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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
EDIT3: The op amp was most likely oscillating, small caps in the feedback paths stopped the crackling sound for the most part.
Hello, I am having problems with my newly built SSTC. So far the bridge "hot" connection is supplied separately from a variac, through a 1000 watt lighbulb. When I turn the variac up above certain voltage, the sparks start making a sharp buzzing/crackling sound when I tune below resonance, at this point the output is just a small corona (in resonance at this voltage, the coil makes ~6-8cm sparks, limited by the bulb). The coil is not interrupted so it is very easy to hear this problem.
I am afraid to turn it up more, as the interference could show up also at resonance and damage components. Everything is designed with respect to shielding, so I have absolutely no idea where the interference could come from. I tried adding a common mode choke in front of the controller, grounding it through a resistor instead of direct ground, changing the 4046 IC and swapping primary connections, but all of this has none effect.
I would be thankful for any help as I need to get this coil running in less than a week, and if the interference problem is not solved, I'll just be looking at a useless complex expensive device with loads of time wasted on it.
Coil data, if of any help: 230V~ mains IRG4PC40W fullbridge with FES16JT reverse diodes, 3.3uF dc block, 330pF + 9ohm RC snubbers, halfwave unfiltered supply 4046 oscillator, 74HC14 deadtime generator (~200ns) Overcurrent shutdown (CT bridge current sensing) Frequency around 150-200 kHz Secondary 11x37cm (4.3x14.6"), 0.25mm enameled wire Primary 32 turns (may decrease for longer sparks), 4mm OD 2.3mm ID solid copper wire with PVC insulation
Here are some pictures and diagrams which could help you:
Many thanks for any help.
-Jan
Edit: I just completely bypassed the modulator circuit, which is a LM358 op amp amplifying low frequency signals (derived from the mains frequency) and the interference is gone for the most part, however a small amount of interference remained. However trying to fix the modulator circuit would be just a "patch for symptom", the cause would be still there.
EDIT 2: I tried putting little ferrite toroids from CFLs on the GDT primary leads, no change. I tried replacing the LM358 with a different one - a change for even worse occured.
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SSTC interference/noise issues, please help [FIXED]
