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DC Resonant Charging Tesla Coil

ZakWolf, Mon Feb 23 2015, 04:28PM

Hi, I'm planning on building a small spark gap tesla coil and was wondering what the difference was between these two circuits ? Also , I was going to try to power it with a rectified resound fly back. I found these high voltage rectify diodes and was thinking I could just use 1 to half wave rectify it. Would that be considered a unidirectional flow ? if I use a full wave rectifier (4 diodes) would that prevent current from flowing back into the cap bank After the spark gap (unidirectional) or would one diode be sufficient to surpress the kick back after the spark gap fires ?

Thanks guys

update: so after reading up on some things I found that I might need a TVS diode.as they are ment to surpress high voltage kickback. I read that some of the diodes are unidirectional and bidirectional. Isn't a regular diode unidirectional becuade it will only conduct in one direction ? Anyway my goal is to shut off the current as soon as the spark gap fires and the spark is extinguished so that know current flows back into the bank

UPDATE #2so I guess I answer my own question about which circuit to use. The circuit that puts the spark gap between the HV output and the capacitor in series with the primary . I read that this one inhibits high voltage kickback and current back into the power supplies secondary.

Re: DC Resonant Charging Tesla Coil
Sigurthr, Tue Feb 24 2015, 01:22AM

Both circuits perform identically, but the key difference is this;

-the parallel gap + series cap blocks RF and HV from being reflected back at the power transformer.

Look up "DC Resonant Charging" if you want to power a SGTC from DC, it involves a lot more than just using a rectifier or diode bridge.

Re: DC Resonant Charging Tesla Coil
ZakWolf, Tue Feb 24 2015, 01:27AM

Sigurthr wrote ...

Both circuits perform identically, but the key difference is this;

-the parallel gap + series cap blocks RF and HV from being reflected back at the power transformer.

Look up "DC Resonant Charging" if you want to power a SGTC from DC, it involves a lot more than just using a rectifier or diode bridge.

Thanks I will take a look

So I took a look at DC RESONANT CHARGING and I found this simplified diagram. So with the adtion of the De Qing diode it will enable me to fire the circuit at any rate i want (dependent on the current /charging capabilities of the HV supply) without the cap. emptying all of its stored energy back into the HV supply.For initial testing I wan to use a static gap and a permanent magnet to help quench it

As far as calculations go dont I need to know what voltage im dealing with from my HV source? If so how do i even measure my Flyback output?

also say I end up with an out put around 5kv, I need to smooth that after the HV Full Bridge. I can use an electrolytic? What kind of capacitance am I looking at for a smoothing cap of that rating, a couple micro farads ?

I eventually want to run this setup on a rotary spark gap

UPDATE: so I need a DC Choke, I do have a massive lamp ballast from a street lamp (bigger then a mot). I read I can wind my own solenoid style on a couple layers of rectangular laminations or use a single coil ballast style. I haven't found anything yet about the rating of this choke, what am I going for ?

Re: DC Resonant Charging Tesla Coil
GrantX, Sat Feb 28 2015, 06:40AM

If I remember correctly, the DC choke needs to have some rather impressive specifications. Several Henries and capable of withstanding a voltage of more than twice the peak input DC voltage. The street lamp ballast might work, but I imagine it could flash over to the core. The core also needs to withstand the DC component without saturating, so a core made from straight rectangular iron laminations should have a large enough air gap for these purposes. You'll probably want a nice big spool of wire to achieve a few Henries of inductance.

Richie Burnett's page on DC resonant charging will give you more accurate info than I can:

Probably a good idea to also build the de-Q-ing diode string with a stand-off voltage of twice the voltage on the tank capacitor, for a good safety margin (voltage on the tank capacitor can be 2x the input voltage, so the diode should be rated for 4x the DC supply voltage).

Re: DC Resonant Charging Tesla Coil
ZakWolf, Sun Mar 01 2015, 03:25AM

GrantX wrote ...

If I remember correctly, the DC choke needs to have some rather impressive specifications. Several Henries and capable of withstanding a voltage of more than twice the peak input DC voltage. The street lamp ballast might work, but I imagine it could flash over to the core. The core also needs to withstand the DC component without saturating, so a core made from straight rectangular iron laminations should have a large enough air gap for these purposes. You'll probably want a nice big spool of wire to achieve a few Henries of inductance.

Richie Burnett's page on DC resonant charging will give you more accurate info than I can:

Thanks Grantx, I have read his site, its a great source of information.

I have seen his DCSGTC videos and his inductor is HUGE, looks like a mot. rewound with a massive spool of magnet wire.

About the diode ratings,

I found these, they are spendy and I have not the slightest clue of what my current ratings are going to be like. Does any one know a better place to get HV diodes?

This leads me to ask, Should I rectify a mot and use that for my HV source, or make a flyback transformer powered by a ZVS driver.

Either method im going to need to know the cont. current so I can get a suitable diode. I think I should first figure out what my HV supply will be and how im going to rectify it (full bridge). then I can get a cap bank and wind an charging choke

UPDATE:Im getting the feeling I really need a scope and an LCR meter for inductance:(

UPDATE#2:So i bought an LCR meter , it measures anywhere from 20H to .1uH. Why do i need a few henerys? is there an ideal inductance i need to aim for and is that based on my power supply ratings?

UPDATE #3: Im worried that if i buy nice HV diodes im going to pull to much current or (not sure if i can do that from a flyback, im aiming for 10kv output) burn them up all at once wasting them. Will I see a major difference in performance if I just use one to half wave rectify it and add a HV smoothing ca after the output.

Re: DC Resonant Charging Tesla Coil
Ash Small, Thu Mar 05 2015, 11:31AM

If you use, say, 4 chokes in series the voltage across each one is a quarter of what it would be with one. Also, as the inductances add, you can use thicker wire for more current capability.

You need to do some maths first, I think, to work out how many Henries you need.

Re: DC Resonant Charging Tesla Coil
ZakWolf, Thu Mar 05 2015, 10:34PM

Ash Small wrote ...

If you use, say, 4 chokes in series the voltage across each one is a quarter of what it would be with one. Also, as the inductances add, you can use thicker wire for more current capability.

You need to do some maths first, I think, to work out how many Henries you need.

You're right ash, I do need to do a lot of calculations but I'm waiting on my inductance meter and here in a week or two I will get a scope. I'm planning on using a fly back for power and I need a scope to calculate the output voltage or I can make a string of 1Mohm resistors to test it.

should half wave rectifying it be ok or would full wave be optimal.

I'm currently making my HV capacitors and will link picture to here after I get one done there gonna be nice !

Re: DC Resonant Charging Tesla Coil
omegalabs, Fri Mar 06 2015, 04:41PM

DC resonant charging is pretty useless in my opinion. You can get the same sparks from a normal AC system. It can be used when you have a very low supply voltage (<4kVAC) or you want to use a 3 phase socket. Or probably one more, when your transformer has a very low drop, the charging reactor can compensate the high current flow into the primary circuit, so you don't need a big ballast before the transformer.

Re: DC Resonant Charging Tesla Coil
Ash Small, Fri Mar 06 2015, 05:43PM

ZakWolf wrote ...

should half wave rectifying it be ok or would full wave be optimal.

As someone already pointed out, these are usually used with lower voltage supplies like MOT'S. Usually a doubler is used.

When I built a similar circuit using a flyback I didn't use resonant charging, I just used a DC flyback and big resistor (330 Ohms if I recall correctly. The DC flyback charges the tank capacitor through the resistor and when the cap is charged to sufficient voltage the gap fires. The resistor stops the HV getting back to the flyback.

I used a length of suppressed auto ignition lead as an HV resistor.

This was for my HF TIG welder.

EDIT: Should read '330 k Ohms', I think.

Re: DC Resonant Charging Tesla Coil
ZakWolf, Fri Mar 06 2015, 11:45PM

Ash Small wrote ...

ZakWolf wrote ...

should half wave rectifying it be ok or would full wave be optimal.

so half wave is fine... since you said you used a DC flyback. DC flybacks only have a single diode potted inside of them making them only half wave...

since I will be placing the gap across the HV in and putting the cap in series with the primary (check first post) I have read this will inhibit and back EMF and voltage into the transformer. Especially if Im using a DC choke and a DeQing diode after the HV supply

If this is true, I do not want to use a HV resistor and instead want to utilize the function of the choke to double the cap voltage before it fires.

Re: DC Resonant Charging Tesla Coil
Ash Small, Sat Mar 07 2015, 01:41PM

A DC flyback has several internal diodes connected in series with the windings.

I think you could use an AC flyback and doubler with the series resistor instead of using resonant charging. I think it makes more sense if using a flyback, this would avoid the need for a choke and de-Q-ing diode, but see what others think

Edit: I'll post a schematic of my design later if you like. I'm posting via my phone at the moment

Re: DC Resonant Charging Tesla Coil
ZakWolf, Sat Mar 07 2015, 07:02PM

Ash Small wrote ...

A DC flyback has several internal diodes connected in series with the windings.

I think you could use an AC flyback and doubler with the series resistor instead of using resonant charging. I think it makes more sense if using a flyback, this would avoid the need for a choke and de-Q-ing diode, but see what others think

Edit: I'll post a schematic of my design later if you like. I'm posting via my phone at the moment

Thanks Ash, Im confused now though. Why use a doubler when I can use the choke to double the voltage? I guess the doubler would be much easier and cost less. In any case I want to play around with different options and see how each of them perform.

I rolled some caps, there bigger then my hand. 130cm L x 50cm Dia (5 1/4"L x 2" Dia)

Re: DC Resonant Charging Tesla Coil
Ash Small, Sun Mar 08 2015, 12:46AM

I rolled my own caps too and they worked perfectly. I think you'd be wasting your money on a choke, but see what others think. By all means do it to experiment with inductors, I do that myself

I'll post my schematic tomorrow when I'm sober for you to look at, maybe you wonn't be so confused then

Re: DC Resonant Charging Tesla Coil
ZakWolf, Sun Mar 08 2015, 01:28AM

Ash Small wrote ...

I rolled my own caps too and they worked perfectly. I think you'd be wasting your money on a choke, but see what others think. By all means do it to experiment with inductors, I do that myself

I'll post my schematic tomorrow when I'm sober for you to look at, maybe you wonn't be so confused then

Thanks Ash, appreciate it

I measured the capacitance it read 11.57nf, that's with two layers of overhead projection sheets. From what I read roughly each sheet is rated for 14kv??? Im pretty sure that rating varies when used with different frequencies, what would you say a good voltage rating would be with two sheets ?

Re: DC Resonant Charging Tesla Coil
Ash Small, Sun Mar 08 2015, 11:27AM

Here's my schematic I drew this from memory ten years after I built it. The only bit you need is the tank circuit, etc.

I've not used OHP capacitors, so can't really advise on them. I used polythene.

I don't see any reason why you couldn't use, for example, an AC flyback and tripler from an old TV, maybe?

(My circuit was designed to 'fire' under argon, so didn't need to be as high a voltage as your's will need to be.)

Re: DC Resonant Charging Tesla Coil
ZakWolf, Mon Mar 09 2015, 01:52PM

Ash Small wrote ...

Here's my schematic I drew this from memory ten years after I built it. The only bit you need is the tank circuit, etc.

Thanks Ash, although i dont think i really understand whats happening there. Can you explain it.

Also, what is a good cap. size for HV filtering? I have never made one before and I just want to do a little smoothing after its rectified.

Re: DC Resonant Charging Tesla Coil
Ash Small, Mon Mar 09 2015, 04:39PM

There is no need for a smoothing capacitor in my design as the tank capacitor gets charged up instead.

When the voltage on the capacitor is high enough the gap fires, and the tank 'rings down', transferring all the energy from the primary tank capacitor to the secondary and top load.

The primary tank capacitor then charges up again from the flyback and the sequence repeats.

The resistor isn't of high enough value to restrict the capascitor from charging, but it is sufficient to stop the HF as the tank rings down, protecting the diodes, etc. You could use an AC flyback and tripler (or any number of multiplier stages) or you could just use a DC flyback. It depends on what voltage you want to charge the primary tank capacitor to.

