LED arrays for use with Tesla coils

[davekni]

Made LED strings specifically for use in AC electric fields such as near Tesla coils.  Idea is to replace more fragile fluorescent tubes.

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Standard LED replacements for fluorescent tubes work OK, but are lower voltage and higher current than is ideal for Tesla coil use.  Most emit light on only one side.  So I made LED strings as an alternative, with different colors on the two sides.  They are visible under HV power transmission lines, and of course brighter in the higher-frequency field of Tesla coils.

One option is to connect pairs of LEDs anti-parallel.  Made a couple small ones that way previously.  However, each LED gets only half the total current (either positive or negative half-cycle).  To improve current efficiency, these strings use signal diodes in a bridge rectifier connection for every 20 or 24 series-connected LEDs.  That way full current passes through each LED.  Pitch is one LED per 5mm (one per 10mm on each side), for 100 and 120mm long sections.

One feature didn't work very well:  A goal of sectioning the strings was to mimic behavior of fluorescent tubes where sliding one hand up the tube prevents the section between hands from glowing significantly.  My issue appears to be insufficient capacitance from LED strings to outside of plastic tube (hand), especially for my SMD version.  LED ECBs are only 7mm x 120mm inside a 16mm OD tube.  Next version needs to have some metal closer to tube walls (and perhaps larger-diameter tube) to increase capacitance.  Not sure how to best accomplish this while minimize light blocking.  The three commercial LED tubes connected in series worked well in this regard, but in large steps of their ~300mm tube length.  I think their opaque back side appears to have metal just under plastic outer layer, so plenty of capacitance.

If there's any interest, I'll post ECB files.  SMD version uses 2835 LEDs and SOT23 series-pair diodes (MMBD1203).  Includes anti-parallel diodes across every 3 LEDs to avoid any reverse-current in case capacitance from hand introduces AC to middle of the string.  Not at all sure such is necessary.  One could populate just the two diode packages, one at each end, that form the rectifier bridge.  (BTW, this is how direct-replacement LED tubes work.  One diode pair at each end of tube rectify incoming AC from ballast.)  Through-hole version is for 5mm LEDs.  Diodes are same, still SMD.  I plan to make an updated SMD version with multiple test-pad holes for the internal DC rails for adding external wires to increase capacitance.

I'd be happy to send MMBD1203 diodes to anyone who needs them.  Saw a good EBay deal for left-over Fairchild parts originally purchased from Mouser.  I now have ~6k parts and no need for that many.

[klugesmith]

Nice work.   Where did you get little circuit boards fabricated?

Have you ever tried Tesla-coil excitation of multiple fluorescent lamps with terminals connected together?
Be interesting to model the circuit of, say, an big triangle of three lamps in field of the TC.

With holiday light season coming up (not counting Diwali) I hope to restore my fluorescent lamp octahedron.
Simple connections at each vertex put the 12 lamps in series, easy to light up with mains frequency NST.
In last round (2019), tubes started to fail in tens of hours, when operated in cold weather without enough current to keep the cathodes hot.
But that's a story for different thread.

[davekni]

Nice work.   Where did you get little circuit boards fabricated?

These ones came from OSHPark (local, though USPS shipping is free to US).  Boards were too small for DigiKey Red.  DigiKey Red is slightly cheaper and a bit faster, but has more requirements and limitations.  For larger boards, I often order from AllPCB (China).  Boards are much cheaper, but shipping eliminates savings for small orders.

Have you ever tried Tesla-coil excitation of multiple fluorescent lamps with terminals connected together?

I haven't kept any long fluorescent tubes around around.  More fragile and hazardous.  LEDs are more efficient too (and come in colors).

With holiday light season coming up (not counting Diwali) I hope to restore my fluorescent lamp octahedron.
Simple connections at each vertex put the 12 lamps in series, easy to light up with mains frequency NST.

Saw pictures of your octahedron.  Looks great.  Could make something visually similar with LED tubes and standard 120VAC (if using tubes with internal ballasts).  Not sure how hard it may be to find omnidirectional LED tubes.  Perhaps back-to-back 12V LED strips would work better.