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Simplest way to drive a flyback. Really.

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Th3_uN1Qu3

Mon Jan 03 2011, 01:33PM

Registered Member #2614 Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26

Here's something i did coz i got bored... and honestly it turned out better than expected.

Take apart your average PC PSU, which uses halfbridge topology. Most cheap ones do.
Locate 12v outputs on transformer (the ones on both sides of the dual diode). Don't remove anything as you need the PSU to work as normal.
Make primary winding of 5 turns on flyback.
Connect primary winding of flyback between the outputs you identified before (NOT between any single output and ground as you will get 2x less power).
Have fun.

Warning: DON'T spark to the PSU ground (since one end of the flyback is at positive, the other at negative potential relative to PSU ground), but instead to the HV return wire on the flyback. This is powerful enough to make flaming arcs so be careful if it's your first flyback project. The advantage is that you're also going to get a somewhat regulated output - since the flyback appears as a load on the 12v output, the PSU will adjust duty cycle to keep it in check.

I'll come with some pics later.

GeordieBoy

Mon Jan 03 2011, 02:56PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

> The advantage is that you're also going to get a somewhat regulated output - since the flyback appears as a load on the 12v output, the PSU will adjust duty cycle to keep it in check.

It won't be regulated unless you take feedback from the flyback transformer output! The flyback output is outside the control loop of the PC supply.

The control loop feeds back the rectified and smoothed 12V output and makes decisions on controlling the duty ratio of the pulses based on this, and usually the 5V output also to some extent. This helps with cross regulation between +/-12V and +/-5V outputs.

You will probably find that the output from the flyback transformer is quite weak until you load up either the 5V output or 12V output of the PC supply. Without a load on the PC supply outputs the duty ratio will most likely idle at a low value which is just sufficient to keep the output capacitors of the supply charged with discontinuous inductor current. Once you draw some current off the supply's DC output, the feedback loop will wind up the duty ratio and start putting some pulses through with a decent pulse width to keep the output voltage up. This will likely increase the output from your flyback transformer because more average power is being delivered with wider pulses.

-Richie,

Th3_uN1Qu3

Mon Jan 03 2011, 06:31PM

Registered Member #2614 Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26

It will certainly be regulated better than in the single transistor or 555 circuits. Sure nothing beats sampling the output directly but that isn't always practical and i don't think it's a good idea to try and regulate the supply when drawing arcs.

And you are wrong that it'll "idle". It doesn't matter if the load is directly on the transformer or on the DC output - the net effect is still that the feedback loop will see reduced voltage on the DC output side and act accordingly. But yes, loading the 5v output helps if the supply is group regulated (you aren't gonna sacrifice one of the expensive units are you).

I did find that i don't like the voltage output however - with the 555 driver it would spark at almost 10cm while it could only draw an arc to ~3cm. The arc was violet with the 555 driver. Now it only sparks at ~5cm, and arcs the same 3cm but this time the arc is yellow flame. And i don't like that because it quickly starts eating the wires. Too high current methinks.

Th3_uN1Qu3

Mon Jan 03 2011, 08:08PM

Registered Member #2614 Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26

Hmm, maybe the first power supply i tried this on had primary current monitoring. I tried another one and indeed i needed to load down the 12v output to get things going. And i also managed to blow it with full 12v load and the flyback at the same time. Back to the other one. ;)

LIGHT

Mon Jan 03 2011, 08:25PM

Banned on 4/6/2011 for unsafe, irresponsible behavior.
Registered Member #3442 Joined: Fri Nov 26 2010, 04:09AM
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Posts: 114

A better way is to use a high wattage CFL ballast.

I used a tesco 11watt ballast with results of a 1.1cm spark.

Dr. ISOTOP

Mon Jan 03 2011, 10:34PM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

PC power supplies can deliver significantly more than 11W, though.

LIGHT

Tue Jan 04 2011, 12:24AM

Banned on 4/6/2011 for unsafe, irresponsible behavior.
Registered Member #3442 Joined: Fri Nov 26 2010, 04:09AM
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Posts: 114

Yes bwang, but this was my first flyback project, you can buy much higher wattage CFL ballasts.

ScotchTapeLord

Tue Jan 04 2011, 01:23AM

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

I'm pretty sure the simplest way to drive a flyback is to wire a battery to the ground side of the internal primary and repeatedly strike the + lead to the other end of the primary. Really. :)

Th3_uN1Qu3

Tue Jan 04 2011, 01:18PM

Registered Member #2614 Joined: Sat Jan 09 2010, 08:57AM
Location: Bucharest, Romania
Posts: 26

ScotchTapeLord wrote ...

I'm pretty sure the simplest way to drive a flyback is to wire a battery to the ground side of the internal primary and repeatedly strike the + lead to the other end of the primary. Really. :)

Hmm. Maybe one can go one step further and automate the striking. A wind-up flyback perhaps?

Anyway, i was correct on the first PSU (a Seasonic SS-200GPX), it has primary current monitoring AND all sorts of protections, over temperature included, as it shuts down after 10-15 minutes of use and i need to let it cool down a little. Good PSU. I had fun burning stuff and lighting up a fluoro tube that doesn't really work with any other method - it has so little gas left inside that even a mains voltage tripler doesn't start it properly.

The damage on the first PSU is minimal as well - switching BJTs failed open - so it won't take much to fix that one. I have to get my act together and build myself a proper regulated bench supply but since uni exams are coming, i'll have to delay it for a month or so.

Antonio

Tue Jan 04 2011, 02:00PM

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

This system drives the flyback transformer as a regular transformer, not in "flyback" mode. The output voltage is at most the applied primary voltage multiplied by the turns ratio of the transformer formed by the added primary winding and the secondary winding. This is much less than what can obtained in flyback mode with the same primary winding, and there is a great possibility of destroying the internal rectifier due to excessive reverse voltage. The excitation frequency may be too high to.

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