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From: rpw3@amdcad.AMD.COM (Rob Warnock)
Newsgroups: sci.electronics
Subject: Re: Flyback Transformers
Message-ID: <22498@amdcad.AMD.COM>
Date: 31 Jul 88 10:01:22 GMT
References: <14741@shemp.CS.UCLA.EDU> <3844@pdn.UUCP> <1052@unccvax.UUCP>
Reply-To: rpw3@amdcad.UUCP (Rob Warnock)
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Organization: [Consultant] San Mateo, CA
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Couple of other trivia notes about flyback transformers:

The high-voltage rectifier -- back in the vacuum-tube days -- was of
course a vacuum tube, and needed filament [a.k.a. heater] current.
But if you consider that the cathode was at the output voltage (look
at how you have to use a diode), that meant that the cathode-to-filament
insulation would have to withstand 15,000-35,000 volts... if they were
separate. So they were connected.

But now you had the task of making a filament transformer that would
withstand 15,000+ volts, and handle a few hundred milliamperes at 60
Hertz. No way! It would be *huge* (and exPENsive). So instead they ran
the filament on the 15 KHz horizontal frequency: the "transformer" was
typically 2-3 turns of (heavily) insulated wire wrapped around one side
of the ferrite core of the flyback transformer. (The flyback was normally
made of a couple of large U-shaped pieces of ferrite butted together
to make a big open "box" shape, with the actual autotransformer coil
all on the top side, and the mounting bracket on the bottom side.)
Worked like a charm.

More trivia: While color sets needed an explicit output capacitor to
keep the voltage stable enough, many cheap black & white sets simply
used the capacitance between the coated inside and the coated outside
of the picture tube as the "filter". Since the high-voltage feed point
was covered with a rubber "nipple", and since the vacuums in the picture
tube and the high-voltage rectifier diode were *good* insulators (as were
the well-insulated wires), it was not uncommon for the high-voltage to
persist for *weeks* after a set was turned off. You could get a *nasty*
shock from a set you pulled out of a trash pile (for example). And in
doing service work (which I did in high-school), you learned (sometimes
the hard way) that the glass of a picture tube had some sort of "memory"
for the voltage (or maybe it was a electret effect), because it wasn't
sufficient to discharge the high-voltage once (which you usually did
by slipping the end of a grounded screwdriver up under the H-V nipple
on the picture tube) -- to be safe, you had to discharge it 4-5 times
at first, and then maybe 2-3 times again over a few minute period.

The safety rules around the shop said that if you were going to leave a
picture tube lying out on the bench unattended for more than a few minutes,
you had to clip a drain wire between the H-V nipple and the ground strap
on the tube.  (Yes, I got zapped at least once by a "discharged" picture
tube that had regained some of its charge. You *don't* want that to
happen while you're carrying it!)


Finally, I once had a need for some high-voltage in an easily portable
package (to power a surplus aircraft strobe light being used as a
"come home" beacon for some campers... uh... don't ask). I took a
flyback transformer -- including a 1A2 (?) diode tube with the filament
"transformer" mentioned above -- and wrapped an additional few turns
of heavy wire around the unused side of the ferrite core (are you
still counting?), and kludged up simple oscillator from a car-radio
audio output transistor. With a 12 volt lantern battery as input,
it took about 15 seconds to charge up the output capacitor (surplus
power company "power-factor" adjusting cap) enough to let the strobe
fire.  (I got enough over-voltage I didn't need a trigger for the strobe.)

Ah, the good (?) ol' days...


Rob Warnock
Systems Architecture Consultant

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