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LAN_403

Dr. Dark Current

Sat Jun 24 2006, 07:41PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

schematic:

driver:

arcs:

plasma globe from big lightbulb:

notes: fets stay cool@120w. hand-wound flyback: 20kv. universal flyback driver.

Marko

Sat Jun 24 2006, 07:51PM

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

hmm,
gates directly on pin3 omg!

Darn good work if nothing dies in few seconds, and mantains such arcs...

I guess you can get much bigger sparks with just small totem pole on pin 3, for higher gate peak-current, but rest of circuit is pretty well-made.

Small bipolar totem pole will give you much more peak current than NE555 output, and increase in switching speed will surely reduce transistent times (very important for voltage gain, V = -L*di/dt). This will also help your fets stay cool. If you have some small gate drivers (like ICL7667) it would be even better.

I like the big bulb used as plasma globe in last pic

good work..

Dr. Dark Current

Sat Jun 24 2006, 07:57PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Firkragg wrote ...

hmm,
I guess you can get much bigger sparks with just small totem pole on pin 3, for higher gate peak-current, but rest of circuit is pretty well-made.

the NE555 has pretty high peak currents by itself (the 200ma in datasheet is not peak current), I think it is actually much over an amp.

if somebody still doesnt get the point of my post: it was that i made THE flyback driver which is "good in everything" (simplicity, efficiency, output voltage/power...) and it looks c00l on the wooden base

J.M.

Steve Ward

Sat Jun 24 2006, 07:59PM

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

Driving the fets from the 555 is just fine for these frequencies. Driving them faster might actually kill them because as you say V=L*di/dt, and V might just get too high. I actually built my plasma globe driver in nearly the exact same manner.

Dr. Dark Current

Sat Jun 24 2006, 08:05PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

...and to make this driver even more perfect, here is long and detailed page on how to build your own driver. (part of my second try on making a personal website index here )

J.M.

Marko

Sat Jun 24 2006, 08:41PM

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

What I fear more is death of NE555 (may pull pin 3high after that, murphy's law and so), fet's may get hot but probably won't die since they are quite overrated.
Simple totem-pole would be a small but great improvement to the circuit.

Driving them faster might actually kill them because as you say V=L*di/dt, and V might just get too high.

Hmm, I used to drive out a hell from a hifi bipolar transistor, at 200kHz, with very fast switching and input of 28V (power about 40W). It was rated 140V and never died. It wasn't even particulary hot.

I saturated a BDT, exploded a mosfet driving it, blew the whole breadboard but main switch remained fine without any clamping.
I had arcs starting at about 6cm, but very low current, I guess I was on edge to kill the flyback..

This leads me to think IRFP250's with 200V rating would be even more indestructible in the same use.

vasil

Sat Jun 24 2006, 08:58PM

Registered Member #229 Joined: Tue Feb 21 2006, 07:33PM
Location: Romania
Posts: 506

Nice driver jmartis.
I observed in my drivers too that using two transistors in parallel helps a lot the brightness of the arcs. maibe the totem pole would not be a bad idea.

Keep the good work!

vasil

...

Sun Jun 25 2006, 04:22AM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

Why use 2 irfp250's in paralel? They already have such a low on resistance (22mOhm if I remeber correctly) you are probably just screwing yourself from the extra gate capacance loading down the 555 and causing switching losses... Assuming you are running at 4amps you can't be loosing more than a few hundred mw in the fets when they are on...

Could you scope the output of the 555 just to get an idea of what it looks like in there?

BTW 500th post

Dr. Dark Current

Sun Jun 25 2006, 06:45AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

... wrote ...

Why use 2 irfp250's in paralel? They already have such a low on resistance (22mOhm if I remeber correctly) you are probably just screwing yourself from the extra gate capacance loading down the 555 and causing switching losses... Assuming you are running at 4amps you can't be loosing more than a few hundred mw in the fets when they are on...

Could you scope the output of the 555 just to get an idea of what it looks like in there?

I use 2 irfp's in paralel, because I found that they heat less. I can scope the 555 output when I dig up my (tube) scope

J.M.

EDIT: dug up the scope, seems to be alive

anyways, here is a shot @operating frequency (~25k) with the flyback connected. It is scoped directly on the fet's gates. It doesnt change much when drawing an arc. i didnt even expect it to look so clean!

Marko

Sun Jun 25 2006, 10:45AM

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

Can you focus up on transistents (rise and fall edges)?
Waveform looks very good considering the load you put to NE555.

On just 25 kHz slow switching isn't that big problem, but you'l certainly get more HV by making them faster (means faster transistents)

One IRFP would be more than enough for 100W operation.
It might also unload 555 a bit as Peter claimed.

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