Author Topic: Small DIY plasma globe using ZVS oscillator  (Read 148 times)

Offline davekni

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Small DIY plasma globe using ZVS oscillator
« on: January 22, 2020, 06:21:31 AM »
This is my latest little project - only 20 watts.  Made a plasma ball from a plastic wine glass filled with about 15kPa of argon.  Works some with air at lower pressure, around 5kPa.  Experimented with argon/helium mixtures, but plain argon was the brightest.  Don't have neon nor xenon around (due to expense), the normal gasses inside plasma balls.  Mine isn't as bright and doesn't have multiple colors.  Still fun.



For the HV transformer, I used a pair of small already-potted transformers from EBay:
https://www.ebay.com/itm/20KV-high-frequency-high-voltage-transformer-ignition-coil-inverter-driver-bNWUS/323974762353?hash=item4b6e668f71:m:mg4RAb8mo-3R0Me2JUcxcfw

The pair are electrically and magnetically in series, both primary and secondary.  Magnetic path is closed with some NiZn ferrite pieces (low electrical conductivity type).  Still some gap due to the potting.  Driving circuit consists of a normal hard-switched buck converter using the same inductor as the ZVS input inductor, 19V to 15V(adjustable), with ~1.2A current limit.  Separate buck and ZVS boards, still connected with clip leads in the pictures:





The HV winding capacitance and load (plasma ball) capacitance alone makes a ~25kHz resonant frequency.  So, I added only a little bit more primary-side capacitance.  This creates an issue with the two possible resonant modes, one at ~25kHz and another (with primary and secondary voltages opposite) at ~530kHz.  To keep the ZVS oscillation from locking to 530kHz, I added a series LRC notch filter across the ZVS output (transformer primary).  Schematic is:


Here are a few waveforms of the ZVS outputs (transformer primary terminals).  First is with low load (nothing touching the plasma ball).  Second and third are with load (hand on plasma ball), showing lower voltage (due to buck-converter current limit) and a cycle or two of high-frequency mode ring at each zero-crossing.  Without the notch filter, the ZVS locks to the high frequency and stays there.




Any suggestions for improvements?

2/2020 update:  After a longer run, the little transformers developed an arc path between the two, burning through the potting material.  I've replaced them with two identical parts glued end-to-end instead, avoiding the series HV being adjacent.  Haven't ran that configuration for long yet.
« Last Edit: February 20, 2020, 05:17:40 AM by davekni »
David Knierim

High Voltage Forum

Small DIY plasma globe using ZVS oscillator
« on: January 22, 2020, 06:21:31 AM »

 


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