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Registered Member #610
Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110
I believe I may have found the answer to my question in another recent thread. I took pictures so I'll post anyway.
My question is about the dips in waveforms while using a GDT as a load. The answer may be that I need diodes across the collector and emitter to absorb the magnetizing energy being shot back?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You don't need them with an actual capacitive load.
But don't confuse a capacitive load with a series resonant load running at a leading power factor. While this draws a leading current at the fundamental, it looks inductive to all the harmonics. The current is a phase-shifted sine wave that will want to flow in the diodes at some times.
A real capacitive load looks capacitive at every frequency. The current flows in the form of sharp spikes at the transition times, and the diodes would never conduct.
Registered Member #610
Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110
What nobody answered yet is why I need diodes with 50% duty cycle with capacitive load.
You're hi-jacking my thread man! No worries though :)
I added diodes and got the same results. Just some obvious checkpoints: NPNs: Added 1N5819 with cathode to collector, anode to emitter. PNPs: Added 1N5819 with cathode to emitter, anode to collector. Transistors are D-PAK: base = pin 1, collector = pin 2, emitter = pin 3.
I was sure this would work. What am I overlooking?
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I don't KNOW what's cauing the glitches, but here's a couple of things to investigate;
The droop on the 'loaded' waveform looks like the GDT may be saturating - 'scope the power supply to look for excessive ripple caused by GDT magnetizing current - try running at two or three times the present frequency (if convenient) to see if the droop decreases - try short-circuiting the 220 Ohm resistors temporarily - 'scope the outputs of the TC4428
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GDT Driver with BJTs
