The third project I have in mind is a high voltage generator, through an old TV flyback transformer (of which there are many examples on the web), and some other parts that can be easily recovered...
Step 1: The Flyback Transformer
I must say the first difficult I encountered in the beginning was that it wasn't so easy to find enough informations about the pinout of the flyback transformer. Indeed, there are few datasheets that one can find on the net, and moreover each transformer has a different pin sequence and a different number of pins too.
Yet luckily, after a lot of seeking, I found the datasheet and the draw of pinout for a flyback transformer very similar to mine.
So, with the help of the draw in combination with this online guide:
I could make the right attribution for the pins and then keep on with my job...
Step 2: Materials and Components
Before proceeding to make the circuit, no need to remember that this present step should be carried out by people conscious of the risks about high voltage managing.
Since tens of thousand of volt can be reached, all the necessary precaution are a must to make things safely.
Electricity is not a joke!
Well, this is how I did and how I suggest to do. The materials I used are:
- 1 flyback transformer from an old tv (even monitor is OK)
- 1 NPN power transistor (suggested: Vce(p) > 600-800 Volt, Ic > 8 Ampere and P(tot) > 30 Watt, medium h(fe))
- 1 5-watt resistor, 220 ohm (also 3 watt is good)
- 1 5-watt resistor, 27 ohm (or 3 watt)
- 1 capacitor, around the range of 10 nF (1 kV)
- 1 capacitor 22-47 uF (50-100 V)
- 1 ultra-fast diode, like UF2007 (this one is optional but recommended for the health of the transistor)
- A suitable case where insert all the elements (optional)
- Some electrical wires (of proper size and it's better, different colors), soldering iron, tin solder
- A piece of PCB board, where components are mounted
- Power supply for the circuit (at least it has to deliver 2 A at 12 V)
Step 3: Basic Principles of the Circuit
The circuit I made (FIRST IMAGE) is inspired by the principles of the power inverters .
In practice, it is based on the fact that oscillators, such as in the "Royer Oscillator" or also in the "Armstrong Oscillator" (IMAGES 2 AND 3), may generate a higher voltage in the secondary coil through a step-up voltage transformer and a feedback coil. This type of circuits represents a feedback oscillator.
The "core" of the oscillator is the primary, the secondary and the feedback windings of the transformer, which, operating through a concerted mechanism, bring the transistor to act like an electrically controlled switch. So it's possible to obtain a DC pulse at the output. This is nothing but a waveform generator to drive the step-up transformer, which then provides the required high-value voltage (in a somehow AC form) on the secondary.
Leaving aside how all this works in detail, in the next step I will show the role of some of the components used in my circuit....
Step 4: The Circuit in Detail
The transistor must be a NPN power transistor, whose characteristics are shown in the previous step. I used a BU4522AF (in the old CRT tv circuit is used for the horizontal deflection) with quite satisfactory results, but also can be used the BU508D, or the common 2N3055. Resistors must be suitable to bear the current of the circuit, like I indicated before.
For the protection of the transistor an ultra-fast diode, like UF4007, is suggested (it can be optional, but it's better to use it) and a high voltage ceramic capacitor, improving the stability of the oscillator. A toroidal ferrite inductor (> 400 uH) was seen to improve the overall circuit since it helps to prevent current spike and determines less power dissipation for the transistor, but as for me I noticed not so useful so I left it aside.
The "core" of the circuit is the flyback transformer, obtained (if I remember well) from an old CRT monitor.
About the right wiring of the transformer, as I said previously, I was a little bit lucky since I could find the schematic for a similar FT. So I could identify both primary and feedback coil, or better I chose them (based on different voltages), and I used as is, without the need to use additional windings across the transformer. I report the image (SEE IMAGE 1 AND 2).
For the power transistor of course is needed a suitable heat sink, as the circuit waste a fair amount of current.
In fact, for the circuit work well, it's necessary a power supply capable of delivering at least 2 Amps at 12 Volt, that yes it can be also a mobile battery charger. Anyway, I used a DIY bench power supply, delivering up to 10 Amps at 12 Volt.
Step 5: The Operating Circuit and the Electrical Discharge
Once the circuit is put at work, suddenly I can notice a brilliant blue spark through the FT electrodes, and in these conditions I estimated the spike voltage to be around 15-25 KV.
I deduced empirically it from the lenght of the spark gap, according to the "Paschen's law" (for needle-shaped electrodes) that establishes with good approximation the relation between the atmospheric pressure and the gap lenght and the generated voltage:
V (KV) = p*d + 0.7
Mine was around 1.5-2 cm. So I can tell an average value of 20 KV.
I must observe that as soon as the circuit was powered on after a few seconds the smell of ozone is well distinguishable. Among other things the concentration of ozone, anyway, depends especially upon the degree of air humidity.
Step 6: Some Considerations: Generating the Spark...and Ozone
What it does happen in the atmosphere if we introduce a high-voltage spark?
When the voltage applied to the electrodes is great enough air ionization is produced, and therefore we can observe the formation of plasma (that is ionized gas). The plasma created by the high voltage field between the electrodes (like in my experiment) is called corona, and the effect of that is an electrical current flow through the medium. Hence the term "corona discharge".
The generation of ozone raise from the fact that our air is composed by 80 % of molecular nitrogen and the rest 20 % by molecular oxygen, being more reactive than nitrogen. For this reason, there is an intermediate formation of atomic oxygen radicals which can react with molecular oxygen to form ozone.
1) O2 ----> 2O* (formation of radicals)
2) 2O* + 2O2 ---> 2O3 (formation of ozone)
The overall reaction:
3) 3O2 ----> 2O3 (it's an endotermic reaction and it requires an electrical discharge to come)
Other by-products may be nitrogen oxides, but in these conditions they are negiglible than ozone.