Author Topic: High voltage Christmas ornaments  (Read 460 times)

Offline klugesmith

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High voltage Christmas ornaments
« on: December 15, 2019, 12:03:49 AM »
First light, this afternoon, for a project that's been brewing for a couple years.  Based on T5 fluorescent lamps that used to be overhead at work, before they were replaced with LED's.

Here's Small Octahedron #1.



Hubs are made from black poly tubing that, by "serendipity of fit", pushes nicely onto the T5 lamp ends.  Too soon to say whether it will crack or slip in cold, heat, rain, or snow.  The jumper wires should be OK for series-connected illumination, because they were cut from a retired string of miniature lights.


Need to find some decorative hub caps, and figure out how to support octahedrons for indoor or outdoor display.  Vertex up is un-natural;  the shape naturally rests with with two triangular faces horizontal.

« Last Edit: December 15, 2019, 12:24:10 AM by klugesmith »

Offline John123

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Re: High voltage Christmas ornaments
« Reply #1 on: December 15, 2019, 03:29:26 AM »
Looks good Richard, very creative. Reminds me of the Christmas decorations they string between the stores on the town high street.

Offline klugesmith

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Re: High voltage Christmas ornaments
« Reply #2 on: December 15, 2019, 04:57:27 AM »
Thank you.  The light has already seen some service, and some rain.  Wish I could tone it down a little bit -- here we see it next to a star-shaped fake neon (white LED) decoration.

That's running on an old fashioned 30 mA NST.  I could dial it down with a variac or dimmer.
A modern high-frequency neon sign "transformer" up close might be safer for strangers poking fingers into decorative light installations.  Should first determine the maximum striking voltage for string of 12 lamps.

More educational, if the rain stops, would be to measure and chart the octahedron's I-V curve using a variac and 60 mA NST.  IIRC, somewhere between about 20 mA and 40 mA there's a sharp drop in voltage, I think marking transition from cold to hot cathode behavior.  Much lower currents might be hard on the cathodes, but tube life measured in 100s of hours would be plenty.

Here are I-V trajectories for a string of 20 much longer T5 lamps, measured one year ago. 
Blue starting with variac at full blast, orange starting at zero. 
Grey dots are measured V,I with various resistors, and NST primary at nominal voltage.
« Last Edit: December 15, 2019, 05:09:02 AM by klugesmith »

Offline Mads Barnkob

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Re: High voltage Christmas ornaments
« Reply #3 on: December 16, 2019, 10:25:20 AM »
Funny idea, reminds me of the Tesla coil powered fluorescent light chain that TeslaDownUnder made, but I can not find the pictures right now, he got a web 1.0 homepage that is hard to navigate :)

Albeit pretty dangerous to hang outside with exposed NST wires?

TeslaDownUnder also made a series of Christmas installations over the years: http://tesladownunder.com/Xmas.htm
http://www.kaizerpowerelectronics.dk - Tesla coils, high voltage, pulse power, audio and general electronics

Offline klugesmith

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Re: High voltage Christmas ornaments
« Reply #4 on: December 16, 2019, 07:28:18 PM »
Brings back memories.  It was tesladownunder (now what's his real name?) who first referred me to the 4hv forum, about 10 years ago when I was playing with a can crusher.

I put better insulation on the wire connections under tree, and put a variac between wall power and the 15 kV NST.
With no electricity alternating faster than 60 Hz, the octahedron lights at less than 25% of nominal on the variac.
It's appropriately bright and not too flickery at 1/3 of nominal, which would be 10 mA, near bottom of electric-shock hazard zone.  Maybe safer than 120 volts with unlimited current, in the rain, over wet ground, as seen in billions of places.


Here's a picture of the darkened tube end state today.  As a possible indicator for tube aging, it can be monitored more easily, if subjectively, than voltage and current and temperature.  Just guessing.


