porosity (overall conductivity), as far as how much magnetic field it absorbs before reaching the copper within.

Tim

5
T3s---:
I don't have any recent experience with "the more complicated circuits", but, regarding the "ZVS Royer circuit", please tell me more about:
Quote
---doesn't always start up at all, or at the intended frequency (more a problem for high frequency oscillators).

I ask because I don't think I have had either of these problems.

Startup is an issue at very low Q factors.  Transistor gain is quite high so it's not usually an issue at common Q factors.

You might just see it with copper coils tightly fitted around steel pipe, or with lossy coils (say a stainless pipe coil used for process heating).

Frequency is a problem for this circuit for example,



which runs around 500kHz, but can lock into other modes in the 400-650kHz range depending on what load you have attached.  The problem is if there is extra capacitance at the load, in addition to what's on the oscillator board itself.  The connecting wires between oscillator and load form another resonant circuit, and the frequency response becomes much more complicated.  In this case, the transformer is a step-up and its secondary has a resonance near this frequency, even if I didn't put explicit capacitors on it.

As I said, you're less likely to have problems with this, at low frequencies where the capacitors are more likely to act together.

Tim

6
For a typical e.g. N87 material, at that frequency and size you will want Bmax ~ 70mT, maybe more or less.  Expect a core around 70mm o.d. (toroid) or ETD50 (E-E style, various cross sections).  This should be a good starting point; vary size larger or smaller depending on how much winding area is needed.

Flux density Bmax gives the volts per turn:
Vpk / N = 4*Bmax*Ae*Fsw
for a square wave with full duty cycle (if this is a full-wave forward converter or the like).  Ae is in the datasheet, and N is the number of turns.

What leakage inductance do you need?  Litz has the regrettable property that, because it's transparent to magnetic fields (that's how it works), all the space within as well as around the cable counts towards leakage inductance.

For example, if you're making a welder with DC output, you'll need leakage low enough that it doesn't burn out your diodes.  That would be, uh, well 0.5*LL*Ipk^2 is the energy in the leakage, so the power dissipation (as diode avalanche or burned in a snubber) is that times Fsw.

If you use about a meter of cable for the winding length, with no interleaving, just overlap typical of a toroidal transformer say, then expect around 0.6uH of leakage.  Which would give, at 80A and 200kHz, 384W of dissipation.  Indeed, 0.6uH would drop 60V at 80A and 200kHz -- this is actually so much leakage, compared to your load impedance (less than an ohm!), that such a transformer would actually be practical in a resonant topology!

If low leakage is a concern, consider using alternating layers of foil on a rectangular style core (EE?).  A planar transformer is also not unreasonable, but gets a bit challenging to design at this scale.

Tim

7
Do you think, there is an principal limitation to the ZVS Royer circuit, like efficency or maximum Power output, compared to more complex used circuits?
Except for the savety and that it's difficult to adjust your output power.

And stability, because it's just a dumb oscillator and doesn't always start up at all, or at the intended frequency (more a problem for high frequency oscillators).

So yes, that's all.  Like how a car is an engine on wheels, with steering.  It's not like you need a throttle, or brakes, or a windshield. ;)

So, that's why we design more complicated circuits.  They're not very complicated really.  Tesla coil drivers have been made worse than what's needed for this.  (My typical controller circuit is implemented in about 200 components, maybe not something you really want to build a kit of, but not at all impossible.)

Tim

8
Electronic circuits / Re: Induction Heater schematic modification
« on: June 06, 2019, 09:08:12 AM »
Hah, I forget that Uzzors used some of my bitmap schematic symbols from back in the day. :)

Yeah, that's about right, though again it doesn't have an enable, so it's always flat out and you don't have any way to stop it if something goes pear-shaped.  Like if the inverter draws destructive current or such.

Tim

9
You may find this of interest:
http://hamwaves.com/antennas/inductance.html

Of course it doesn't give loaded Q, but unloaded L and Q at least are nice.

From Q and applied voltage, you can calculate coil losses, and then idle current, and get an idea of what loaded current consumption will be.

Loaded Q is best guesstimated from ratio of enclosed areas, and typical values for materials.

Tim

10
Electronic circuits / Re: Induction Heater schematic modification
« on: May 27, 2019, 09:18:54 PM »
Depends on the load.  The most extreme possible case would probably be around a 2:1 change, for a steel (~half freq) or copper (~double freq) pipe fitting tightly inside a solenoid coil.  The change under operation would be due to, say, melting a slot in said pipe, breaking the circuit.

Which would also be a fairly sharp transition when it breaks.

Frequency is normally starting high, but this circuit has no enable or startup condition so it reduces to a power-on case, which is around zero since the filter capacitors are ground referenced.  Which is, I forget what the 4046 does, is that minimum frequency?

Tim

11
I wouldn't be concerned about frequency, or stirring.  At least not until you're looking at aerospace alloys.

0. Use a thick, fairly conductive crucible (e.g., steel) to shield the melt from magnetic field.  (Marginal for aluminum, and not practical for cuprous alloys.)  Note that graphite has to be quite thick to provide much shielding.
1. If stirring causes oxidation, prevent oxidation (duh :) ).
1a. For cuprous alloys, use a cover of slag.  This should be mostly silica, lime and soda; you're really just going for a melted glass of modest viscosity.  Straight-up borax is too thin and will pour out with the metal.  (Add sand, alumina, clay, whatever to thicken it.  Add fluxes (borax, soda, lime..) to thin it.)
1b. Use a charcoal cover, or other reducing agent.  Using a graphite crucible to begin with, already helps a lot (graphite burns at red-hot temperatures, slowly but surely -- the thin blue flame is carbon monoxide burning off).
1c. Without any need for air in, or exhaust out, there is the option of using inert gas, or even vacuum. :D This is probably harder to pull off (e.g., you have to diffuse the inert gas into the porous refractory first, otherwise it's not doing much), or difficult in general, and expensive (vacuum hardware, plus feed-thrus to handle everything under vacuum?), but absolutely a possibility, something to think about in the long run maybe.
2. If stirring causes hydrogenation, just do a degassing step.  This is normal practice with aluminum.  May be harder with cuprous alloys.  Maybe the hydrogen can be burned out, say by dropping some CuO into the melt (push the slag aside so it doesn't dissolve into that)?  Maybe that doesn't work with zinc present, dunno.  Inert gas sparging will work for both, in any case.
3. Stirring probably causes a lot of slag entrainment (in aluminum, aluminum bronze, etc.), which is a good reason to use a flux (for aluminum, NaCl+KCl eutectic is fine, or you can get commercial mixes that include some fluorides as well).  This may introduce more hydrogen, so again, consider degassing.

Regarding removing caps -- for the same power, you need probably proportionally higher voltage, or matching equivalent to this.  Keep this in mind as the ZVS oscillator output voltage corresponds to supply voltage, and is limited by component ratings.

Tim

12
Electronic circuits / Re: Induction Heater schematic modification
« on: May 26, 2019, 09:37:39 PM »
It may be closed loop eventually, but as mentioned it will be very slow.  Conditions on the tank can change much more rapidly than that (say if the coil gets shorted out).  Or if the CPU crashes.

Which is another matter: with care, this can all be done in software, but one tiny slip-up and the inverter explodes.  I wouldn't recommend an Arduino beginner to do that, at least unless they want to spend the money and time to replace all those transistors!

Tim

13
FYI, gypsum (the active ingredient in plaster) dehydrates in two stages: first to hemihydrite around 170°C, and then to anhydrite around 700°C.

Actually, looking it up, it seems it goes straight to anhydrite around 170°C, at least that's what the text claims.  But the amount lost is just less than the total.  But more than hemihydrite.  It's like it's... quintahydrite, as it were (i.e., 1/5th of an H2O). The final total is plausibly correct (the exact figure is 20.93% water in stoichiometric gypsum).  So now I really wonder if this paper is missing something, or if it has to do with material purity (it was only "90% pure"), or if no one really knew the truth about gypsum dehydration and just kind of went along with it all these years because, who cares it's just gypsum, right?

Anyways-- I've never had problems when heating it to dull red hot.  But I've always seen exactly the bubbling you describe: not violent, but still too much for the material porosity and venting to handle without leaving bubbles.

FWIW, I usually use 1:1 to 2:1 sand (fine or sifted sand preferred) and plaster, for simple casting investment.

Protip: when the metal has fully solidified, drop the mold in a bucket of water.  The plaster will spall off, freeing the casting in no time. :)

Plaster-based investment is also reusable, just dehydrate in the oven to get back to the active hemihydrite form (hardens when water is added).  Needs to be smashed up finely, of course.

Tim

14
Electronic circuits / Re: Induction Heater schematic modification
« on: May 26, 2019, 04:11:48 AM »
Don't use slow rectifiers for D5/6, use 1N4148.

D4 isn't really doing much.  It will draw most of the current through R10, that would otherwise flow entirely into U3's ESD diodes (and in turn from +15V*), but a schottky (e.g. BAT85) will do better.  Can also place a series resistor from D4 into U3 (say 1k) to ensure current takes the intended path. :)

*Little known fact about CMOS ESD diodes: they act like BJT emitters, with the base tied to VSS and the collector to VDD.  A fraction of the ESD current is drawn through the base, but the remainder is "cascoded" straight to VDD.  This can greatly increase the power dissipation, of a chip that otherwise shouldn't really be dissipating anything at all.  Activation of ESD diodes can also interfere with internal functions; you'd have to test the CD4046B (or whichever version you're using) to see if anything else happens.  In short, ESD diodes can be used on a repetitive basis but should be used sparingly and at low currents (< mA).

