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Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
I was over at a friend's house today when I spotted an old plasma globe in the corner, all dusty and neglected. Of course, I couldn't let that wonderful piece of HV hardware go unnoticed, and asked him why he didn't have it plugged in, and he said it was broken. So I plugged it in, and sure enough, I got one or two thick, fairly bright solid streamers to the glass. Knowing what it should look like, I knew this wasn't right, and I heard some whining from the step-up board.
Now that I have the thing home and taken apart, I found its a simple TL494 driver into a mini AC flyback transformer. The vacuum in the globe seems fine, as the driver board got the same performance with a burned out light bulb. I found one of the transistors gives a resistance reading of close to zero (or even negative) and then slowly works its way above my multimeter's range, about 2Mohms. Is this an indication of its failure? Also, there seems to be a place for a large polarized capacitor next to this transistor, could it be possible that this cap was left out accidentally? All of the caps give a infinity reading on the meter, and the resistors give nominal resistances. I would like to repair this thing, as it seems nothing is terribly wrong with it. It also seems like it would be a fun thing to drive a CW multiplier off of.
Registered Member #1559
Joined: Mon Jun 23 2008, 10:17AM
Location:
Posts: 10
Look up the datasheet for the transistor (just google the number that's written on it) and see if it says anything about internal resistance or other specs that might intrest you.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I would say that the gas inside the globe has leaked. If the HV is working then most probably it is working as it should. Otherwise you would see smoke, blown fuse etc.
Registered Member #1153
Joined: Mon Dec 03 2007, 07:27PM
Location: Croatia
Posts: 213
I can help you little:
I collect plasma globes and all variations of them. There is always place for electrolytic capacitor on input but its never populated and i populate it myself and it makes performance better.
Are you sure there is a TL494? In every unit i disassemble ever there was HEF40XX inside, except one used feedback winding.
Here is what brakes on 99 % of this plasma globes:
The nipple where was air pulled out and gases pushed in is on bottom side. Usually by assembly the nipple comes exactly next to FET heat sink. Trough time there is a small discharge from nipple to heat sink and glass slowly "crack" until air escapes inside. All plasma globes i have i put the globe few cm up (its in rubber stand) to escape from this problem.
I don't think you circuit is dead. I will be on your plasma globe and that it let air in. Second thing to try is to input little bigger voltage, see if it will work fine. Those Chinese transformers that come with it overheat and die soon.
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Yes, its a Texas Instruments TL494CN to be exact. One transistor is a TIP122 (heatsinked) and the other is an NEC D882. What kind of cap do you usually put in Kreso? The other caps on the board are 10 or 1uF 50V along with a 3.3nF film foil and a 10nF ceramic.
I tried hooking the output of the board to a burned out light bulb, and performance was about the same, given I had my fingers close to the glass. Without a nearby ground, there weren't even streamers, though I do not know if this is a fair comparison or not. In any case, if the globe is broken, there are all kinds of other things to do with this board.
Registered Member #1153
Joined: Mon Dec 03 2007, 07:27PM
Location: Croatia
Posts: 213
Firefox wrote ...
Yes, its a Texas Instruments TL494CN to be exact. One transistor is a TIP122 (heatsinked) and the other is an NEC D882. What kind of cap do you usually put in Kreso? The other caps on the board are 10 or 1uF 50V along with a 3.3nF film foil and a 10nF ceramic.
I tried hooking the output of the board to a burned out light bulb, and performance was about the same, given I had my fingers close to the glass. Without a nearby ground, there weren't even streamers, though I do not know if this is a fair comparison or not. In any case, if the globe is broken, there are all kinds of other things to do with this board.
Just any used not need electrolytic i have. 16-25 V 1000-4700 uF.
Very interesting that its a TL494 i never seen it.
If you connect a light bulb and there isn't o sparks inside (until you put a finger there) then there is still a hope your globe is fine. One funny thing that could happen is the frequency determing cap dying and its working on some strange frequency which is not good for plasma globe?
Or your flyback transformer is busted. If you had one more plasma globe you could know fault in 2 minutes. BTW what is size of plasma globe? Dont tell me its tat 25 " i forever want . Can you scan or post photos of both sides of board, i would be glad to see how its made
Registered Member #1389
Joined: Thu Mar 13 2008, 12:50AM
Location: Pittsburgh, PA
Posts: 346
Haha, the globe is a 6", not anywhere close to being a 25". In any case, I will try putting one of my nice big electrolytics on it right now. I may have some caps to replace the originals as well. Strange that the mini transformer could be busted, as I can still draw faint 3 mm arcs to an ungrounded screwdriver on the end of a plastic stick.
What frequency are these things supposed to operate at? Like I said in my original post, one of the transistors whines when power is supplied, and I don't think the manufacturer would have intentionally left it in the audible range.
Registered Member #902
Joined: Sun Jul 15 2007, 08:17PM
Location: Pacific Northwest USA
Posts: 1042
Firefox wrote ...
Haha, the globe is a 6", not anywhere close to being a 25". In any case, I will try putting one of my nice big electrolytics on it right now. I may have some caps to replace the originals as well. Strange that the mini transformer could be busted, as I can still draw faint 3 mm arcs to an ungrounded screwdriver on the end of a plastic stick.
What frequency are these things supposed to operate at? Like I said in my original post, one of the transistors whines when power is supplied, and I don't think the manufacturer would have intentionally left it in the audible range.
they generally wont be in the audible range (who that is not an HV enthusiast wants that noise for a little gadget?), I have a spare board around here and it still works as far as I know... I will see if I can determine the frequency, but first I have to borrow some things from a teacher at my school and I can let you know, but there is a good chance that someone here knows...
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Plasma Globe Repair
. Can you scan or post photos of both sides of board, i would be glad to see how its made
