Simple High Voltage Flyback inverter [without feedback coil]

9 Steps
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!

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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Step 1: Components

What you'll need:
- a Power MOSFET (I used a K2996 - Datasheet: -link- )  (any MOSFET should work, it doesn't need to be exactly this one)
- a good heatsink, the MOSFET will get hot
- 2x 1kohm resistor (0.25W)
- a Flyback transformer (no feedback winding required)
- wires and a soldering iron, or aligator clips and or a breadboard.
- a power supply or battery (12V DC)
- a healthy mind, we're working with High Voltage here!

Flyback transformers can be found in devices with a CRT screen (Cathode Ray Tube) or on Plasma Globes. They generate high voltage, and release the electrons in a cathode ray tube (your screen) When the electrons land on the Fluorine layer on your screen, it lights up.

MOSFET's can be found in many devices. I found mine in magnetron's and also in LCD screens.
Normal transistors will probably also work, but I havn't tested it yet.
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Liam.great98 says: Apr 27, 2013. 11:10 AM
I just made this and I notice my MOSFET gets quite hot which means SOMETHING is working but the flyback doesn't make a sound. Im using my own primary made with 9.5 turns of copper wire because I couldn't locate the pins for the priary. My flyback is from a monitor, not a tv. It is big and has the horseshoe array of pins plus an extra three in a line beside. It won't arc to any of the pins.
Electorials (author) in reply to Liam.great98Apr 29, 2013. 11:59 AM
do you get a spark just at the moment when you connect the power supply to the circuit?
Liam.great98 in reply to ElectorialsApr 29, 2013. 12:59 PM
Not at all...
mgingerich says: Sep 21, 2012. 7:00 PM
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
42kwisatzhaderach says: Jan 7, 2012. 7:19 AM
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.
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.
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?
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.
Electorials (author) in reply to ElectorialsJan 7, 2012. 11:24 AM
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?
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.
No problem. If there's any information you need, just ask it ;)
highvoltageguy says: Dec 22, 2011. 9:24 AM
Electorials (author) in reply to highvoltageguyJan 7, 2012. 9:40 AM
What's not working?
highvoltageguy in reply to ElectorialsJan 7, 2012. 10:58 AM
the whole circuit won't work
geckomage says: Oct 26, 2011. 11:44 AM
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) in reply to geckomageOct 26, 2011. 1:04 PM
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.
geckomage in reply to ElectorialsOct 27, 2011. 9:15 AM
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) in reply to geckomageOct 27, 2011. 10:57 AM
aah that's cool

and no problem ;)
geckomage says: Oct 26, 2011. 11:38 AM
Electorials (author) in reply to geckomageOct 26, 2011. 1:01 PM
That is indeed a good way for finding it ;)
geckomage in reply to ElectorialsOct 27, 2011. 7:14 AM
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
geckomage says: Oct 27, 2011. 6:56 AM
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. :)
geckomage says: Oct 26, 2011. 11:45 AM
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) in reply to geckomageOct 26, 2011. 1:07 PM
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?
Umniscient says: Aug 24, 2011. 5:48 PM
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?
Electorials (author) in reply to UmniscientAug 25, 2011. 2:23 AM
in step 6 you can see a yellow tag around 'the primary windings'

That winding has 2 connections. Changing the polarity of that coil means reversing both connections.

You try it once with pin1 connected to +12V and pin2 connected to drain, and after that, you try once with pin1 connected to drain and pin2 connected to +12V.

If none of both seem to make high voltage on the secondary windings, you really have a problem :/
Are you sure your setup is the same as in the schematic?

I suggest you try this:
Connect everything
Measure the voltage over the Primary winding
Measure the voltage over the primary winding while Gate is Disconnected

If there is any change, it means your MOSFET is working correctly.

results should be:
Gate connected -> V-meter: 12V
Gate NOT connected -> V-meter: 0V
Umniscient in reply to ElectorialsAug 25, 2011. 2:09 PM
I did what you said, and it turns out that my MOSFET was broken.

Would this work well instead, or is the current rating too low?

Electorials (author) in reply to UmniscientAug 25, 2011. 4:34 PM
That mosfet should be fine. I recently used an IRF630, and that one worked great, so if you could also buy that one on radioshack, that's good. But the one from your link should also work fine.

What were te results of the test? You said your mosfet is broken, but are you really sure about that? there might also be another problem...
Umniscient in reply to ElectorialsAug 25, 2011. 4:40 PM
I first measured the voltage over the primary coil: 12V
Then I measured it when the gate was disconnected: 12V
There was no drop to zero, so I figured there was something wrong with the MOSFET.

After I realized this, I plugged in another random MOSFET and tried it. No HV on the secondary, but a drop from 12 volts to 2 volts when disconnecting the gate. Does this mean the transistor is working? Or does it have to go to 0V?
Electorials (author) in reply to UmniscientAug 26, 2011. 3:15 AM
Hello,

The 2v might be normal because the gate is connected to nothing.
Try connect the gate to the source (0v) and measure again over the coil. You should now get 0v there instead of 2v
Umniscient in reply to ElectorialsAug 26, 2011. 3:20 PM
Yes, that was the result.
I went ahead and purchased the IRF510, and I'm going to try it soon. Just a question: is there a minimum voltage required to operate a MOSFET? That is, can I theoretically use any voltage under 12 and still get an output from the inverter circuit?
Electorials (author) in reply to UmniscientAug 27, 2011. 12:29 AM
The IRF510 needs 4V (= Vgs) at the gate to work properly.

(x2 for voltage divider + some extra) -> you'll need at least 10V

maybe you could use a 9V battery if that's what you meant with "something lower then 12V" but it won't work so really good. The battery will also drain quickly.
Umniscient in reply to ElectorialsAug 27, 2011. 9:07 AM
All right.
Well, I read the IRF510 datasheets and connected it into my circuit accordingly.
Measurement on the primary: 12V. Disconnected the gate, still 12V.

Now, I'm not sure if the inverter is even outputting AC, because my multimeter shows 12V AUTO. When I manually switch settings to AC, I only get a few millivolts. Does this mean that the recorded 12V is in DC? It would explain the lack of output on my secondary coil.
Electorials (author) in reply to UmniscientAug 27, 2011. 9:16 AM
Yes, that indeed means you have 12V DC.

When you just connect the circuit, do you get a small spark then? (I mean really tiny, like 0.5mm)
If you don't get that, I think your flyback transformer is broken.

You better test this without the mosfet, just connect 12V straight to the primary coil at a fast frequency.
(I mean connect 12V and inmediately disconnect it, then connect again, and go on like that. This will create a 'fake' AC - alternating - voltage)
Umniscient in reply to ElectorialsAug 27, 2011. 9:54 AM
I don't get a small spark, but when I repeatedly connect and disconnect the 12V, there is an output of a few volts on the secondary.
Electorials (author) in reply to ElectorialsAug 27, 2011. 9:19 AM
oh, and about your mosfet, you don't have to worry about that if might have gone broken. If your wiring is correct, it won't be broken. There's just something else wrong, as I said, probably the transformer.

You also tried with reversing the primary coil right? (it matters since many flyback transformers have a rectifying diode at the secondary coil).
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