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.
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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Signing UpStep 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.
- 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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do you mean the voltage over the primary coil?
I have no idea why it gets so hot :/ Is your heat-sink large enough?
My heatsink is about 8in by 4in and seems pretty big; it had 5 separate mosfets/ transistors on it originally.
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.
But that's something totally different and a lot more complex.
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.
and no problem ;)
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?
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?
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
Would this work well instead, or is the current rating too low?
http://www.radioshack.com/product/index.jsp?productId=2062618
Thanks for your help!
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...
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?
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
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?
(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.
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.
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)
You also tried with reversing the primary coil right? (it matters since many flyback transformers have a rectifying diode at the secondary coil).