Generating ozone and negative ions in the air can be done by a corona discharge. For this we can use a sharp tip of negative polarity, acting against a flat metal part of positive or neutral potential. The device generates negative air ions and ozone. The ozone acts against rot and mould, and can refresh the unhealthy air of humid cellars or whatsoever air charged with bacteria, fungus or bad smell.
A cascade, generating a high DC voltage from grid voltage, is simple, silent and has a far higher efficiency than the typical flyback converters. Here is what we call in Germany a Greinacher- rectifier. In the English-speaking world the schematic is rather called a Cockroft-Walton multiplier, probably independently invented.
Schematic of the ozone generator circuit
The capacitors were put on the upper side and the diodes on the underside of perforated boards without copper islands, just the pure plastic board. In the negative branch are 15 Capacitors 68 nF 630V (0.068uF), in the positive branch 14.
We need 29 diodes.
In the range of high voltages, the connection spots were carefully rounded and deburred to avoid corona discharge at unwanted locations, and then all sharp edges were painted over with epoxy glue. This has a relative dielectric constant of about 5 and lowers the field strength to below the corona level.
The diodes can be 1N4007, but the first one has to be 1N5408 or similar. Very important! Because when you switch the device to the grid, the first uncharged capacitor draws a current peak which is limited only by the impedances and resistances of the grid, cable, fuse, diode.
A 1N4007 will be damaged, and fuses below 2A will blow. This is usually not taken into account with the schematics in books and in the internet.
I made my own fully isolating and touchproof casing from plexiglas, glued together with UHU-PLAST. The screws for lid and the PC-board are from Polyamide.
You need to make also a touch-proof discharge chamber; else the 7 kV is too dangerous.
The first discharge elements were graphite inserts for rechargeable pencils on the negative side and a brass ring on the positive side. A glass chimney was built around the discharge parts, which is so high that you do not reach any parts when pushing your finger inside. On the lower end of the chimney is an opening, which is so narrow that you don’t get your fingers in. So the device is touchproof, but not tamper-proof. Someone could theoretically push a knitting needle inside or a spoon or whatsoever. If there are small children in the same room, switch it off and take it away.
The graphite pencil cores were glued into a brass holder with a mixture of graphite powder and “banana-oil” lacquer, which conducts fairly well after the lacquer has dried. This worked for about a year and then caused trouble, because from time to time a graphite tip became red hot glowing and then caused a flash-over, destroying the fuse.
So I converted the discharge elements to hardened steel nails acting against zinc-plated steel washers. This arrangement also gives a good ion wind and an updraft in the chimney. So we dont need any ventilator.
The distance is to be adjusted to give a visible and audible corona discharge in normal air conditions, but no spark-over in higher humidity. In the dark, the tips shall develop a visible slight blue light. About 5-7mm is correct.
The here described device is enough for a large living room. It is too powerful for a small room and continuous operation. It takes 0,35mA from a 230V 50Hz grid.
If there are no humans or animals in the room, you can generate as much ozone as you like, but need to switch if off before staying there.
The correct concentration of ozone is, when you enter the room and just feel that the air is “somehow fresh” but you cannot make out a distinct ozone smell. The ozone concentration is too high and becomes unhealthy, when you get cough, tears in the eyes or a running nose. It is slightly above the limit when you can recognize ozone content in the air. The “measurement” has to be performed by staying outside for a while, then entering the room, taking a deep breath and judging. Because you get used to it when staying inside.
After some time (6-12 month) the tips become dull and the zink on the washers is converted to zinc oxide (this effect has no relevance). Also dust collects. So it is a good idea to clean the device and re-sharpen the tips once per year. If you can get stainless steel washers of seawater-resistant quality (maybe from a boat shop), they will last longer.
I have also made an about 8 times more powerful device, which is used for disinfection of an underfloor range of a building (no contact to living space) and will report about it later.