I am doing a lab project where I need to ionize an 8W fluorescent tube with negative, rather than AC, voltage.
The power supply is a standard small inverter that runs off 12VDC.
To clarify, there will be a coil in contact with the lamp that is fed with a separate AC signal. The ions within the lamp, and hence the electric charge on the tube's surface, must be negative with respect to that coil.
I suppose this means it must be driven with negative DC or a negative going waveform. But I am unsure how to proceed. For instance, can the inverter be adapted to do this?
The voltage need only be sufficient so as to ignite the tube.
Do you mean DC? I suppose the problems would be similar to those encountered using positive DC, except that you'd have to turn the lamp around. ;-)
Seriously, are you trying to power the lamp with DC, or are you just concerned with the polarity of the triggering pulse (I'm thinking along the lines of flash lamp triggers here)?
Current regulation will be a problem if you power the lamp from a DC supply, since discharge lamps' resistance drops with increasing plasma temperature, resulting in still higher lamp current (and power) one triggered.
Perhaps the illuminati over on sci.engr.lighting can provide some insight into your problem.
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I agree. There is no such thing as "negative voltage." Voltage is the potential difference between two points. With DC voltage, one point is negative or positive with respect to the other. However, since the universal ground plane of the world can be one of the two points, it is possible to have a terminal that is negative with respect to ground and some might call that a negative voltage.
Powered a low pressure Hg rare gas discharge with DC will eventually drive the Hg to the negative end.
An active DC current regulator could be used, or for a lab experiment instead of a real lighting system, a resistor could be used.
Can you post a diagram. In a fluorescent lamp type discharge, some of the light and small electrons diffuse quickly to the wall, creating a negatively charged sheath that slows down further electron diffusion to the wall while increasing the diffusion of the heaver and larger positively charged ions. The net effect is to balance the flow of electrons and ions to the wall, which is required for stable operation. Are you trying to modify the diffusion rates of the ions and/or electrons with the external field?
Again, negative with respect to what? Perhaps you want the wire on the wall to be negative with respect to the lamp electrodes. In that case all you need to do is make a DC power supply from your inverter by using a properly oriented diode rectifier.
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There are two voltages in play here. There's a voltage between the ends of the tube, usually 120VAC ballasted through an inductor. And it seems you want a different voltage potential difference between the in side of the tube and your surrounding coil.
If the second voltage is much larger than 120 volts, you can probably keep the tube running off its regular power source. The middle of the tube will be wandering between +60 and -60 volts (RMS), but if you're applying many hundreds of volts to your coil, you might be able to ignore the AC ripple. Otherwise if you need a really steady potential difference, you'll need to run the tube off DC, which means it won't last very many hours. And as other have noted you 'll need some way to limit the current, with a current-limiting power supply or just a resistor. As a rough guess, an 8 watt tube runs at about 70 milliamps, so if you start with say 200 volts DC, you'll need about a 1,000 ohm resistor, at 5 watts or so.
So to make the tube more negative, you'll hook your HV inverter's negative lead to one end of the tube, the positive lead to your coil. Piece of cake.
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