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Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
jmartis wrote ...
Tonic, IMHO you can get decent ferrite cores from any equipment that contains a higher power SMPS (TVs, monitors, photocopiers etc.etc.). Sometimes they are in the line filter, sometimes they have a few turns of the ground wire on it. I found some large cores in an old photocopier SMPS, they are excellent and for SSTC frequencies I need just ~4 turns on them.
Damn, once time I had three monitors for ripping and didn't care about small cores. Beside those monitors, I don't have nothing from which I would get off a core. Whatever, tomorrow I shall receive ordered cores, but I will keep in mind what you said in future :)
wrote ...
I think the NE555 will work up to few MHz, if you dont like the slow rise/fall times you can always put a schmitt trigger on the output (e.g. parallel all 6 inverters of 74HC14 to drive your transistor output stage)
Thanks for advice. *me going to read what a Schmitt thing is*
Very nice sparks, Linas :)
Returning to my plays, how much MOSFET heats when it's plugged directly to NE555's output? I've tried with it without heatsink and transistor was heating rapidly in few seconds to temperature I couldn't resist. It's a IRFP450 with two Zeners diodes across gate and source, with damping resistors. Without plugging +30V to primary consisting 10 turns, the square wave is fine, 12V, off time is three times larger than on time and frequency is 25 kHz. When I plug +30V, the off part of wave gets crazy and probably pushs transistor into linear region, causing heating, instead of shutting it off. Is it possible that too large gap between cores is a reason of it? The space between cores is something aroung 0.5mm.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Linas wrote ...
but what would be the difference, if I use feedback? Would the spark be longer????
Probably yes because it would automatically tune as the streamers load the output down.
Tonic wrote ...
Returning to my plays, how much MOSFET heats when it's plugged directly to NE555's output? I've tried with it without heatsink and transistor was heating rapidly in few seconds to temperature I couldn't resist. It's a IRFP450 with two Zeners diodes across gate and source, with damping resistors. Without plugging +30V to primary consisting 10 turns, the square wave is fine, 12V, off time is three times larger than on time and frequency is 25 kHz. When I plug +30V, the off part of wave gets crazy and probably pushs transistor into linear region, causing heating, instead of shutting it off. Is it possible that too large gap between cores is a reason of it? The space between cores is something aroung 0.5mm.
I don't know how are you plugging the FET directly to 555 output, are you loading it down or what?
The "off" part of your wave gets crazy because it is not clamped> it can fly inductively to the full 30V. Power your BD transistor driver from the same 12V as the 555 and put the clamp diodes on it as I said earlier. This should clean the waveform up.
BTW when driving GDTs to drive FETs, try to keep the duty cycle as near to 50% as possible, otherwise this would result in a DC component on the GDT output so you could either overvoltage the FET gates and/or the FETs would not turn on properly.
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
jmartis wrote ...
I don't know how are you plugging the FET directly to 555 output, are you loading it down or what?
The "off" part of your wave gets crazy because it is not clamped> it can fly inductively to the full 30V. Power your BD transistor driver from the same 12V as the 555 and put the clamp diodes on it as I said earlier. This should clean the waveform up.
BTW when driving GDTs to drive FETs, try to keep the duty cycle as near to 50% as possible, otherwise this would result in a DC component on the GDT output so you could either overvoltage the FET gates and/or the FETs would not turn on properly.
Gate with damping resistors is connected directly to BD's output (there is of course DC blocking capacitor). Source is connected to NE555's GND. One of primary's lead is connected to drain. Second lead is connected to +30V. +30V comes from bridge rectify, which also powers NE555 generator through LM7812. Since NE555 is switching gate, +30V should flow to primary, then transistor drain and next grounded source.
About diodes, I did use them, though they're Zeners, not Shottky. They're placed back to back and across gate and source. Didn't help much with this ringing on off part when I'm drawing arc.
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
So you're running the IRFP450 directly from the BD1XX output? Then the DC blocking capacitor is the problem. Since you cut out the GDT a DC component isn't a problem anymore. Also the DC blocking cap will halve the voltage seen at the mosfet gate, increasing it's on resistance and making it heat more.
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
Uzzors wrote ...
So you're running the IRFP450 directly from the BD1XX output? Then the DC blocking capacitor is the problem. Since you cut out the GDT a DC component isn't a problem anymore. Also the DC blocking cap will halve the voltage seen at the mosfet gate, increasing it's on resistance and making it heat more.
Removing DC blocking capacitor didn't help a bit. I've tried without air gap and again no success. Wonder if ever I'll get it working Hm, I will try with separate power supply for HV transformer. Is it needed to connect both PSUs (for transformer and NE555) grounds?
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
So youre driving the TC with just one transistor? If so I'm not surprised it gets hot.
To the diodes: Zeners will not help (much), you need to clamp the output voltage from your BD driver to supply voltage. You don't need Schottkys (though they are preferred), a standard fast diodes (e.g. UF4007) will do it.
Registered Member #528
Joined: Fri Feb 16 2007, 10:32PM
Location: Warsaw, Poland
Posts: 166
jmartis wrote ...
So youre driving the TC with just one transistor? If so I'm not surprised it gets hot.
Not TC, but AC flyback transformer from TV.
Two new superb probes and two cores have arrived today. Those probes are very convienent, now I can scope two waveforms at one time :D The situation with GDT is a bit better, the waveform at secondary is very similiar to one at primary. The only problem is that square wave has stair shapes, just like on pic (my dad did take my digital camera, so no real photo to evening):
I've removed the 560ohm resistor between NE555's pin 3 and BD's bases. Decreased small ringings a bit, but still it's not this. NE555's output is a square wave. BD's for some reason deforms wave to stair shape. The possibility of reason lying in GDT is small because I've tried with those two new good quality cores with different turns (from 2 to 15) with bifilar and twisted pair of wires.
jmartis wrote ...
To the diodes: Zeners will not help (much), you need to clamp the output voltage from your BD driver to supply voltage. You don't need Schottkys (though they are preferred), a standard fast diodes (e.g. UF4007) will do it.
To clarify, I have to do like as Richie did with UF4001? His scheme:
EDIT:
Whatever, I did with 1N4148 like as Richie did and I must say that now I don't see any ringings. But still problem described above exists.
For testing, I did put a 10kohm resistor in series with GDT. Primary had normal square wave, but secondary's voltage has dropped to 0V. I've seen only a line with ringing in places where switching at primary side occur.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The "staircase" waveform you see is absolutely normal, it is because when driving inductive loads, the load draws current from the supply as well as returns it.
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NE555 interrupter and GDT problems
Hm, I will try with separate power supply for HV transformer. Is it needed to connect both PSUs (for transformer and NE555) grounds?
