DISCLAIMER:

This is an extremely dangerous project, and it should only be attempted by people with experience in electronics, and specifically, High Voltage. If it's your first time seek help for your own safety.
Homemade High Voltage supplies are unlikely to meet any international standards, the safety and correct operation is NOT guaranteed at all, and will depend on the skill level of the builder, the effort put into it and most importantly, the common sense.
This is not intended to be a tutorial on how to build a high voltage PSU, I only intend to show how I built it. This PSU has been built according to my criteria, and it can be useful if you're looking for inspiration, but I discourage anybody who wants to replicate it blindly without applying his/her own common sense. I'm not an expert on the subject.
I'm not responsible for any damage or injuries caused by the use of this information.
Please, be extremely careful with High Voltage.

Step 1: Intro:

This high voltage power supply has been designed to output a fixed voltage of around 50kV, it could easily be converted to an adjustable supply by connecting a variac in case of using transformers or by adding some extra circuitry to regulate the power going in. I initially thought about a high frequency PWM to regulate the power going into the capacitors, but I abandoned the idea. I found that adjusting the frequency is enough to make the voltage vary by a significant amount, allowing some control over it, this happens because the flyback must operate at a certain frequency in order to maximize the output.

The total cost of building it is around 10 to 15€ since most of the parts (transformer, bridge rectifier, heatsink, flyback, switch, connector, cables...) have been salvaged, the only parts that I bought are the components of the 555 driver, the connectors and the capacitors. This shows the importance of having a big pile of electronic junk, specially old stuff with chunky electric and electronic components, it doesn't matters if you have to pick it from the dumpster, it can save you tons of money on the long run and by repurposing these devices you're being Eco-friendly. A good practice it to save the tin when desoldering and avoid throwing it into the trashcan and when you're done with the board or there are no more valuable components you can take it to a place where it can be recycled properly.

Caution has been taken in order to isolate the high voltage output from the user and the internal circuitry.

Step 2: Materials:

Power input:

Transformer + bridge rectifier + capacitors

Switching PSU

Both must be rated rated 5 amps and 20 volts at least for higher voltages.

Separate* input for the driver
Switch of choice and connector
Shrink tube

Driver circuit:

Perforated board of PCB to be etched
555 timer
8pin socket
7812 (if the power intput to the 555 is > than 14.5 or lower than 35)
Small heatsink for the 7812 or other regulator (if needed)
2*100nF
1*1uF
1*10nF
1*68uF (or 100uF if you wish)
2*4148 diodes
3*10k
(1 per MOSFET) 10R
1*680R
1*470R
1*10k pot
1*100k pot
2* pot knobs
1*2N2222 and 2N2907 (or other NPN - PNP pair)
1*Infrared sensor
1*Infrared LED
1*BC547 (or similar e.g: 2N2222 or 2N3904)
2*Banana female connector (or isolated high voltage connector)
MOSFETS (I used 3* IRF540N but I recommend 1* IRFP260)
Heatsink for the mosfets (and fan if needed)
Pushbutton

High voltage:

Flyback transformer from an old TV or computer monitor
Thick copper cable (around 1 meter)
Epoxy

*You can try using the same supply used to power the flyback to power the driver, but I'm not sure about if this would cause the driver to operate under rough conditions and lose overall efficiency. I've used a 12V 1A PSU from an old router. Remember to connect both negative terminals of the power supplies together, otherwise it won't work.

Step 3: Calculations

The only calculation we'll need to perform is to obtain the value of the capacitors (in case you're using a transformer and not a switching PSU to power the flyback) we use the formula provided by the picture.

In my case I used 20000uF, this causes quite a lot of ripple, but it isn't really important since the flyback won't be picky about it, I'll maybe add 10000uF or 20000uF more just to see the effect on the output. I discourage wiring the driver to the same source than the flyback, the ripple created by the changing currents could alter the correct working of the driver, resulting into lower efficiency and smaller arcs.

Step 4: Building the Case

Every power supply needs a case to hide the components from the user, and this is a high voltage power supply, so extra care must be taken. The obvious choices are wood and plastic or variants, although plastic is preferable. Metal is not a good option for a beginner, specially without further isolation and using proper rated connectors, cables... even one of the sides being metallic adds a tremendous amount of danger for you and the circuitry.

Even though the use of plastic is encouraged, I chose wood because I couldn't find a plastic case big enough, this has allowed to further isolate the high voltage parts avoiding corona discharges and other potential dangers.

After applying my basic woodworking skills I came up with this, the steps are detailed in the pictures.

Note: If you plan to paint the case, check the paint isn't conductive at high voltages when dry, oil based paint is recommended if it's too dense mix with a thinner (e.g: acetone). Water based paint is obviously forbidden.

WARNING: Although wood is a very good isolator it can't be compared to plastic due to it's ability to retain moisture. Using wood is discouraged if it's going to be in a very humid environment, and extra care must be taken in order to keep the wood away from water. It's a good advice to let the wood dry in an oven before applying paint or treating it.

Step 5: The Driver Circuit:

I've made this small driver circuit it revolves around a 555 timer with adjustable frequency and duty cycle (from 5-50kHz and 5-50% duty cycle), it has it's own 12v input independent from the transformer, which has an 8 volt output, unfortunately this one doesn't delivers enough current to charge the gates fast enough, leading to efficiency problems which translate into lower output voltage at the flyback.

The MOSFETS are three IRF540N in parallel, I used these because I had them lying around and they're relatively cheap, you can also use the popular IRFP260N. In this configuration they barely get warm even under full load.

I was going to add an optocoupler to the circuit but I decided not to, Instead I just wired a button with a 1k resistor where the IR sensor should be placed. You can edit the circuit and remove the transistor, leaving the button connected to pin 4 with a 10k resistor going from it to ground.

Note: To make the duty cycle go from 5 to 50% (and not from ~5% to 100%) you have to place a 10k resistor at the potentiometer as shown in the picture, this resistor must be placed in series with the diode facing the capacitor. If you connect it in series with the other diode you'll end with an adjustable duty cycle from 50 to 100%.

You can download the Eagle schematic and the board files below if you plan to etch a PCB.

Step 6: Wiring Things Up:

After making sure the circuit works correctly I wire the MOSFETS in parallel, to do this I join all the drains and sources with high amperage cables, add a 10Ω at each gate and joint them together.

Note: If you parallel MOSFETS add a 10Ω resistor at each gate and bridge the 10Ω resistor on the PCB.

I also attach a mains connector and a switch to the case and wire them, it is very important to use heatshrink tube or other kind of protection when making connections with the mains, you don't want exposed connections around. I screwed the ground connection to the case just in case I want to wire something afterwards and so it doesn't moves.

If you want to make an optocoupler to further improve the safety of the supply you can make one with an IR LED and sensor and a piece of heatshrink tube (white is better since it reflects light better than black), wire the IR LED to a batery pack in series with a resistor and a button and you're good to go. Just make sure this circuit isn't in close proximity to the other one or you'll defeat the purpose of the optocoupler, which is preventing you from getting shocked. Without an optocoupler current can travel through your body in case you touch or get near the high voltage output, reach the button and enter the circuit again.

WARNING:A pushbutton is extremely preferable over a switch, in case of accident the button will spring back and shut the circuit down. NEVER use a switch to turn on the high voltage output unless everything is always under special conditions that make it impossible for an accident to occur.

Once the driver has been assembled I can start testing without having assembled the PSU yet.

Step 7: Wiring the Power Supply:

After I found a suitable power source I attach it to the rest of the circuit. I connect the 12V PSU to the grid along with the transformer with a single pole switch wired as shown in the pictures. I connect the transformer to the bridge rectifier and then to the capacitors using plenty of heatshrink tube.

Wiring the power supply to the flyback and MOSFETs is shown in the next step.

Step 8: Preparing, Wiring and Isolating the Flyback

To prepare the flyback I just need to find the negative pin, this is usually done by powering the flyback up, then swing the high voltage cable around the pins until it arcs to the right pin. I solder some thick wire and cut and cover all the pins with epoxy, this is a good practice, since it avoids corona discharges and other nasty things, hot glue works too, but epoxy is better since it soaks better.

Then I wind around 10 turns of thick wire around the core of the primary. The positive output of the PSU is connected to one of the ends of this wire, the other end gets connected to the drain of the MOSFET, finally, the source is connected to ground. You can use a terminal block to swap the connections around (see picture), it's quite convenient.

One very important thing to notice is flyback transformers have polarity, they have a built in diode which only allows current to pass in one direction. To test which way you should connect it just power the flyback (lowering the power just in case) and place the positive and negative wires at a certain distance, then reverse the connection, compare both spark lengths, if the spark or corona discharge is bigger one way or the other you'll know you've connected it right, I usually mark the negative side with a permanent black marker in order to avoid confusing them.

To isolate the flyback I pass the high voltage wire to the next compartment though a small hole, roughly the diameter of the wire, I solder the wire to the banana connector and then stuff the cavity with plastic film, this will ensure there's no arcover or corona discharges, i cover the plastic film with some plastic foam. I cover the flyback compartment with the same kind of foam.

You've probably noticed that small cup around the connector, I drilled a small hole and passed the connector though, then I bolted it to the case, it might not be as good (and expensive) as a standard high voltage connector, but this cup is a good protection against accidental arcovers.

EDIT: I added a 22uF 250V capacitor across (or parallel to) the primary in order to achieve some resonance, this has improved the current output and I would say the voltage is a bit higher now, I might be wrong, but the transformer and bridge rectifier don't seem to get as warm as before, or at least I haven't felt any bad side effects of using the capacitor. I recommend using a non polarized high voltage rated capacitor since the voltage spikes at the primary can be nasty.

Step 9: Test Run:

After everything is connected you can test if everything is working fine. Be extremely careful when doing your first test, it's extremely advisable to set a spark gap and a wood stick to push the button just in case. If you're using two separate power sources for the driver and the flyback, remember to join the negative terminals, otherwise it won't work.

After completing the first test without problems I would test it again in total darkness to see if there are corona discharges where there shouldn't be and fix this with epoxy, corona dope or some other kind of insulating material.

To optimize the flyback's output power we'll need to adjust the frequency and duty cycle, to do this you can set a spark gap and adjust the knobs until the spark length is maximum. You can also measure the current flowing through the secondary in and adjust the potentiometers to obtain the maximum amount.

Step 10: What Can I Use It For?

X-ray tube PSU
Gas ionization
Ozone generator
Nitric Acid generator (Birkeland-Eyde process)
Metal vaporization and deposition
Smoke precipitator
Jacob's ladder
Franklin's bell
Ionocraft
Electrostatic motor
SG tesla coil PSU
More random experiments (and burning things up)

And the list goes on...

You could possibly convert it into a plasma speaker by connecting an audio input to pin 5 of the 555, you can add a small amplifier if the sound isn't loud enough.

Step 11: Some Safety Advices:

I found this instructable which explains in detail the procedures and guidelines when dealing with high voltage as well as identifying potential electrical dangers.

I also found this pdf I already posted in other high voltage related project.

It's extremely advisable to read and comprehend this information, you shall not proceed with any high voltage project without knowing the basic safety precautions and procedures and without previous experience.

Special equipment like properly rated rubber gloves should be worn.

Also, when high voltage is mixed with vacuum there's a chance to produce X-rays, vacuum tubes and some low pressure lightbulbs are known to produce X-rays when a high voltage is applied. Be careful an follow general radiation guidelines to avoid accidental exposure to ionizing radiation.

Step 12:

I hope this has been useful to obtain some inspiration and gain experience about experimenting with flyback transformers and high voltages.

Thanks for watching and please be safe.

Introduction: 50,000V High Voltage Power Supply