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DRSSTC - cannot get oscillating

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vasil

Tue May 30 2006, 09:01PM

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

Bad solders on the PCB? It was one of my problems....
Did you conect right the pins of the 74 HC 14? All the chips are in right position? sometime in a hurry it is possible to make it wrong.
The input for the drive has to be very well filtered. Thats why i bought a dedicated 12 V for it.
Where did you get from the GDT cores?

Marko

Tue May 30 2006, 09:31PM

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

Bad solders on the PCB? It was one of my problems....
Did you conect right the pins of the 74 HC 14? All the chips are in right position? sometime in a hurry it is possible to make it wrong.
The input for the drive has to be very well filtered. Thats why i bought a dedicated 12 V for it.
Where did you get from the GDT cores?

Guys, give me a break a bit..

The board works, it was tested lots of times before I used it for this coil.
I put my signal generator to very good use there.

I made gate drive signals to perfection far before I actually hooked the board to IGBT gates.

And I surely didn't do things like turning diodes wrongly or something.
The Coil oscillates nicely with antenna, giving some 2,5cm arc (at 30V in).

Since I get nice sparks form external 1:100 osciloscope CT (without burden) I guess one in cascade on the bridge probably got shorted (I receive only a small amount of noise on output). Bridge will get an autopsy next weekend

(Im leaving town tomorrow and I can't continue much work now)

HV Enthusiast

Wed May 31 2006, 01:02AM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

2.5cm output arc with only 30V in sounds reasonable.

Marko

Fri Jun 02 2006, 08:51PM

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

I removed and disassembled both CT's, and put hem to test separately.
One works with oconsional arcing to core, while the second seems to be totally shorted.
I can get shocks just from single CT when coil is operating just at 30V, and I didn't take some great care for insulation! (just a bit of tape).

Probably II also managed to damage thick enameled wire I used for second CT, since output was complete dead-short...

Thanks for help anyway, I'm now glad I didn't try to push the coil more in this state..

Steve Ward

Sat Jun 03 2006, 01:22AM

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

This is another reason i switched to using 2 cascaded CTs so that each would have only a few 10's of turns. This allows you to use plastic insulated wire, greatly reducing the risk of shorting out the winding! Usually, just winding the wire up against the ferrite (even if it has the slightest edge) will damage the insulation enough to make it useless. You have to coat the entire core with something plastic, or just use plastic insulated wire.

Hopefully you will get it working and not be led to think that my circuit is a phoney

Marko

Sat Jun 03 2006, 11:11PM

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

Yep, it works now.
I rewound CT's with PVC wire and hooked them up, but now I was able to get only about 5mm spark, giving my best with tuning.

Frequency match is almost perfect (says osciloscope) but for some reason the coil doesn't wnat to draw much power and secondary rings down almost imediately after beginning of the cycle.
Maybe it is time to put some more volts to the bridge and see if it blows up.

Dr. Drone

Sun Jun 04 2006, 01:32AM

Registered Member #290 Joined: Mon Mar 06 2006, 08:24PM
Location:
Posts: 1673

Steve Ward

Sun Jun 04 2006, 07:32AM

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

Only 5mm sparks at 30VDC input? Can we have the full specs of the setup? Operating frequency, number of cycles per burst, tank L and C values (approximate is fine, just to get an idea). IF you are running only say 5 cycles of RF, then you will only get puny sparks at low input voltages. If you can, scope the output voltage of the half-bridge and compare it with the current output, make sure its reasonably close to ZCS. If that looks ok, then you are probably alright at higher voltages. A variac is still really nice to have... 30V to 320V is quite a jump!

Marko

Sun Jun 04 2006, 02:25PM

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

It seems to repair a bit at 100V input to bridge.
Now I have some 7-8cm spark, either using primary feedback or antenna.

Specs of coil are:

Primary cap: 125nF, 3,2kV
Primary inductance: about 10uH
Secondary: 110*450mm, PVC tube, o.2mm wire for inductance of 80mH

Topload is 10pF, self capacitance 8pF, and resonant frequency about 120kHz.

After some 100us of ON time I get almost no more spark length.

Coil draws only ten watts at 100bps, seems quite low!

I tought it will overload my tiny 40VA isolation transformers but for some reason it is not fery power-hungry (even in perfect tune)
I need to mock up an additional CT to measure primary current precisely.

I guessed my pri mary was overcoupled, so I raised secondary about 5cm more.
Interestingly it didn't affect performance or power draw by any mean.

HV Enthusiast

Sun Jun 04 2006, 03:31PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Are you running it with a 40VA isolation transformer???

If so, thats probably your problem. The wire in these is quite thin at that power level, and you are in essence "choking" your dc bus cap and not allowing it to charge up enough when operating. Get rid of the isolation transformer and try again.

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