The atmosphere at Oscilloclock.com has been charged lately. Mails have been pouring in from folks who want to generate high voltage for their CRT projects, but have instead ended up with high tension from frustrated attempts. The primary culprit? Lack of a decent HV transformer.
HV Transformer basics
CRTs require high voltage, to coax electrons out of the electron gun and then accelerate them towards their fiery demise at the screen. This voltage can range from hundreds of volts for small tubes, to tens of kilovolts for large tubes! In the case of the Prototype and Model 1 CRTs, around 3kV was needed.
How can we create such a voltage?
Well, everyone remembers the old wall-warts, right? The horribly heavy black boxes that plugged into your outlet and supplied power to your cassette deck, answering machine, or fish tank pump? WHY were those things so heavy? Because their main component was a massive hunk of solid iron, with some coils wound around it. This was a transformer, and it served to change the line voltage in your house to whatever voltage your fancy equipment desired.
Wikipedia does better justice in explaining how a transformer works, but the basic principle is that an alternating current flowing through one coil (the primary) creates an electromagnetic field in the heavy iron core. The other coil (secondary) is swathed in this glorious field, and electron flow is induced. The more turns the secondary coil has around the core compared to the primary, the higher the voltage you get!
How high can you go? Well, if you have a turns ratio of 1:10, and your input is 240V AC, you would get a nice fat 2400V AC output. Rectified, this would power some CRTs beautifully! Most oscilloscopes prior to the transistor era used these so-called ‘heavy iron’ transformers with high voltage windings, at ratios of 1:5 to 1:30 depending on the voltages needed.
These transformers are very large – but to make matters worse, it is not just the tranny that’s clunky and heavy! The other components needed to rectify into DC and smooth out all the nasty ripples are almost as massive.
Slimming Down, Speeding Up – Magnetics
The gross obesity of these old components is really down to the frequency of the input. Running at 50/60Hz (or 400Hz if you reside on an aircraft), these components act like an ice skater twirling around on one leg with arms fully extended – a nice slow spin, a nice large surface required. Now, imagine the skater pulls those arms in and squeezes into a pencil-like shape. