The Discrete SSTC / Tesla Coils / Forums

Forums

4hv.org :: Forums :: Tesla Coils

« Previous topic | Next topic »

The Discrete SSTC

1 2 next

Move Thread

LAN_403

uzzors2k

Wed Jul 25 2007, 01:53PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

The thing is I don't have any gate driver or buffer chips atm, but I would like to build a SSTC. Almost every coil I've seen uses the same design; feedback -> buffer -> gate driver -> GDT. Basically a bunch of custom ICs and very few common junkbox components, now where's the challenge in that?

Putting together a discrete gate driver is easy enough, but the feedback buffer seems to be the real problem. I know most antenna based designs use a high impedance input buffer, but are they really necessary? How strong is the antenna feedback signal from a common SSTC? Or what about base current feedback coils, I imagine it's feedback would be strong enough to run a signal transistor.

I made a schematic of a concept coil which I haven't tested yet, can the antenna drive the 2N2222s or should I try base current feedback instead? Or will anything work at all without buffers? The current GDT driver is weak, and will be upgraded if necessary.

thedatastream

Wed Jul 25 2007, 02:56PM

Registered Member #505 Joined: Sun Nov 19 2006, 06:42PM
Location: Yorkshire!
Posts: 329

You could try using small signal N-FETs as the input stage for greater sensitivity...

Steve Ward

Wed Jul 25 2007, 03:35PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Id go all with fets, but then again they arent as common in a junkbox (i only have a few tiny fets!).

Coronafix

Thu Jul 26 2007, 01:13AM

Registered Member #160 Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938

Excuse my ignorance if i'm wrong, but if the antenna drives the
circuit, how does the antenna get the first signal?
I suppose i'm saying there is no start up oscillator, will it still work?

Tom540

Thu Jul 26 2007, 04:47AM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
Location:
Posts: 617

I think when the power comes on it'll send a little spike or glitch through the coil. Either that or the antenna will pick up some random noise and start up that way.

uzzors2k

Thu Jul 26 2007, 07:39AM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I'm putting together a test lash-up today to see if the antenna input stage is sensitive enough. A few n-fets are a luxury I wish I had, nothing I scavenge has any.

Actually the start up is quite reliable, a part of the gate drive system; with no input signal to the antenna both transistors are off. Now the "top" 2N2222 sends an inverted signal to the diode BD139, turning it on. GDT is pulsed and a MOSFET is turned on. Now this pulses the coil, and if the antenna doesn't catch anything the GDT just saturates and the half-bridge is turned off again. However if the antenna sees something both transistors are turned on; the diode BD139 gets an inverted signal and is turned off. The "bottom" 2N2222 turns off the next one beside it, and the following signal to the final BD139 is high. The GDT receives a current reversal and the first MOSFET is turned off and the second turns on. The coil is pulsed again. Once the reversed signal is received by the antenna things repeat from T0.

Marko

Thu Jul 26 2007, 09:56PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I don't think this is going to work. There is not enough amplification for the antenna signal wich is wery weak, and the way the circuit is made with pullups is simply wrong.

The input is current instead of voltage driven, and system could find itself operating in linear mode if antenna current is too small.

Transistors should be in emmiter follower configuration, not common emmiter to be able to source high peak current. Apart from that, BD139 are very slow and probably too large.

If I needed to build something like this, I would go for base current feedback clipped with iodes. This gives nearly perfect squarewave and much more initial power. I would use two secondaries on the CT.

Then I would simply use one or more totem poles to drive the GDT.

Small mosfets may reduce delays even further, but you'll need complementary (P and N ) pairs. Such a halfbridge is inverting so you need even number of them in series.

uzzors2k

Sat Jul 28 2007, 09:16PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

I see all sorts of flaws in the schematic now, base biasing, weak input, both BD139s always being on, etc. Meh. How about the new version, simplified and with better feedback. On second thought it resembles the Skori coil a great deal. It's untested since I'm busy moving these days. Next time I'm online again (one month from now

) I should have it working, so I'll let you guys know what kind of circuit I used.

flannelhead

Fri Jan 25 2008, 04:35PM

Registered Member #952 Joined: Mon Aug 13 2007, 11:07AM
Location: Finland
Posts: 388

Hey, how are the experiments going? Here in the Nordic countries it's not so easy to get gate drive ICs. So keep up the good work!

Dr. H.

Fri Jan 25 2008, 06:35PM

Registered Member #931 Joined: Mon Jul 30 2007, 05:25PM
Location: Bulgaria
Posts: 486

Hi guys. About the strength of the signal - recently i built a SSTC using one of Steve's circuit (the one with the 74HC14). Because at that time i didn't have any of those i used the 74LS14. Because it is TTL i had to decrease the 100k resistor (the one from the start-up oscillator) down to 4.7k and the coil run perfectly, so i suppose that the signal from the antenna must be quite strong

Cheers Huben

1 2 next

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob