Instructables

Simple High Voltage Flyback inverter [without feedback coil]

Intro
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.
 
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Kutluhan2 months 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.

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.
Electorials (author)  42kwisatzhaderach2 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)  42kwisatzhaderach2 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)  42kwisatzhaderach2 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.
-max- Electorials4 months ago

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

Electorials (author)  Electorials2 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)  42kwisatzhaderach2 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)  42kwisatzhaderach2 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.

copyjam1 year 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 copyjam6 months 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.

THATS WAY OVERPOWERED.

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- msalko4 months 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.)

dillonxti copyjam5 months ago

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. :)
-max- geckomage4 months ago

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
-max- mgingerich4 months ago

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.

-max-4 months ago

You should add a few TVS diodes, MOVs, or at least some zener diodes between the gate and drain of the MOSFET, to prevent a high voltage transient from destroying the fragile Metal Oxide Layer between the gate and the rest of the Semiconductor Field Effect Transistor.

msalko6 months ago

I'm not sure but i think there was one type of these transformers that had secondary coil connected to feedback coil. I can only imagine the guys face when he gets 1kv out of nowhere :D

(removed by author or community request)
Electorials (author)  Liam.great981 year ago
do you get a spark just at the moment when you connect the power supply to the circuit?
can you please help this won't work!
Electorials (author)  highvoltageguy2 years ago
What's not working?
the whole circuit won't work
geckomage3 years ago
I know this somewhat defeats the purpose of your instructable (which i will try tomorrow and see how well it works :) then post results) but do you know of any good way to detect a feedback coil in a flyback? im thinking of using a 20 volt 2 amp freq generator to make a square wave at around 20 khz and using an osciloscope to find which set of pins gives me the right wave form for the feedback ( i did get some crazy waves comin off of the secondary, but no arcing :[ )
Electorials (author)  geckomage3 years ago
what do you mean with crazy waves? I hope you didn't connect your scope to the secondary, because it can only handle 400V max!

and I don't really know a good way for detecting the feedback. The only real way I know, is to apply an alternating voltage, of which you know that part is working correctly, to the primary windings. (like you said with the functiongenerator), and use a volt-meter to measure the AC feedback voltage. It should be just several volts, 1 to 3 volts I think.
haha no worries. we have a pretty heavy duty oscilloscope aside from the regular ones and have several special probes for it. from bumping up really small voltages to where you can clearly see the waves to taking up to 30 kv and bumping it back down to see the waves without frying the osciloscope ;) (had a bunch of sponsors donate generously to fund the program) got a full electronics lab, the full autodesk, mastercam, and multisim swuites on some powerful computers. a full wood/metal shop with lathes and mig and tig welding machines included. also a large 3 axis cnc mill, a smaller 4 axis one, a smaller 3 axis prototyping one and a cnc plasma cutter. :) ill only have acess to it till the end of next semester though, so if you have anything you'd like me to try to build for a project that you'd like to test let me know ;D we have a circuit board printer too but thats not quite working yet :( and thanks for your speedy replies and advice! :D
Electorials (author)  geckomage3 years ago
aah that's cool

and no problem ;)
geckomage3 years ago
On most semi-modern flyback transformers there is a built in diode on the primary, as well as a rectifier on the secondary (helps smooth out the voltage). So the polatiry does matter (as you have said). One method i found to work well for finding the primary coil's polarity is this. After finding the pins for the primary, solder a few wires onto the pins (does not have to be permanent). Next, take your voltmeter and hook the positive probe (the red one) to the HV output wire (the big thick usually red wire on the top of the flyback) (this can be done with alligator clip probes or by using alligator clips on your probe tips. if no alligator clips are available just use some electrical tape to hold them on or something) then take your negative (black) probe and hook it to the HV 0v pin which you have already found. If your volt meter is like mine and automatically adjusts itself to measure the proper voltage, you should be fine. if not, i have never had this create more than a readable 35 volt spike. Then take your 9v battery and hold one of the connections on one of the terminals of the battery, then take the other and rapidly tap it with your finger (or something else if you dont want to risk shocking yourself, but i had no issues with this) to simulate pulsing DC current. Keep a watchful eye on your volt meter. (if its hooked up the wrong way you will get next to no voltage) then try flipping the polarity of the 9v battery. which ever one gives you a larger voltage spike is the correct polarity of the coil! enjoy :)
Electorials (author)  geckomage3 years ago
That is indeed a good way for finding it ;)
lol just thought i'd help anyone that needed it :D then you can see fairly easily which way it is (i dont know if you get arcs when you hook it up backwards) but ive always believed in starting with smaller voltages first ;D
geckomage3 years ago
I do not know if anyone had anything similar occur but with this same setup i applied 24 volts to it (as i found on a different instructable) and only got about 12 volts on the HV 0v pin. but none of the other pins showed any voltage. hope this helps someone!
Electorials (author)  geckomage3 years ago
do you mean with the same setup as in the picture here?
It's weird to get only half the voltage.. Really weird actually!
It would mean that the resistance of the coil would be as large as the internal resistance of your volt-meter, which is several mega ohms.
and that would mean your coil is no coil but an insulator.

Could you explain more in detail what you did there?
Umniscient3 years ago
I don't understand what you mean by "the polarity of the primary coil".
Does it mean which side of the primary is connected to the resistor/12 volts and which side is connected to the Drain?

I've tried both directions, and seen no corona discharges at my HV +. The secondary measures at a few millivolts. In fact, I believe my circuit is inputting DC to the primary coil.

Or is my understanding incorrect?
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