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tarakan2

Wed Jun 19 2013, 03:57AM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
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Posts: 179

Do Neon Sign Transformers create sparks when an insulated metal object (like screwdriver with a plastic handle) is brought to their uninsulated terminal that is energized but not connected to anything?

This is a very interesting effect. I have a use for this effect.
Modern fly back transformers have a circuit to prevent this from happening in their body, as I understand.

Do you know any NSTs that are cheap yet can be easily modified so the central tap of the secondary winding is not grounded?

(I am looking at powerful high frequency transformers in the range of $50-$150 from Ebay and from neon lighting transformer suppliers online.)

PhilGood

Wed Jun 19 2013, 08:29PM

Registered Member #3806 Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259

tarakan2 wrote ...

Do Neon Sign Transformers create sparks when an insulated metal object (like screwdriver with a plastic handle) is brought to their uninsulated terminal that is energized but not connected to anything?

That might happen under certain conditions, but at very close distance (less than 1mm) and sparks will be really very small

tarakan2 wrote ...

Do you know any NSTs that are cheap yet can be easily modified so the central tap of the secondary winding is not grounded?

You could just not conect the center tap to mains earth, but in some NSTs center tap might be connected to the metal body so you would have to mount the whole NST on insulators to keep center tap floating. I'm not sure if some problems could occur when operating an NST this way...

tarakan2 wrote ...

(I am looking at powerful high frequency transformers in the range of $50-$150 from Ebay and from neon lighting transformer suppliers online.)

You can get old NSTs for free or almost free from some neon signs installers, just ask them... For this it's better to go visit them directly rather than askin by mail or thru phone...

GrantX

Thu Jun 20 2013, 03:19AM

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

My 15kV/60mA 50Hz NST does make some very tiny sparks to an unconnected screwdriver, but not very noticeable. When doing this I did not hold the screwdriver handle, I taped the screwdriver to a short PVC conduit, which would have reduced capacitance to ground (holding the screwdriver handle would greatly increase the capacitance to ground). A 1000V-rated insulated screwdriver is unlikely to conduct even at 15kV, but I'm not gonna bet my life on it!

I believe high frequency units would have a much better effect, since the higher frequency allows much more reactive current to flow.

And modern flyback transformers produce a DC output, so capacitive coupling to an unconnected screwdriver won't create a continuous arc.

tarakan2

Thu Jun 20 2013, 03:56AM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
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I am not sure what I am looking for, really. It has to be a powerful transformer giving out 15000 volts or so.

I don't like the idea of buying a finished unit, yet I am not so sure that I can build a driver myself. I had a bad experience with a mozillini flyback driver that gives out around 3000V with a modern flyback driver - a lot less than I expected.

This effect with sparks jumping onto other conductors is indeed very interesting. I learned how to make use of it and I want a transformer with this quality.

TBJ

Sat Jun 22 2013, 08:54PM

Registered Member #5374 Joined: Mon Jun 18 2012, 06:54PM
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OK, firstly, any two conductive objects (like the screwdriver and ground) act as plates of a capacitor. This means that the screwdriver is 'capacitively coupled' to ground - you can model it as having a capacitor connected between it, and ground.

A capacitor connected in a series circuit has a property called "reactance". This is a measure of the impedence of the capacitor in ohms and is a function of the value of the capacitor, and the frequency of the signal across it. The reactance is calculated like this:

Reactance = 1 / (2 x pi x F x C)

Where F is the frequency in Hz of the applied signal, and C is the value of the capacitor.

Therefore you can use the reactance to work out the current that your capacitor (screwdriver-ground combo) can pass, and therefore the size of the arc you can draw, using the voltage available from your transformer, and ohm's law.

You don't really need to calculate any of this, but what you do need to know is that as the frequency increases, the reactance of the screwdriver-ground capacitor DECREASES. This means higher frequency = more current = bigger arcs.

tl;dr: To get bigger sparks from your screwdriver, you need a higher frequency power supply, and of course more voltage.

Neon sign transformers operate at 60 (50)Hz and that is nowhere near enough. The mazilli circuit with a flyback would be a good choice for a high frequency high voltage supply - it sounds like your one wasn't working properly.

tarakan2

Sun Jun 23 2013, 02:49AM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
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Z=R+jX

where j=-1^0.5
and
X is reactance

So we get a triangle that has one side dedicated to R and X is on imaginary axis.
The hypotenuse of the triangle will be Z the impedance that increases with frequency.

I am looking at power triangles and reactance triangles.

1) So as f approaches infinity, Xc approaches 0 or also approaches infinity?

Inductance XL is not involved in this?

2) Are there any condition when reactances X multiply in electronics? X1*j*X2*j = X1*X2*-1
3) In a theoretical oscillator circuit where R=0, X=Z?
Are there any conditions in electronics where ordinary resistances multiply?

By using diodes this process can be guided in a very odd direction. A source with no ground can charge a capacitor to a very high value through two diodes. I have done this experiment. Eventually a capacitor will blow or some circuit has to make use of this DC charge. I made a circuit that uses a thyristor and a zener diode to make use of this energy to power several LEDs.

One strange device that I had seen in my time is a 60hz igniter transformer that can give out 18mm sparks to ungrounded objects.

Maybe this transformer uses 60Hz to stimulate some higher resonance. Is that possible?

Dr. Dark Current

Sun Jun 23 2013, 12:06PM

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

Or the transformer is pulsed on the primary side

tarakan2

Sun Jun 23 2013, 06:43PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179

Does reactive power add parasitic load to the power grid or does it create resistance that the current has to overcome?

tarakan2

Sun Jun 23 2013, 06:45PM

Registered Member #3859 Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179

Dr. Dark Current wrote ...

Or the transformer is pulsed on the primary side

Pulsed with a halfwave of the 60hz AC? So the collapse of the magnetic field creates resonance that makes this transformer work at higher frequency?

Dr. Dark Current

Sun Jun 23 2013, 07:01PM

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

tarakan2 wrote ...

Dr. Dark Current wrote ...

Or the transformer is pulsed on the primary side

Pulsed with a halfwave of the 60hz AC? So the collapse of the magnetic field creates resonance that makes this transformer work at higher frequency?

I think it works in a similar way like a Tesla coil. There may be an SCR or TRIAC which discharges a capacitor into the primary winding every half cycle, so the transformer is really a pulse transformer, not a conventional mains frequency one.

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