First of all I'd go back to using a 10-15 ohm resistor in series with the 12v supply, it will still work and produce 180-200v out for test purposes.
Don't bother putting another mosfet in just yet, you need to fid out why it's being fried first. Probably either the mark/space ratio is too long or it's being driven on permanently. What's the DC voltage on pin 3?. You could try connecting one pin of a small cap, maybe .1u to pin 3 and connecting your meter probe to the other pin, if it's oscillating you should get a reading therough the capacitor. Or try a cheap AM radio next to it, you may pick up harmonics. I would check around the feedback circuit and Q2, beyond that you really need a scope to see what the gate drive looks like. It could be very long rise/fall times though if they're failing almost instantly it's probably just the 555 output is stuck high. Tony. On Nov 18, 11:31 pm, Imbanon <[email protected]> wrote: > Just tested my FETs. I have 2 new in working order, the other 3 are as > good as a broken nixie. Now I'm afraid of using them, they are my last > x) > > I can't test the inductors, but they should be 100uH, 1watt max. I > can't test them, nor do they have any markings on them. Just plain > black cover. > The diodes are UF4004. I suppose they should work.. > > Can I use some other inductor, like 220uH? > Also, how do I check if my 555 is oscillating without an oscilloscope? > > Thanks! > > On Nov 18, 10:29 pm, Quixotic Nixotic <[email protected]> > wrote: > > > > > > > > > I've made up dozens of these supplies in all kinds of layout > > configurations and have found them to be bomb-proof. In fact I have > > never had a failure yet, they work every time. The circuit will work > > without the snubber R6/C3. > > > There must be some simple answer to your problem. It seems to me as > > if the 555 might not be doing any switching, or doing it incorrectly. > > Or as you say the FET is not all it's cracked up to be, or the diode > > is not fast enough. Check the value and rating of your inductor. > > > The FET should not get too hot. Usually it will run cool enough for > > you to hold it. > > > John S > > > On 18 Nov 2011, at 19:43, Imbanon wrote: > > > > Hi all. > > > > Yeah, another thread about this supply..http://www.ledsales.com.au/ > > > kits/nixie_supply.pdf > > > I know I could and should have made that Maxim power supply, but I > > > didn't. So now I'm stuck fixing this with loads of spare parts I've > > > got... except for the FETs.. > > > > Anyways, the supply isn't working. The first one I made worked just > > > fine for 15 minutes, powering 6 IN-14 at 170 volts (multiplexed). Then > > > I set it to 200 volts (to get RMS of 2mA), which worked for 5 minutes, > > > and then smoked and failed. > > > I made another on a quick PCB, which just smoked immediately when I > > > switched it on. The same thing happened to the newest, breadboarded > > > one. > > > The MOSFET seems to overheat instantly and smoke out. Having nothing > > > to lose (except the molten hole in the breadboard), I left it powered, > > > to see the diode smoking next. Aparently, its still in working order > > > (so says the multimeter). > > > Heat sinking FET only makes it last longer in not working state before > > > it smokes. > > > > I tried replacing inductors, 555s, diodes, transistors.. > > > What the F**K do I do people? Losing my head here! > > > > And yeah, I'm powering it at 12 volts, tried 5. Output is always 3-6 > > > volts. > > > > Thank you for any help provided! > > > Imbanon > > > > -- > > > You received this message because you are subscribed to the Google > > > Groups "neonixie-l" group. > > > To post to this group, send an email to [email protected]. > > > To unsubscribe from this group, send email to neonixie-l > > > [email protected]. > > > For more options, visit this group athttp://groups.google.com/ > > > group/neonixie-l?hl=en-GB. > > >  > > > JS Design > > 54 Brambledown Road > > Wallington > > Surrey > > SM6 0TF > > > T+F +44 (0)20 8773 4779www.jsdesign.co.uk -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
