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Building first flyback: some questions.

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spectral532

Tue Mar 09 2010, 03:38AM

Registered Member #2725 Joined: Tue Mar 09 2010, 02:31AM
Location:
Posts: 6

Hello all, this is my first post here. I recently pulled a FBT from a 17" color monitor, and I'm almost ready to power it up. However, I have a few questions before I finally put it all together.

1.) Why do people wind their own primaries/feedback coils instead of using the built-in coils?
2.) Does it make any difference whether I use regular or enamel wire to wind the coils?
3.) How much current do the coil wires need to be rated for?
4.) What should I do with the focus/G2 lines coming from my transformer? Can I just pull them out?

Thanks in advance

Zeus

Tue Mar 09 2010, 03:55AM

Registered Member #2316 Joined: Tue Aug 25 2009, 03:04AM
Location: Bendigo, Australia
Posts: 107

1.) We wind our own coils on flybacks for two reasons.
The internal coils don't have the right turns ratio, and they won't burn out as easily.
2.) Enameled wire will get slightly closer to the core, but it doesn't matter.
3.) About 4 amp wire should do .
4.) Just cover them with hot glue.
Good luck with getting your flyback driver working. Can I ask, is it a single transistor driver.

hboy007

Tue Mar 09 2010, 03:02PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

welcome to 4hv!

1) the original primaries often lack the center tap needed for simple oscillator circuits
4) after connecting one or two other bottom pins to ground that correspond to the potentiometers you can use one of the voltages to monitor the total output voltage and/or
create some sort of feedback regulation, they should be around 300V / 1kV

spectral532

Wed Mar 10 2010, 01:31AM

Registered Member #2725 Joined: Tue Mar 09 2010, 02:31AM
Location:
Posts: 6

Thanks for the answers. Though now that I've started assembling the flyback circuit I have a few more questions

5.) Does it matter which direction I wind the primary/feedback coils? I assume that the feedback coil is wound in the same direction as the primary.
6.) About how many turns do I need on the primary/secondary coils? I've read different sources that say anywhere between 2 -15 turns on the primary.
7.) Is there any specific ratio of primary to secondary turns?

Thanks again for the help.

@Zeus. Yes, I'm building the single transistor driver shown on HV wiki. I figured that I should start with something simple to begin with. Once I get the circuit working I'll consider building more advanced drivers.

ScotchTapeLord

Wed Mar 10 2010, 02:41AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

5) No, but it matters which way you connect the leads to your circuit. This is simply done by trial and error- if you have weak or no output, switch the leads and hope for the best! I guess the same goes for feedback, though I don't know for sure, since I've always used a VCO.

6) As few as possible without overheating your transistor! Start with a lot of windings and remove one at a time, observing changes in performance. Often times, there is a "sweet spot" unique to your setup/core/etc.

7) Not if you wind your own =)

Good luck! Flybacks are a lot of fun!

Zeus

Wed Mar 10 2010, 10:07PM

Registered Member #2316 Joined: Tue Aug 25 2009, 03:04AM
Location: Bendigo, Australia
Posts: 107

It may help the performance of the circuit if you add this

to it

spectral532

Sat Mar 13 2010, 06:04AM

Registered Member #2725 Joined: Tue Mar 09 2010, 02:31AM
Location:
Posts: 6

Thanks for the help everyone, I managed to get my flyback working last night. However, it seems to be a bit weak compared to videos I've seen of other flybacks. Additionally, the spark length seems to get shorter the longer I keep the circuit powered. The spark strikes at close to 3/4" when I first power the circuit on, and then drops down to maybe a 1/4" after a few seconds on. I'm not sure if this is a limitation of my flyback, or of the driver circuit I am using.

Currently I'm using the single transistor driver shown on HV wiki (I substituted the 2N3055 with a FJP13009). I used 2 turns for the feedback and 4 for the primary. I would appreciate it if anyone could help me figure out why my flyback is so weak.

quicksilver

Sat Mar 13 2010, 07:45AM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

Just my opinion:
I have made quite a few drivers and the one Zeus recommends is the one that got me the best performance with the lowest input energy (12Vdc 3a). However I will say this; that it is VERY easy to blow your transistor w/ such a driver and 4 feedback 10 primary but you WILL get some pretty arcs. It's just life that as you raise performance, the reliability slips. Of course this is very dependent on the transformer but if it's a newer style, try 4/10 & make the windings clean & neat.
The ZVS driver needs current - 8a + but will preform outstandingly. but there you have some trial & error with more than just the windings on the transformer. Taken all into consideration a good balance is made by the one above.

Matt Edwards

Sun May 09 2010, 05:23PM

Registered Member #2838 Joined: Fri Apr 30 2010, 07:55PM
Location: tehachapi, CA
Posts: 333

Try the Mazilli ZVS driver! It is a bit more complicated but will produce MUCH better results. Although this circuit is good for beginners, the 2n3055s seem to burn up pretty quickly and produce vary limited results. Here is a link to a good page with some info.

If you would like to continue with the design that you are presently using, try switching the 2N3055s for better mosfets. Perhaps you could even try the push-pull setup with two mosfets.

If you decide to go with the ZVS I recommend using this circuit. It has produced vary good results for me. I would say pushing 48v 5A+ it produced about 6 inch arcs before toasting several components. (I am using a pretty large flyback.)

What are you using for a power supply? If you use the ZVS circuit keep in mind that it will need a few amps or more. I was using 8 lantern batteries connected and melted one. I switched to a 25v 2A transformer and it was overheating. I am currently using a rewound microwave transformer at 36v. It can safely handle 5-10A without getting warm.

quicksilver

Mon May 10 2010, 05:39PM

Registered Member #1408 Joined: Fri Mar 21 2008, 03:49PM
Location: Oracle, AZ
Posts: 679

The Mazilli ZVS driver is perhaps the toughest and I agree with Oidium45 that replacing 2N3055's is pretty much a demand. They are included in many driver designs because they are so common, can be bought at Rat Shack, etc. But replacing them is worth it.
I have driven the ZVS driver with low current (3A) & at that level it's performance is similar to many others. However once you get over 5-8A you see a real difference.

Frankly, I think that a great deal of the blown components in the ZVS is due to the need for better heat sinks and working up to a level where you see if there is need for alterations in other components. The design is great. but when you have 8+ amps, the weakest link in the chain will bust. If you're lucky enough to have a pretty beefy PS then it's really worth it (ZVS).

This design CAN be made to work quite well however. It's VERY simple to make, inexpensive and flexible...

What is needed is a selection of higher watt resistors (10W) a MUCH better transistor (MJE13009) & a larger capacity cap rated for higher energy (50v @ 7000+ uf) but it won't give you what the ZVS will at higher energy levels; so a lot depends on what power supply you are going to use.

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