D3 is clamping the tank input to about -0.6V.  Is this intended?  Why not symmetrical? (i.e., to -V, with a TVS diode on that supply to ensure it doesn't get cranked way down.)  Also, if there's no shunt resistance, just the series resistors (which I suppose are for current limiting), then does U5 really need gain at all (it's set for G = 101)?  Seems like it could be an LM311 instead, or not used at all (just cap-couple to U3)?

The output driver is fine, but you may want an enable signal into it, to turn the inverter on/off and provide an input for fault protection (which in turn can be something like peak current detect).

The big problem though, is that it's open loop.

1. Why use a 4046 if you're not using it as a PLL?
2. Why drive the 4046 VCO from a -- very slow -- PWMDAC?
3. Why read the 4046 PC from a -- very slow -- ADC?
4. Why not do it entirely digitally?  Granted, this may not be very easy in the Arduino framework, but an ATmega is more than capable of generating the signals used here, and can respond to them much faster (and can be reprogrammed much faster than the wiring on a PCB).
5. Why not measure PC with a timer routine?  (Again, I don't know how difficult this is in Arduinoland.  I've got to imagine someone's written a routine or library for a function this basic.  Incorporating that function into the overall program, I don't know.)
6. Why not any voltage or current measurement, or controls?  (Okay, you want to keep it simple for starters.  But you also want to be able to expand later, and expanding in very likely directions is worth some thought beforehand.)
7. Why not any PWM?

The last one is actually a two-parter:
a. You need some dead time between gate drives, otherwise the inverter transistors may be on simultaneously, and that's a problem (assuming the usual, voltage-sourcing inverter, and depending on what exact gate drive circuit is used).  Normally, a small delay is introduced to one of the switching edges, effectively reducing the duty cycle of each channel, and this sets dead time.  Alternately, you can use a PWM generator like TL494, which has a maximum duty cycle (per channel) of 45% or thereabouts.

TL494 doesn't have a PLL.  It is still a VCO.  As you aren't using a PLL right now, it would be viable from where you are now.

b. There's actually a good reason to run at maximum PWM% all the time, depending on the type of inverter used.  In short, it's not actually very feasible to control PWM with a half or full bridge, and simple on/off (per cycle) PWM.  The problem is this: when the inverter is turned off from all sides, the output impedance is high, and that changes what the load is doing (namely, it allows it to ring down, discharging some of its energy into the supply rails, and shifting phase angle with respect to where your oscillator is still running at).  You actually have to use a three-level inverter (shorting the output to 0V instead of leaving it open circuit), or a full bridge with phase-shift PWM (which is hard to generate); either way you need four transistors to do it.

So, even as a professional, I tend not to do that, and just stick with full-wave operation, leaving PWM at max (i.e., whatever is needed for dead time, 45% or whatever).  I would rather vary power output by varying driven frequency (which is presumably what you're going to do here), or varying supply voltage (which requires a completely separate buck converter stage, but it does offer big wins for controllability -- the frequency-shift method is actually quite tricky to control).

Tim

15
Is that green core even ferrite?

Protip: don't take pictures so close your camera can't focus on them!

Tim

16
Again, you need a bigger transformer core, or more turns.

Tim

17
Hi, could you post a sketch of your circuit - to better understand how it looks/works.
I found identical gate drive circ described on page 53 of this paper:
https://pdfs.semanticscholar.org/1265/19da946b5df9b8941a521d5a7019720bcd0d.pdf
However not fully studied it yet...

Looks to be a cromulent paper as such, but seems to lack motivation, innovation and results (the poor efficiency at the end is particularly unimpressive).

That is to say, it looks to have all the trappings of an academic paper; but doesn't seem to have much analytical or engineering value.

Tim

18
Why do people make this circuit with all the extra junk?

It's an analog circuit.  It only works because it it has real linear gain at the resonant frequency.  If you try to sharpen up the gate drive edges, or, I'm not even sure what's being done here, some biasing/level shifting or local feedback thing, you're just making it more liable to oscillate at other frequencies, potentially very high frequencies that make it burn up!

The underlying circuit by the way is the Baxandall oscillator (or more accurately, one of the equivalent topologies presented in his titular paper); it's also commonly misnamed "Royer" but that actually refers to the voltage-fed, saturation-commutated version, with an otherwise very similar circuit.

Here's an example of mine which is unfortunately a terrible example for discussing the circuit,



it has basically the same circuit except Q401, Q405, D403 and R416 are deleted (Q401 shorted through C-E), and R408 = 1k.  And equivalent for the other side of course.  There's also a 0.47uF for the VCC bypass.  Runs at ~600kHz, a pair of FDP33N25 I think.

Tim

19
Electronic circuits / Re: Car starter on PWM
« on: April 06, 2019, 09:04:32 AM »
No catch diodes across the motor? :o

Tim

20
Solid state Tesla coils / Re: GDT Experiments
« on: March 20, 2019, 01:18:29 PM »
The one thing missing from Burnett's advice: reduce the driver impedance.

Was that testing with a function generator (50 ohms)?  Looks about right for it.

Tim

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