In mine I had a 1:1 ratio between primary and secondary, but you will want a much higher turns ratio in order to achieve the voltages required for 'breakout' in air at STP.

Also, I'd suggest using a more powerful flyback driver that the single 2N3055 driver that I used.

See if you can now come up with a schematic based on the above

EDIT: As for tank capacitor value, I used 20nF, but anything from 10nF to 50 or even 100nF should be ok.

EDIT 2: This is the bit you need to look at. This has a DC flyback, but an AC one and doubler, tripler, etc would be a lot simpler than resonant charging

(It takes several cycles of the flyback to charge the primary tank cap. Newton Brawn once worked out how long it takes to 'ring down', it was a surprisingly short period.)

I might even build one of these using one of Fiddy's flybacks and a doubler or tripler.

Re: DC Resonant Charging Tesla Coil
ZakWolf, Thu Mar 12 2015, 04:29PM

Ash Small wrote ...

There is no need for a smoothing capacitor in my design as the tank capacitor gets charged up instead.

When the voltage on the capacitor is high enough the gap fires, and the tank 'rings down', transferring all the energy from the primary tank capacitor to the secondary and top load.

The primary tank capacitor then charges up again from the flyback and the sequence repeats.

The resistor isn't of high enough value to restrict the capascitor from charging, but it is sufficient to stop the HF as the tank rings down, protecting the diodes, etc. You could use an AC flyback and tripler (or any number of multiplier stages) or you could just use a DC flyback. It depends on what voltage you want to charge the primary tank capacitor to.

In mine I had a 1:1 ratio between primary and secondary, but you will want a much higher turns ratio in order to achieve the voltages required for 'breakout' in air at STP.

Also, I'd suggest using a more powerful flyback driver that the single 2N3055 driver that I used.

See if you can now come up with a schematic based on the above

Ash the schematic you posted is what Im going for, except that the resistor will be my HV Choke.

As far as the maths go I dont think i can perform any till i get an oscilloscope... I am at a loss as far as calculations go. I know that im making a DC resonate Tesla coil so that must require the primary to be tuned (by the HV cap) to res. frequency of the secondary. Im not to sure how the inductor comes in to play as far as resonate charging goes. I know that it builds up a field while the cap is charging and at the moment the current stops flowing through it, collapses and causes it to almost double the voltage before the spark gap fires.

So, what calculation would i need to perform/what is the point of having an "X value" inductor. Is it really specific or is it just a minimum Henry rating?

I now have a multimeter and an LCR meter, still working on getting that scope though.

Re: DC Resonant Charging Tesla Coil
Ash Small, Thu Mar 12 2015, 05:05PM

Resonant charging is normally used with something like a couple of MOT's, around 2kW of power. Your flyback will put out ~1/1000th of that. The MOT's re-charge the capacitor 100 times a second. While a choke 'may' provide protection for the charging circuit, it will not serve any other purpose if used as you are describing. unless the smoothing capacitor is much larger than the tank capacitor.

I don't think that a flyback can keep the smoothing cap charged sufficiently to sustain resonant charging. I think a better way would be to use a doubler to get the same voltage, and charge the tank capacitor directly. The coil will 'fire' each time the tank cap voltage exceeds the breakdown voltage of the gap. I guess it will still fire several times every millisecond, certainly in that ballpark.

My design does work, I used it in my HF TIG welder, but you will need a higher voltage to achieve 'breakout' in air at STP. (This is just a question of increasing the spark gap, the flyback will charge the capacitor until the breakdown voltage is achieved. and, obviously, having a lot more turns on the secondary.)

The choke and large smoothing capacitor are not required, in my opinion.

Others may have other views

EDIT: The easiest way to 'tune' the primary tank circuit is to adjust the inductance of the primary coil. You will have to do some maths first, though.

EDIT: I think I need to re-read Ritchie's page before I'm certai that what I stated above is correct. I'll re-read it and then re-post or further edit this post.

EDIT: Yerp, I'm as sure as I can be that I'm correct.

Re: DC Resonant Charging Tesla Coil
TeslaHighvoltMan, Tue Mar 24 2015, 01:41PM

Anybody already checked out this 12kW DC resonant charging system? Seems to have an amazing performance: