Picture of Simple High Voltage Flyback inverter [without feedback coil]
In this instructable I will guide you trough all the things you need to do to get your flyback transformer working. I hope everything is explained in the instructable, but if there is anything you don't understand, you can ask it here. It's a very simple project with very few components, and it's a lot of fun to make!

About a flyback transformer:

A flyback transformer is a transformer with a low number of primary windings, and a high number of secondary windings. The inductance of a flyback's primary coil (this is the resistance of that coil) is really low when working at low frequencies.
When a voltage is applied to that low resistance coil, it will draw a lot of current. To lower that current, the resistance of the coil needs to become higher. We can do that by using a higher frequency. Xl = 2.π.f.L Where Xl is the resistance of the coil, and f is the frequency. (L is the inductance of the coil, which is constant).
This high frequency will be provided by our transistor. It will switch the coil on, at a frequency around 30kHz. We have now limited the current, but it still isn't perfect, so the transistor will get very hot.
That's why it needs to be cooled with a heat-sink. We will also use a MOSFET (Metal Oxide Screen Field Effect Transistor) because it can switch high currents, and is almost indestructible (it can handle high currents, up to 10A, 30A peak).
The core of the flyback transformer is made out of Ferrite, because ferrite works a lot better then iron when working with high frequencies.

Warning: High Voltage!
This guide is meant for people who have experience with high voltage. I gave a lot of safety instructions in all steps, so that people know what can be dangerous and what is safe. Please read my instructions about safety on every step, it's important. I am not responsible for any accidents.
1-40 of 106Next »
I can get pretty purple sparks if I connect this circuit intermittently, but if I hook it up straight the mosfet will get really hot and stop working (I'm on my 5th one on a large heatsink). I'm using an 18.5V 2.7A transformer with a IRF640A mosfet and the recommended resistors (I was using the same mosfet as you which worked even better but eventually burned out too). Is there any way to shield the mosfet maybe with bigger resistors (or a choke ??) or something? I also only measure about 1v through the HV0 (using 18.5V) could that be part of my problem? Thanks for any advice; it is an interesting project.

Connect the gate with reversed diodes to + and - so it wont get overly high voltages and add a zener diode which is rated for around 30v in the conducting direction of the mosfet, then it doesnt gets overly high voltages anymore. (or buy a 800v mosfet as someone stated in another comment)

Electorials (author)  42kwisatzhaderach3 years ago
What do you mean with the HV0 ?
do you mean the voltage over the primary coil?

I have no idea why it gets so hot :/ Is your heat-sink large enough?
When I am trying to locate the secondary coil with the volt meter I put in 18.5 and only get out about 1v from the one little pin on the bottom of the flyback transformer, no response from the others. If I put in 12v I can't get any reading. 32v in will give me about 13v out but I don't want to use that much since 32v is beyond the mosfet rating. The flyback transformer also makes clicking noses sometimes.

My heatsink is about 8in by 4in and seems pretty big; it had 5 separate mosfets/ transistors on it originally.
Electorials (author)  42kwisatzhaderach3 years ago
Ok... That's weird. Are you sure the schematic is correct?
I'm pretty sure it is correct; I took it apart and reassembled it a few times now. It is partially working but just over heats when it is continuously hooked up. I am using alligator clips so it is a bit of a mess but I don't think I have any shorts. Do the Amps matter on the input transformer? Also does your flyback transformer make clicks/screeches?
Electorials (author)  42kwisatzhaderach3 years ago
My flyback transformer doesn't make clicks now because I'm using my new flyback driver.
I must admit that driver you're creating now isn't the best thing you can do.
It should work though.

The amps of the input transformer won't do anything bad.
If it's too low, you'll get tiny sparks,
if it's too normal, you'll get normal sparks
if it's too high, you'll still get normal sparks. It's not a problem.

Well, if it is too high, (as in 10A RMS or greater) you will have overheating problems that can melt the flyback.

Electorials (author)  Electorials3 years ago
Have you also tried with another flyback?
I have not tried with a different flyback, I will the next time I come across one. Do you have a different circuit suggestion that is more reliable?
Electorials (author)  42kwisatzhaderach3 years ago
Not really, except for this one: http://www.instructables.com/id/The-ultimate-PWM-driver-for-many-applications/
But that's something totally different and a lot more complex.
Ok, thanks for the time and info. I appreciate the quick responses. I did get quite a bit of entertainment out of this with just an old monitor, I will probably try a different flyback or circuit when I can. I also picked up a breadboard today so maybe that will help.
Electorials (author)  42kwisatzhaderach3 years ago
No problem. If there's any information you need, just ask it ;)

If you get it to oscillate, you can bring the output to the different pins to see which pin the output tends to arc over to. Also, I like to just touch 12V to other different pins until I see one that causes a spark, indicating an inductive load.

If you have an oscilloscope, check to see if it is oscillating properly. To me, it sounds like the MOSFET is halfway turned ON, so it is dissipating a LOT of power, but since the oscillation is weak or non-existent (you only have an output by interrupting the circuit) you may need to try different value resistors, and change the windings around, or the amount of windings. Try winding a custom primary if you can.

Has anybody taken into account the enormous inductive kick (AKA the flyback pulse) that occurs every time the MOSFET switches off? It is this voltage spike that gives the flyback its name. The inductance of the secondary winding, together with its self capacitance forms an underdamped LC circuit that resonates at a specific frequency. When the MOSFET switches on, the inrush DC current through the primary winding is limited by inductance. This current ramps up to the point at which the ferrite core magnetically saturates and that electric energy is converted to potential energy stored in the magnetic field surrounding the ferrite core. Because this magnetizing current is opposed by inductance, the magnetic lines of force cutting the secondary winding increase relative slowly after the MOSFET switches on, limiting the voltage output from the secondary to a fraction of its peak value. After the core saturates, no more energy can be stored in the magnetic field and the inductance of the primary winding vanishes. The MOSFET current is now limited only by its internal resistance (<0.1 ohm) in series with the resistance of the primary winding (again <0.1 ohm) at 12 volts, the MOSFET is passing 12volts / <0.2 ohms or >60 amperes! Of course, this will load a wall wart down to almost zero volts, but a beefier supply such as a car battery will happily supply that current, heating the MOSFET to the point of failure in short order. Adding an ohm or two of ballast resistance between the source terminal and ground will eliminate this overcurrent situation and protect the MOSFET.

The real action begins the moment the MOSFET switches off. At that instant, the device impedance transitions from a few milliohms to an open circuit in just a few microseconds. With no current flowing through the primary, the magnetic field surrounding the ferrite core collapses like a steel bear trap, dumping a large pulse of stored energy into the core. The energy stored in the field couples back into the core, magnetizing it rapidly in the opposite direction and inducing very high voltages of reversed polarity in both the primary and secondary windings. In a no-load condition, such as when the flyback secondary isn't connected to a load, the self-resonance of the secondary winding causes the flyback to ring like a bell at its self- resonant frequency. This happens when the impulse energy couples from the magnetic field if the flyback's inductance into the electric field of it's self capacitance. This current flows back and forth tens of thousands of times per second, losing a little energy on each pass because of the coil's resistance until all of the energy is lost as heat in the coil or in the load connected across the secondary winding. This is how the flyback transformer generates extremely high voltage, high frequency AC.

This is also how under rated transistors can quickly fail in these oscillator circuits. Remember, at the moment the magnetic field collapses, very high voltages are induced into both the secondary windings. These voltages can peak at 5-10 times the voltage switched by the MOSFET during the forward conduction phase and they do so in both directions. Most MOSFETS have reverse-biased diodes called free wheeling diodes connected across the source and drain within the case. These serve to divert the reverse current spikes around the device and protect it from destructive reverse current. The diode can do nothing to protect the FET however on the succeeding half-cycles of the ringing waveform. If the device isn't rated for the high kickback voltage, it will fail in short order. There are several ways to prevent this short of installing an 800 volt MOSFET. The easiest is to shunt the device with a zener diode with a knee at 2-3 times the supply voltage, but less than the MOSFET's breakdown voltage. You can also clamp the reverse energy by wrapping a few turns of 18 gauge wire around the ferrite core and inserting a reversed diode (called a damper or flyback diode) across the winding. The diode should be a fast recovery type (<100 ns). This method has two advantages. First, the diode conducts, shorting the winding acts only during the reverse portion of the ringing, thereby storing energy in the winding's own magnetic field, swamping out the high reverse voltage across the FET. Secondly, during the following forward half-cycle, the damper diode conducts, dumping the stored energy of the winding back into the core. This energy shows up at the output of the secondary, boosting the efficiency of the flyback circuit.

So, I suggest two modifications to this circuit, Source resistance at the MOSFET and a reverseddamper diode across a third winding.

I am using a transistor form an old PC PSU in a 200v generator by simply charging an inductor to 12v, then it makes a "flyback" of around -200v which i extract by a diode into a capacitor. And i am using the same circuit, but instead of the simple coil a very small flyback transformer (from a plasma globe) to make around 8 kV and it doesnt gets hot (once i tried it with a smaller transistor, as big as the one in the picture, but a transistor and i nearly burned my fingers (it hurt like hell already)).

Great comment. Would it be possible for you to create a modified schematic with your suggested improvements?

RedstoneM14 days ago

Safety tip: Put one of your hands on your back to prevent heart damage and/or hold the cable with one hand on the supply, so you can disconnect it really fast by simply pulling it away.

I'm planning on build this, but i need to know the PSU current spec. What's the minimum required to run this?

Some amperes, i would use a 12v battery with at least 4 Ah (i use one with 7 but with a 5x smaller flyback transformer)

njkl444 years ago
Hey thank you again i can finally play with high voltage EASILY!!! what is the voltage output on this thing (volts?)
pnpnpn njkl445 months ago

A simple way to measure voltage for such devices is to measure maximum spark length, each cm is about 30kV .

As example 2cm spark means its's about 60kV.

Electorials (author)  njkl444 years ago
I think mine gives about 5kV
but I'm looking for what I can do to make it even higher.

I'm sure that a higher input voltage will do that, but I'll check my mosfet first, for the maximum ratings, and I'll have to recalculate the resistors.
usually 100,000 volts is feasible not sure how easily tho
100,000 volts is the opposite of feasible. especially for a flyback alone.
were you thinking of a tesla coil?
Electorials (author)  spark light4 years ago
indeed, it's impossible for a flyback to create such a high voltage.
Maybe not alone, but a voltage multiplier on the output side might work.

actually it can, you just have to do it right, im not sure how but look it up online you'll see 100kv sparks
at 100,000 volts, the flyback will internally arc, causing a meltdown. i'm 100% sure it's impossible because the ferrite core will be an instant path between the two ends of the coil. even under oil, it is highly likely that it will arc internally. the most i've ever seen is about 70 - 80kv. anyone who claims they've gotten to 100kv is most likely optimistically overestimating.
pnpnpn made it!5 months ago

I made almost the same circuit, except I'm using the feedback coil, and in my version there is NPN transistor, you can check my construction on:


The flyback transformer HV generator is a really stuff nice for experiments, for example Jacob's leader (my construction: http://robertgawron.blogspot.com/2015/03/jacobs-la... ), Kirlian photography or Franklin bells. I also think, it's possible to build ion engine with it, but I didn't check if voltage would be sufficient. Anyway if you build one a device presented in this article or similar, those are interesting and relatively simple experiments wort to do.

In addition, if you have're going to obtain flyback from an old CRT TV or monitor, grab also a transistor that drives it, then there is no need to worry what kind of it should be used.

A device as presented in this article, when spark is present emits ozone, nitric oxides, X ray and UV rays, all are not good for your health, so don't run it for long period of time and preferably also not in closed not ventilated rooms.

Kutluhan1 year ago

Hey people this instructable is ONLY for black wired flybacks. Red wired ones not work with this circuit. And this circuitry melts flybacks coat very much. So keep that in mind. :) It's simple but not cheap you can kill your flyback well. I want you to know this so sorry if this comment wasn't nice.

copyjam2 years ago
I couldn't get this circuit to work. I tried it with two different MOSFETs that I had readily available, the IRF510 and the IRFP260. When I connected/disconnected my power supply, I could hear a sharp click coming from the flyback, but there was no HV on the output. The MOSFET became painfully hot in just a matter of seconds.

Any idea what might be happening here?
msalko copyjam1 year ago

hi. it's not like you can use any mosfet like the tutorial says. in your case the IRF510 has maximal current between Drain and Sinc (also called: idss) 5.6 A and the inner resistance of it is 0.54 ohm. and that with 12 vdc gives us 23 amps.


IRLZ34N should do the trick. just put 1ohm resistor on drain, just to be sure if you're using superconductors as wires :D (no really put one there some wires might have lover resistance than you think.)

-max- msalko1 year ago

There are a few other important considerations, however, it is important to know that the MOSFET is not driving a load with zero impedance (similar to resistance, but more general. Impedance includes AC.) It is driving an inductive load. If the figure of 30KHz as the operating frequency is correct, then the inductive reactance (essentially the resistance (or more correctly the impedance) at a certain frequency) will be higher.

Looking at the schematic for the operation of this circuit, it appears that the transistor will never saturate fully anyway. I beleive the output waveform and/or the feedback waveform will be very erratic and contain many harmonics and junk, and since it is amplifying this, it is never fully switched on, so it is truly semiconducting!

Also, the current rating you have given is for steadystate conditions. The peak pulse currents the device can handle go as high as 20A, so as long as it is not left saturated with 20A flowing through it, things should be fine.

With all these simple HV circuits, especially those utilizing MOSFETs, it is important to note that the killers of the MOSFET as overheating of the junction, high voltage transients on the gate (that DESTROY the thin Metal Oxide layer), activation of that parasitic NPN transistor inside all MOSFETs (leads to overheating and catastrophic failure). So, it is important to add at the minimum, a few voltage suppressor devices on the gate so the voltage does not rise higher than the recommended gate voltages in the datasheet. (zener diodes, TVS diodes, MOVs, etc.)

irf3205 might work it can handle 110amps rms

geckomage3 years ago
actually i did a different set-up. i hooked the positive end of the volt meter to the hv. out. the low end of the volt meter to the low end of the power supply, and then used the positive end of the power supply to test around and find it. i found this on another page and it was described there to usually lower the voltage. but generally the secondary coil has a huge amount of resistance (hence why you must use a voltage and power to find it) because otherwise you would need a fancy volt meter to find it. but this resistance goes down when voltage is increased, and the frequency is increased. and most driver circuits run these at 15-25khz ish (correct me if im wrong) and since both the voltage and frequency is increased it lowers the resistance of the secondary coil while in operation. :)

Also all the HV diodes in there have a forward voltage drop, collectively, of amount 15V! Once the voltage exceeds this, the diodes are brought into conduction, and the remainder of the voltage is then has to go though the high impedance coil!

mgingerich2 years ago
For some reason wall jacks always output way more volts than they say they will. I think it's just shoddy manufacturing. It could be that they're designed for a really specific load though

If the reciprocals are outputting ~169.6 volts, that is perfectly normal. You are measuring the peak voltages, not the RMS (Root-Mean Square) voltage. RMS is essentially the average of the absolute value of the AC waveform. It is literally defined as the equivalent voltage/current/power delivered to a resistive load, such as a heater or Edison lamp. Also they are not designed as labratory power supplies, so they are crude in the fact that they have high tolerances.

1-40 of 106Next »