# 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 '20 at 18:33

• 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 '20 at 20:56