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ZakWolf

Mon Feb 23 2015, 04:28PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
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Posts: 608

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.

Sigurthr

Tue Feb 24 2015, 01:22AM

Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597

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.

ZakWolf

Tue Feb 24 2015, 01:27AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

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 ?

GrantX

Sat Feb 28 2015, 06:40AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

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).

ZakWolf

Sun Mar 01 2015, 03:25AM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

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.

Ash Small

Thu Mar 05 2015, 11:31AM

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

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.

ZakWolf

Thu Mar 05 2015, 10:34PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

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 !

omegalabs

Fri Mar 06 2015, 04:41PM

Registered Member #1521 Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128

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.

Ash Small

Fri Mar 06 2015, 05:43PM

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

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.

ZakWolf

Fri Mar 06 2015, 11:45PM

Registered Member #3114 Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608

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.

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