A friend 3D-printed this experimental socket, with an inclined flange for making an octahedron hub.  It attaches by clamping the electrical terminal pins, instead of relying on friction.  Promising but expensive.  Socket ID came out too small and had to be enlarged.  Might be _partly_ attributed to OpenSCAD having rendering the cylinder as a 36-faced prism in STL file.  As counted on outer surface of printed part.
« Last Edit: December 16, 2019, 07:47:38 PM by klugesmith »

Offline shrad

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Re: High voltage Christmas ornaments
« Reply #5 on: December 17, 2019, 09:33:46 AM »
when I see this I'm worried about the mechanical stress put on the ends... a floating jumper seems to be better there, with the mechanical coupling you have there is will probably snap the ends at one time

Offline Hydron

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Re: High voltage Christmas ornaments
« Reply #6 on: December 17, 2019, 03:16:54 PM »
Would a higher frequency HV source maybe work better for this (especially stopping flickering at low power levels)?
As an example, I have a switchmode oil burner ignition transformer that probably outputs high frequency AC (though I have not measured it) at a sensibly low current, though I suspect it's rated duty cycle is well under 100%.

Offline klugesmith

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Re: High voltage Christmas ornaments
« Reply #7 on: December 18, 2019, 07:44:25 PM »
Electrical failure at the beginning of fourth operating session.  :(
I'm guessing that operating at low currents and ambient temperatures took a toll, much faster than expected, on coiled cathodes and/or on accessible mercury in the lamps.

Near the end of third session, Monday night, it was freezing cold and some of the lamps were much dimmer and oranger than the others.

When switched on Tuesday night, HV arcs appeared in unintended places.   After fiddling unsuccessfully with wire spacing and connections, I brought out the "digital NST kilovoltmeter".  As Variac was turned up, external arc-over began in one place or another on the way up to 9 kV indicated, with lamps barely glowing.  Normal indicated voltage is around 1 kV with all tubes lit.

With everything back indoors, I tested each lamp individually, connected to NST with alligator clip leads to the exposed pin at each end. 
Variac at 50/120 of nominal.  Indicated voltages were mostly 0.15 or 0.16 kV, with a low of 0.12 and high of 0.22 kV; some lamps were dim and orangey at one end.
Lamp #4, the exception, didn't light even with variac at full blast.  At standard 50/120 setting, the NST terminal voltage was 2.15 kV, and the other 11 lamps (still all connected in series except at the original feed terminals) all glowed faintly.  With #4 bypassed, and NST re-connected to the feed terminals, octahedron lit up and the indicated voltage was 1.06 kV.  Turned up to 100/120, indicated voltage was 0.74 kV.

A spot check for filament continuity found some opens, including one end of the bad tube, and the dim end of the tube that had registered 0.22 kV.

When time permits, probably not in this calendar year, this project needs more electrical I-V characterization.  On individual lamps.  Before, during, and after low current operation. In cold weather, as long as that's available.  Fortunately I have about 100 longer (~ 1.1 m) used T5 lamps.
« Last Edit: December 18, 2019, 08:08:26 PM by klugesmith »

Offline klugesmith

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Re: High voltage Christmas ornaments
« Reply #8 on: December 20, 2019, 08:27:15 PM »
Here is the small octahedron soon before failure.  Web says the pink color is argon glow, revealed when mercury is depleted.


To put that in perspective, popular accounts are dumbed down when they say "mercury vapor with argon".
In normal discharge there are about 1000 atoms of argon for every atom of mercury.
Let's say argon at 2 to 5 torr (mbar) and mercury at its vapor pressure, 0.003 torr at 30 °C.

One maker of T5 lamps claims to use only 1.4 milligrams of Hg per lamp.
The amount in vapor form, with nominal cold spot temperature of 40 °C, is only 12 micrograms.
(The lamps are designed to run warmer than T8's and T12's, in fact luminaires enclose the lamps to reduce air circulation.)

I learned that T5 lamps are designed for use only at high frequency (order of 50 kHz), with a standard current of 170 mA.

In my outdoor display with only 10 mA exercise, and glass at 5 °C, the volume of one tube can hold only 0.7 micrograms of Hg vapor, at partial pressure of 0.0003 torr.  With cathodes really cold but plenty of voltage, maybe the cathode emission mix and then tungsten are eaten away by sputtering.  Could mercury be trapped in the dark deposits?

Round 2: a large octahedron made with twelve nominal 28 watt lamps, each 1163 mm long.
Here displayed in a cubicle farm.  This variac set to 90% of 0 to 140 V, so around 30 mA at 60 Hz. 
Indicated voltage is 2.84 kV and hasn't changed in an hour of continuous operation.

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Re: High voltage Christmas ornaments
« Reply #8 on: December 20, 2019, 08:27:15 PM »

 


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