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Flyback transformer questions

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LAN_403

Sun Mar 29 2015, 03:17PM

Registered Member #54664 Joined:
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Hi, I'm new to the forum. I have noticed that there are already two threads about flyback transformers made a few days ago, but I'm going to ask you some different questions, so I opened a new one anyway.

I disassembled an old CRT television, and I desoldered the flyback transformer from the main PCB. With the help of this guide, I identified the 0 V pin of the secondary coil and the pins of the primary. There are several things I did not fully understand, even because I found many different opinions on the internet.

If I connect a 12 V power supply to the primary, as soon as I disconnect it I should see a spark between the HV output cable and the 0 V pin of the secondary. I also read that the high voltage pulse period is so short that it may not be able create an arc. In that case, I can extend that period by adding a capacitor to the primary. Is all this correct? Considering that the resistance of the primary is of about 1 ohm, I should be able to use a 14 A power supply without risking to blow its fuse, since 12 V / 1 Ohm = 12 A, right?

This instructable teaches how to create a simple driver using a 2N3055 power transistor. It is quite clear, but there is a thing I didn't understand: why does he wind some wire around the core of the transformer and basically use it as primary coil / input? Can't I use the actual primary coil of the transformer of which I found the pins?

Thank you very much for your help.

Teodor Zafiroski

Sun Mar 29 2015, 05:55PM

Registered Member #48007 Joined: Wed Jun 18 2014, 01:00PM
Location: Prilep, Republic of Macedonia
Posts: 33

Because the real primary of the transformer can be hard to adjust for optimal operation, and if you push enough current through it it may melt inside the epoxy, rendering the coil useless... Because of this things mentioned above, most of the high voltage enthusiasts are winding their own primary, because the number of turns can be easily adjusted and the wire thickness can be chosen to suit the need. I reccomend winding your own primary and starting with 6 primary and 4 feedback turns for the simple 2N3055 driver... Good luck with your first HV project!
P.S. The coil would have a higher impendance tu pulsed current, so a resistance of 1Ohm wouldn't necessarily mean 12Amps through it, also, if the coil pulled 12Amps through it, it would probably burn out...

Sun Mar 29 2015, 06:05PM

Registered Member #54664 Joined:
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Teodor Zafiroski wrote ...

Because the real primary of the transformer can be hard to adjust for optimal operation, and if you push enough current through it it may melt inside the epoxy, rendering the coil useless... Because of this things mentioned above, most of the high voltage enthusiasts are winding their own primary, because the number of turns can be easily adjusted and the wire thickness can be chosen to suit the need. I reccomend winding your own primary and starting with 6 primary and 4 feedback turns for the simple 2N3055 driver... Good luck with your first HV project!
P.S. The coil would have a higher impendance tu pulsed current, so a resistance of 1Ohm wouldn't necessarily mean 12Amps through it, also, if the coil pulled 12Amps through it, it would probably burn out...

Thank you for your help! I've ordered some parts online and I'm waiting for them to arrive. I'll let you know as soon as I make the circuit.

Ash Small

Sun Mar 29 2015, 09:24PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

There are lots of different types of 2N3055 I remember 'E' and 'H' suffixes off the top of my head. They also vary from one manufacturer to another. Motorolla ones with Mexico stamped on them seem to be pretty good, but there are fakes on the market as well. Use a reputable (read 'expensive') supplier, and consider getting the more expensive ones they offer.

EDIT: Also use a BIG fan cooled heatsink. (like from a computer CPU).

EDIT 2: On second thoughts a CPU heatsink would need modifying, so would require some metal working skills, if using the TO3 package, but you get the idea.....Big heatink and fan. (normal computer fan is ideal)

Antonio

Sun Mar 29 2015, 11:01PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

The normal primary is dimensioned to operate from about 100 V in most flybacks, to allow operation at high frequency with the required energy and low primary current. With 12 V it works too, but the connection has to be kept for long time (many ms) to accumulate enough current. A new primary with less turns allows faster energy accumulation, but requires higher current for the same energy. The 3055 driver seen everywhere was probably invented by 3055 makers, because it is really excessively simple. In most cases it destroys the transistor in seconds. If you want a reliable flyback power supply, copy the schematic from the original board.

Teodor Zafiroski

Mon Mar 30 2015, 03:56AM

Registered Member #48007 Joined: Wed Jun 18 2014, 01:00PM
Location: Prilep, Republic of Macedonia
Posts: 33

Antonio, This was my first HV project too, and my experience with this driver was rather satisfying. I've pushed this driver to 20V @ 70W no problem using an adequate heatsink, the only disadvantage of this driver is that a lot of the energy is dissipated as heat, and I mean A LOT! But for a first HV project it is excellent because of it's simplicity and low cost. Also, I used some of the counterfeit transistors in my build and I had no problem but the high power dissipation...

Fri Apr 03 2015, 09:04AM

Registered Member #54664 Joined:
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All the components arrived, I built the circuit but... it doesn't work. There are no sparks between the HV wire and any of the pins of the transformer.

This is a basic drawing of the circuit
http://i.imgur.com/8JWKN9l.png

And here are some photos:
http://i.imgur.com/kA745ov.jpg
http://i.imgur.com/59AYqds.jpg

Any idea of what's wrong?

Ash Small

Fri Apr 03 2015, 11:32AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

This is the circuit I used, it works. I used ten turns and four turns initially. This tends to work for most flybacks.

First thing to try if you are certain it is wired up properly is to reverse the connections to the primary, in case it is back to front. This may be the same with the feedback winding too.

If you want to test the 2N3055, just replace the primary with a small (5 to 50 Watt) 12V lightbulb and remove the feedback winding. If the light comes on when you connect the two wires that go to the feedback winding together, then the 2N3055 is 'good'.

(A light bulb added in series with the primary can be used to limit the peak current while testing, too)

Keep all the wires as short as possible, twist the wires that go to each winding together, and twist the power supply wires together as well. This reduces EM 'interference'.

mister_rf

Sat Apr 04 2015, 08:10AM

Registered Member #4465 Joined: Wed Apr 18 2012, 08:37AM
Location: Bucharest, Romania
Posts: 145

I've spotted an error in the diagram accompanying Instructables article.

We need to pay special attention to the voltage polarities of each coil winding, see the attached image as example.

1428134741 4465 FT1630 2n3055 Flyback Example

Sat Apr 04 2015, 09:44AM

Registered Member #54664 Joined:
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Posts: 0

mister_rf wrote ...

I've spotted an error in the diagram accompanying Instructables article.

We need to pay special attention to the voltage polarities of each coil winding, see the attached image as example.

Thank you, I'm going to modify my circuit and see if it works.

Edit: differently from before, now I the 2N3055 gets really hot, but there are still no sparks. With my voltmeter I could measure about 2 V between the HV cable and the 0V pin. There's still something wrong. :(
Should I try reversing the primary connections too?

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