Simple High Voltage Flyback inverter [without feedback coil]

Step 8 Polarity of the primary coil

As said in step 5, it's not really easy to find the polarity of the primary coil, but this a way to do it:

- Choose a random polarity, and make the schematic from step 6.
- Make sure that the Negative side of the HV coil is connected to the MOSFET's Gate pin.
- Plug it in, and don't touch anything.
- There will be corona discharges (purple plasma around sharp edges of the wire, like in the picture) on the positive HV side, if you wired it up correctly.
- If there are no corona discharges, invert the polarity of the primary coil.

Comments

[Umniscient]

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]

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]

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?

http://www.radioshack.com/product/index.jsp?productId=2062618

Thanks for your help!

[Electorials]

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]

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]

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]

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]

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]

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]

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]

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]

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).

[Umniscient]

Also, I doubt the transformer itself is broken.
You see, I have a signal generator capable of outputting 5 volts. When I set it to 40 kHz and plug it into the primary, I get about 600 volts on the secondary, with lots of sparks and buzzing noises.

[Electorials]

That's really weird!

When a 5V AC signal creates nice sparks, a 12V (pulsed) DC signal should also creates sparks..

At least we now know that the flyback transformer is working correctly.

I would say that your 12V supply is broken or the power rating is too low but that's also not possible because you said you measure 12V over the primary coil of the flyback :S

oh, and is there a way we could chat somewhere while you're working on your flyback? it would be a lot easier. Because this way it will take years :)

[Umniscient]

"oh, and is there a way we could chat somewhere while you're working on your flyback"
Sure... do you have a Gmail account?

[Electorials]

no, I have hotmail. Is that ok?

[Umniscient]

Sorry for my delay!
Yes, it's okay. I have Hotmail as well.

[Electorials]

No problem ;)
I'll speak to you soon then!

[Umniscient]

Hey, I managed to get it working using a slightly different arrangement with a 2n3055.

[Electorials]

:D nice
Can you post a picture? I'd like to see it :D

[Umniscient]

Yes, I've tested the primary polarity each time.