2002-10-26
Flogging a Flyback
Here are some pictures of a evenings worth of sparks and arcs with a flyback transformer and a pair of 2N3055s mounted on a heat-sink. There is the odd car ignition coil in there too.
I did not use the ubiquitous 2N3055 push-pull circuit from the Information Unlimited people. I was stupid enough to buy their "Build Your Own Working... (sexy sounding project)" book from Jaycar many years ago. That design should be taken out and shot. Every high school kid's HV page lists it as their own 'cool design'. Truth be known, it is a horrible design!
The 2N3055 is a fairly pathetic transistor, it has very low beta, especially at high collector currents. The classic circuit has a pair of them being biased via a pair of power resistors that are burning up a good quarter of the supply current. The only reason the 2N3055s are surviving the back EMF is that they are huge devices, really overkill for the job, but basically unkillable in this circuit. A better design would use a MOSFET or at least a modern bipolar power transistor perhaps one of the power Darlington designed for automotive ignition service. A higher supply voltage is more suited to the typical flyback design, they run in the TV or monitor off around 250 volts. Using at least 48 volts would reduce the heating on the transistors. Some free-wheel diodes would be a damn good idea too! One day soon I'll build a H-bridge MOSFET flyback driver, I've ordered some TL494 switch-mode control chips, a rail of big MOSFETs and big (ie TO-220 cased, switch-mode service designed) Schottky diodes especially for the project.
After saying all that, I ignored my own advice. The 2N3055 is cheap, and the power oscillator configuration has its advantages (e.g. always resonant drive), but I choose this time to use a pair of 2N3055s in Darlington configuration to chop my bench PSU into a winding on the flyback, at a rate determined by my bench sine/square oscillator. The entire circuit was assembled with alligator clips. Ugly as hell, but this was only for a bit of fun, not a serious piece of work.
Of course my bench power supply really isn't designed for this kind of punishment. I am pulling 3 Amps at 35 volts out of it, the fun only lasts several more minutes once I grab the ignition coil. The back EMF spikes eventually blow the BD139 Darlington driver for the 2N3055 in my bench power supply's +ve rail. Fortunately just before that happens, I get some pictures of running a HeNe tube off the induction coil output!
The tube is actually lasing quite well, the output is visible on the ceiling. You can see the power supply drop-out light is on, all the hash from the coil is getting back into the supply and upsetting it. The HF overrun is what eventually destroyed the BD139 in it.
Note the fluro light starter guts used as a target in the sharpening of the hobby knife shot. If the arc was struck from its glass surface, the glass glowed a soft blue-green and huge amounts of ozone were produced. I am not sure but I think a lot of X-rays were being produced as well. I'd imagine at this voltage they can't be too hard. There was a lot of short-wave light produced, I experimented with exposing glow-in-the-dark material to the arc light, it charged it very well.
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