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Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
Well, if you want an improvement get rid of those god awful 2N3055 transistors. They are what, about 30 years old? You can get MOSFETs which are cheaper, and a hundred times better in performance, although you'll have to modify the circuit somewhat.
"Use a more modern transistor like the 2N3773 instead of the 2N3055. It has higher current (30 A) and higher voltage (140 V) ratings as well as higher Hfe thus requiring less base current.
Use a single feedback winding (2 turns) with each end connected to the base of one transistor and over a 150 ohm resistor connected to Vcc. (I'm not quite sure exactly what this means but my guess is to connect a 150 ohm resistor from one of the bases to Vcc to provide the startup base drive. --- Sam.)
Add an inductor (about 0.4 mH, can be more but not less) in series to the flyback circuit. Since both transistor are switched on for a little moment (when there is no voltage in the feedback), so there will be high currents (causing losses resulting in excessive power disspation in the transistors) The coil prevents the current spike without wasting power like a resistor. "
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Ruskie wrote ...
klugesmith wrote ...
Instead of investing in uncommonly high-wattage lamps (with regular screw base?), how about wiring two or three sockets in parallel? Then with an assortment of various wattage lamps, you can finely tune your ballast.
Especially if your primary interest is in radiography, and that your first adequate HV power solution will be your last.
Yes, I could do that, thanks. For now, however, I carefully calibrated the HV adjustment pot and also added a stop to it so I won't exceed the threshold. This little extra step of the stop has saved the unit so far and I have been able to power it off and on to full voltage many times since then.
I'm still going to try and add circuitry to protect the 3055's though. The circuit suggested earlier I said didn't work, but this may have been because one of the components leads hadn't soldered properly to one of the transistor terminals. Several mentioned they were going to test though and I would like to see their results before I install it.
I mentionded somewhere recently that I'm building a ~300W variable resistor for pretty much this purpose. The reason I decided against using light bulbs is that you get a surge when you switch on, before the bulbs warm up, which sort of defeats the object.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Ruskie wrote ...
How are you going about building that?
At the moment I'm working with fan heater elemants, cut into ~1m lengths. I made someting similar a few years ago that was within 5% tolerance over the usable range, but I'm trying to improve on it this time, maybe oil cooled. I'm also trying to design out any stray inductance this time.
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