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Gregary Boyles

Sun Apr 19 2015, 02:57PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

Just finished my plasma globe driver:

Just realised from this photo, I stuck the wire from the ground pin of the flyback transformer inside the globe instead of the HV line which is laying on the floor.

The power source is inside the plastic crate in the background. It is a modified microwave transformer with 11.5V, 23.9V and 36V AC outputs. I fused the primary at 3.15A.

At the minute 23.9V AC is going though a voltage doubler/rectifier to give about 77V DC going into the flyback primary.

I need to change a resistor/LED combo on my rectifier board in order to try the 36VAC. The reslting 100VDC would burn up the indicator diode with the resistor I have for it at present.

The plasma globe was discarded in a municipality wide rubbish day so I grabbed it at the time - the electronics didn't work but the globe was still good.

I have audio modulated the 555 through pin 5 via a class A amplifier, but it does not work properly.

I used this configuration:

All I get is a bit of a rhythmic crackling when you plug in an audio source.

Any suggestion why this might be the case?

I will have to play around with my bread board and a speaker and figure out where I went wrong.

Russ Edmonds

Sun Apr 19 2015, 03:47PM

Registered Member #46264 Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61

I think most plasma globe drivers output high voltage high frequency AC to the globe. The center electrode is capacitively coupled to the gas inside the globe.
Russ

GrantX

Mon Apr 20 2015, 01:48PM

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

With your AC flyback transformer: You'll get more plasma in the globe if you connect the secondary's earth wire directly to mains earth and the HV output to the globe. With one wire connected to the globe and the other wire floating you are limiting the capacitive coupling between the HF HV electrode and the surrounding environment a little. With the secondary connected to earth you'll have a ground-referenced supply, and that means a very high output voltage could puncture the glass if you touch it ;)

I've used a similar globe a few years with a little IRFP260N ZVS oscillator and an AC flyback. With the ground-referenced HV secondary I was getting some rather hot and violent discharges in the globe (the original drivers for these ~6 inch globes are only 4-6W and put out roughly 4-8kV at 18-25kHz, versus 15kV+ ~75W at ~45kHz from the ZVS oscillator :D).

Gregary Boyles

Mon Apr 20 2015, 04:00PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

GrantX wrote ...

With your AC flyback transformer: You'll get more plasma in the globe if you connect the secondary's earth wire directly to mains earth and the HV output to the globe. With one wire connected to the globe and the other wire floating you are limiting the capacitive coupling between the HF HV electrode and the surrounding environment a little. With the secondary connected to earth you'll have a ground-referenced supply, and that means a very high output voltage could puncture the glass if you touch it ;)

I've used a similar globe a few years with a little IRFP260N ZVS oscillator and an AC flyback. With the ground-referenced HV secondary I was getting some rather hot and violent discharges in the globe (the original drivers for these ~6 inch globes are only 4-6W and put out roughly 4-8kV at 18-25kHz, versus 15kV+ ~75W at ~45kHz from the ZVS oscillator :D).

Mmmmmm......

Perhaps I will leave it floating - I don't want to damage my globe nor give myself a jump start if I touch it

Eleccentric

Mon Apr 20 2015, 04:59PM

Registered Member #33460 Joined: Tue Aug 27 2013, 06:23PM
Location: Seattle
Posts: 46

If you don't ground the high voltage return, you are more likely to end up giving yourself a shock or damaging the electronics. If the output is strong enough to threaten burning a hole through the glass, you are better off reducing the DC supply voltage or increasing the number of primary turns to reduce power. The flyback transformer is designed to have the high voltage output ground referenced. Its insulation at the "ground" end of the secondary coil is not sufficient to withstand the full high voltage. What is likely happening is capacitive coupling to ground through the primary wiring and drive electronics, roughly ground referencing the high voltage output.

GrantX

Tue Apr 21 2015, 04:31AM

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

As Eleccentric said, its probably best to ground the secondary and reduce the input power. You might risk damaging the globe if the output is too high, but thats cheaper than leaving the output floating and replacing the transformer when it breaks down internally. Grounding the secondary and reducing the DC input will give both the transformer and globe a longer lifespan.

Gregary Boyles

Tue Apr 21 2015, 05:47PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

GrantX wrote ...

As Eleccentric said, its probably best to ground the secondary and reduce the input power. You might risk damaging the globe if the output is too high, but thats cheaper than leaving the output floating and replacing the transformer when it breaks down internally. Grounding the secondary and reducing the DC input will give both the transformer and globe a longer lifespan.

OK, will do then. I'll have to find a chord with an earth wire in it - the one I am using is from a TV se that does not have an earth wire.

Gregary Boyles

Wed Apr 22 2015, 07:44AM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

What sort of wire would you suggest to make a connection with the wall socket earth? I am thinking it could probably be just about any wire since the HV will simply take the path of least resistance rather than creating corona through the insulation.

GrantX

Sat Apr 25 2015, 04:45AM

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

Any wire should be fine, I usually use the same sized wire as the active and neutral. There shouldn't be any corona or any voltage at all since its directly connected to earth. Ideally, there will be almost no current flowing so voltage drops shouldn't be a problem, even with a relatively high switching frequency.

Gregary Boyles

Sat Apr 25 2015, 12:18PM

Registered Member #9039 Joined: Wed Dec 26 2012, 03:31PM
Location: Epping, Victoria, Australia
Posts: 117

GrantX wrote ...

Any wire should be fine, I usually use the same sized wire as the active and neutral. There shouldn't be any corona or any voltage at all since its directly connected to earth. Ideally, there will be almost no current flowing so voltage drops shouldn't be a problem, even with a relatively high switching frequency.

Have done it and it makes a difference, but if you leave your finger on the glass for too long you would end up with a nasty little burn.

I have implemented a darlington transistor FET gate driver for the time being until my TO-220 TC4422 arrive. But geez the darlingtons get mighty hot to the touch. I would say there is a loss in a efficiency here but it will do for now.

I also added some ultrafast rectifier diodes as protection on the Es of the darlingtons, as suggested, and they are clearly catching some ringing coming through the GDT because they are also hot to the touch.

No wonder my pdip TC4422 fried when I powered the FETs with 100V.

I added protection diodes

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