# swapping the ground reference on a transformer's secondary winding

I'm looking to build a [simple] high voltage power supply in order to make electrets via corona discharge. I'd like to be able to fabricate both positively and negatively charged electrets using this method, ideally with the same power supply requiring only minor configuration changes.

Right now, I'm leaning towards using a "wasted spark" automotive ignition coil which has both ends of the secondary winding exposed as such:

The logic behind opting for such a coil is that they're plentiful (i.e. cheap), can generate 30+ kV, and some are easily interfaced with needing only a 12V supply and a logic-level signal for pulse triggering.

Based on my understanding of the corona discharge method used to make electrets (but not the "triode" method), the insulator that you're making the electret from sits on a grounded metal plate, and then some electrode (be it a wire, or needle/s) sits above it at either a high negative or positive potential depending on the charge on the electret desired, and produces a corona discharge.

So my question essentially boils down to this: assuming nothing on the primary side of the transformer changes, is swapping the polarity on the secondary as simple and switching the two output leads, and grounding the appropriate leg?

Also, I may want the output to be DC. If I'm to add a blocking/rectifying diode and filter caps, can I still easily swap the polarity? If so, would I do that after the diodes+caps?

• I ended up finding an Applied Kilovolts brand precision variable 100 - 30,000V PSU with reversible polarity on eBay for a reasonable price. So while I technically don't need an answer to my question, I'd still appreciate one mainly out of curiosity. I'm fairly confident that the output polarity relative to ground can be selected based on which lead of the transformer's secondary winding is grounded, but I'd like confirmation.
– mes
May 23, 2020 at 18:33

A CRT flyback makes the acceleration voltage high-voltage DC, using a combination of a step-up transformer and voltage multiplier. A copier or laserprinter corona generator works similarly. Maybe repurpose one of those?

Otherwise, what might be possible is to use such a coil to drive a Cockroft-Walton type multiplier. Here would be the first step: drive the coil. https://hackaday.com/2019/09/14/subaru-coils-make-a-great-hv-power-source/

Here's another idea: electrostatic air purifier supply, like this one: https://www.amazon.com/Voltage-Electrostatic-Cleaner-Supply-Output/dp/B073GGCFSF

• I have considered using a flyback-based HV PSU kit off eBay. I don't think those are rectified/filtered, but have both legs from the secondary exposed. If I used a unit from a TV, I'm not sure how I'd switch polarity. Is it just a matter of swapping it at the primary? Using an ignition coil, I think the voltage will be high enough, so I doubt I'd need to add a multiplier. Funny that you mention that hackaday blog post because I found it yesterday while reading up on this.
– mes
May 22, 2020 at 19:21
• TV units makes a positive acceleration voltage. Was thinking it might be possible to float the unit's power input such that it could be used to make negative voltage. Same thing with a copier corona unit. That said, the coil with the CW multiplier could be wired either way to make the voltage. May 22, 2020 at 20:34
• Added another idea: air purifier electrostatic gen - link to 20kV unit. May 22, 2020 at 20:38
• I was reading up a bit on those "ionic" air filters yesterday for circuit ideas, or even buying one to repurpose. For slightly under $200 I could buy a used lab/industrial-grade HV PSU off eBay. The problem is that they all tend to be only a single polarity. OTOH, I can buy a used ignition coil for probably$10 at a junkyard. Then with a 555 and some other scraps lying around, I can build one. Also, I don't think I can simply swap the primary-side polarity on a CRT flyback that has an integrated multiplier, since the diodes would be reverse biased.
– mes
May 22, 2020 at 20:56