Question about the half bridge in this circuit

[boylesg]

Sorry I misunderstood.

I had a look at your post #7 link. I believe the half-bridge and capacitors is a substitute for a full H-bridge. It is simpler to design and operate a power supply this way.

Switching frequency depends on how fast the mosfets are driven. They are switched according to what arrives at the antenna (at extreme left in the schematic).

My guess is that the antenna picks up EM radiated from the tesla coil, as the secondary resonates (or perhaps self-resonates). In this way the system automatically fixates on the resonant frequency.

I don't believe you need to adjust capacitor values to achieve resonance.
The capacitor values only need to be large enough to store juice during one half of the power cycle and then release it during the other half, without dropping too much in voltage.

If you read his text on the page with his sequential schematics, as he improved it, the antenna effectively overrides the input into the FET gate driver from the 555 square waves if they are not precisely at the resonant frequency of the secondary.

But what about what I observed in the circuit simulator? There seemed to be bugger all current through the primary coil if the frequency of the square wave input to the FET gates did not at least roughly match the indicated frequency, according to the equation, of those capacitors combined with the primary coil/inductor?

Surely if they do not roughly match the FETs will be fighting against the LC circuit formed by the primary coil and two capacitors?

 

[BradtheRad]

But what about what I observed in the circuit simulator? There seemed to be bugger all current through the primary coil if the frequency of the square wave input to the FET gates did not at least roughly match the indicated frequency, according to the equation, of those capacitors combined with the primary coil/inductor?

Surely if they do not roughly match the FETs will be fighting against the LC circuit formed by the primary coil and two capacitors?

I'm trying a simple simulation. It displays the same thing you state, several Amperes through the primary during one half of a cycle. These are ringing oscillations.

I doubt my values match those in the schematic. The transformer has several unknown specs.

This may be the chief resonance. It might just require slightly different values, and then the entire circuit might resonate at this frequency.

I see this scenario:
(a) On powerup, the 555 generates pulses, just to start things going.
(b) This induces ringing in the transformer.
(c) The ringing becomes the dominant frequency.
(d) The secondary winding radiates EM at this frequency.
(e) The antenna picks it up and feeds it to the amplifier. (The 555 pulses are overridden.)
(f) The mosfets are driven continually at the resonant frequency.

I guess this means you can choose any reasonable value for the capacitors.

I have no experience with tesla coils. I don't know if the secondary winding has a different frequency that it tries to resonate at. Etc.

 

[boylesg]

I'm trying a simple simulation. It displays the same thing you state, several Amperes through the primary during one half of a cycle. These are ringing oscillations.

I doubt my values match those in the schematic. The transformer has several unknown specs.

This may be the chief resonance. It might just require slightly different values, and then the entire circuit might resonate at this frequency.

I see this scenario:
(a) On powerup, the 555 generates pulses, just to start things going.
(b) This induces ringing in the transformer.
(c) The ringing becomes the dominant frequency.
(d) The secondary winding radiates EM at this frequency.
(e) The antenna picks it up and feeds it to the amplifier. (The 555 pulses are overridden.)
(f) The mosfets are driven continually at the resonant frequency.

I guess this means you can choose any reasonable value for the capacitors.

I have no experience with tesla coils. I don't know if the secondary winding has a different frequency that it tries to resonate at. Etc.

I believe that the top load effects the resonate frequency of the secondary as well so I will need to set that up before I try and measure mine secondary coils.

There is a terrific and simple little circuit that some one put up and that I have used to measure the approximate resonant frequency: https://www.rmcybernetics.com/projects/DIY_Devices/homemade_tesla_coil_tuner.htm.

I have tried it previously it it works great. I set mine up, pretty roughly, in Chinese food container. I should get a proper enclosure for it.

The several amps you observed is a worry - I only saw a 100mA or so. Did you use the 680nF caps or some other value?

If that is the case in the real circuit I will need to carefully limit the duty cycle and use a fuse to prevent it reaching those levels as it would burn out the power supply and my connecting wires. Either that or upgrade them.

I could run it from some old car batteries instead rather than run it from a mains powered power supply. Less convenient though.

I will start off by limiting the duty cycle and include a fuse and see how I go.

 

[BradtheRad]

The several amps you observed is a worry - I only saw a 100mA or so. Did you use the 680nF caps or some other value?

Capacitor values 1 uF.
I put 1 ohm in series with the transformer primary.
Power supply 160 VDC (similar to Steve's High Voltage Mini SSTC schematic). So several amps is no surprise.

Now I reduced the supply to 12V. Current went below 1/2 A.

Of course my figures may not be optimum for running a tesla coil.

 

[boylesg]

Capacitor values 1 uF.
I put 1 ohm in series with the transformer primary.
Power supply 160 VDC (similar to Steve's High Voltage Mini SSTC schematic). So several amps is no surprise.

Now I reduced the supply to 12V. Current went below 1/2 A.

Of course my figures may not be optimum for running a tesla coil.

Actually the salvaged xmas light transformers I am using put out 36V AC / 50V DC (after I rectified it). I am going to connect 2 in parallel to my rectifier circuit where the rectifier can handle about 15A. The transformers were about 50VA or so in power output.

So I won't get quite that many amps, but I could still fry my power supply and I will still put some fuses on either side just in case.

I have a heap of 3.15A fuses I salvaged from old TVs and that sounds about right for the DC side of my power supply.

 

[boylesg]

I found an answer to this question on this forum: https://4hv.org/e107_plugins/forum/forum_viewtopic.php?168961.last, where all the tesla coil builder hang out.

Those who suggested that it is not a resonance issue were of course absolutely correct - I was inappropriately using the concept of resonance.

The amount of current flowing through the primary is not a resonance issue but rather an issue of the impedance of the caps at a given frequency.

So what I was observing with the low current level flowing through the pretend primary coil in the Java circuit simulator, was in fact due to the fact that the 680nF caps had a very high impedance at the square wave frequency I was applying to the FET gates.