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Registered Member #1696
Joined: Sat Sept 13 2008, 06:01PM
Location:
Posts: 21
Hello everybody. Last weekend I decided to build a new flyback circuit. I used the simple 2n3350 circuit, which most of you are familiar with. I used the 2 resistors, 27 and 220 Ohm, but, those were rated for a max of 0.25 Watt. I boxed the circuit, and the flyback worked fine (arcing up to 2,5 cm). But, after some time, for some reason i decided to take a look inside, and found out I completly burned the resistors beyond recognision(but the circuit still worked). So, I bought 2 new ones, rated for 5 Watt. I thought my flyback would work better with the new resistors, but now it only arcs up to 1 cm, and the resistors get REALLY hot. Anyone got an idea why this happened?
Registered Member #2040
Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180
If those are the wire wound type then you have introduced some inductance into the circuit. I can't recall the schematic from memory to tell you how much it matters, so I'll let you figure that out. I would definitely check that out if I were you.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
I would measure the resistance of the old and new resistors and make sure they match, the old resistors may have changed their resistance after they "burned in"
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Diagnosis by smell!
You should never use power resistors in a sealed cabinet. They need plenty of ventilation, and many partly depend on their own leads to sink heat.
I don't think that's what your problem is here, but it will become one if you run resistors hot, and let them start heating up everything else in the box.
It's quite OK for wire-wounds to feel very warm to the touch, but if you over-heat them the ceramic will start to crack off, as well as damaging nearby components.
Registered Member #1696
Joined: Sat Sept 13 2008, 06:01PM
Location:
Posts: 21
Ow oke ^^ Yes they are wirewound. I now installed a fan, which cools te heatsink of the n3350 transistor, but also cools the inside of the box. Now the arc is max. 3 cm :D I havent got a clue why, but hey, it works ^^ (this is not the real spirit of a scientist though...) thnx for the replies!
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Weaker flyback with better resistors!
