Re: Step Up Transformer
dexter, Fri Jan 13 2017, 05:36PM

since you're using a half bridge you can easily use just 2 diodes in a voltage doubler configuration

Re: Step Up Transformer
Hydron, Fri Jan 13 2017, 07:05PM

A standard voltage double won't work for this (he's not running a fixed DC bus, but is instead using the AC line to give half-sine shaped voltage impulses), though the Villard circuit (as used in microwaves) might be worth investigating/simulating, as it level shifts the AC waveform rather than giving a DC output.

Re: Step Up Transformer
alan sailer, Sat Jan 14 2017, 03:11AM

Hydron is right I'm using the AC input as a small ramp.

If worse comes to worse Ill just use the existing transformer.

Cheers.

Re: Step Up Transformer
dexter, Sat Jan 14 2017, 10:05AM

Sorry... totally missed the RSSTC and stacato part...

Since I'm fixen on "no transformer" upgrade the bridge to full bridge to utilize the full mains voltage :)

Re: Step Up Transformer
alan sailer, Sat Jan 14 2017, 03:03PM

Dexter,

Yes I'd like to avoid the transformer. But 220 outlets are rare where I live. I only have one that I put in years ago for a table saw.

I have another puzzling data point. Last night I put in a smaller transformer ~200VA instead of the 500VA unit I am using. Foolishly I turned it on at full voltage (I'm using a variac during the testing stages) and the half bridge became a no bridge.

I'm curious why this would happen. The output voltage (no load) for both transformers is the same, about 250VAC.

Cheers.

Re: Step Up Transformer
dexter, Mon Jan 16 2017, 10:30AM

Sorry... totally missed the RSSTC and stacato part...

Since I'm fixen on "no transformer" upgrade the bridge to full bridge to utilize the full mains voltage :)

Re: Step Up Transformer
Uspring, Mon Jan 16 2017, 02:38PM

alan sailer wrote:

I have another puzzling data point. Last night I put in a smaller transformer ~200VA instead of the 500VA unit I am using. Foolishly I turned it on at full voltage (I'm using a variac during the testing stages) and the half bridge became a no bridge.

Transformers can cause a significant overshoot on the secondary if turned on at the wrong time, i.e. at the peak of the primary sine wave. Peak secondary voltage can be up to twice its normal value.

Re: Step Up Transformer
klugesmith, Tue Jan 17 2017, 03:56AM

>> Transformers can cause a significant overshoot on the secondary if turned on at the wrong time, i.e. at the peak of the primary sine wave. Peak secondary voltage can be up to twice its normal value.

Are you sure about that? What makes it happen?
For core saturation and high inrush current, zero voltage crossing is the worst time to switch on a transformer. (One slope is worse than the other, depending on the direction of remanent flux from the last time transformer was switched off.) Core flux settles down to its normal, symmetric-around-zero swing soonest if the primary is switched on at a voltage peak.

Re: Step Up Transformer
Uspring, Tue Jan 17 2017, 10:27AM

You're right, klugesmith. I made a gross error in my calculations. I think, correct is, that when you turn on at 0 crossing, there can be a tiny bit of overshoot of about 5%, due to primary resistance.

I haven't thought about saturation effects, but it looks to me, that the effects on the secondary side, as alan sailer worries about, might be less dramatic.

Re: Step Up Transformer
alan sailer, Wed Jan 18 2017, 01:20AM

If I get a few spare IGBJT's I may try messing withe the smaller step-up transformer and try and see what caused the death of the devices. I'd monitor the CE voltage on the devices at a low voltage and see what is going on. I have a nice diff probe that will allow this.

Back to the original (500VA) transformer, I've been turning it on at full input voltage (130VAC) for weeks now and have had no problems. Sometimes the coil doesn't start up but I'd assumed it was because I randomly turned it on at a zero voltage point.

The coil has no explicit method of starting.

Cheers.

Re: Step Up Transformer
Steve Ward, Mon Feb 20 2017, 01:17AM

A level shifter (like what VTTC and microwave ovens use) might be a good option for a ramped SSTC as you still get a sinusoidal output that is simply offset. You need a large capacitance to hold up a SSTC's energy consumption, so an electrolytic type would be most convenient. It must be large enough that it does not get over-discharged, which can result in reversal of the charge polarity on the cap! You could add a diode to protect the capacitor from accidental voltage reversal, but if it suffers from this issue, then the capacitor is significantly too small and should be bigger.

For a coil that draws 20A peak off a ~300V peak line, i find a 2200uF shifter cap to be suitable, and up to 3300uF being about the point of no extra benefit. The diode sees significant peak current, so it should be oversized for 80A peaks. The capacitor needs to have a voltage rating of at least the peak line input voltage (so 200V probably). You should be able to scale these values based on the current demand of your coil. I've attached an LTspice simulation of the simplified circuit which includes line parasitics and protection diode and assumes the tesla coil looks like a constant current sink (at least its roughly true).
]ramped_sstc_level_shifter.zip[/file]

Re: Step Up Transformer
alan sailer, Mon Feb 20 2017, 04:54PM

Steve,

Thanks for the information. I'll look into learning about the concept.

The .zip file opens up an .asc file which I can read using TextEdit on a Mac. But the file means nothing to me. Is there a program that runs an .asc file?

On a side note, thanks so much for all the information that you have posted about tesla coils. So far I have found it very useful and accurate. I did a version of your Mini SSTC that is working fine. (after I sorted out various grounding issues and blew up at least three Mosfet's).

Cheers.

Re: Step Up Transformer
hen918, Mon Feb 20 2017, 05:30PM


Yep:
LTspice:

Re: Step Up Transformer
alan sailer, Tue Feb 21 2017, 01:32AM

hen918,

Thank you! I can't believe that I did not see that earlier. I'm trying to do to many things as once...

LTSpice? You mean I have to get smarter now? It could be painful

Cheers.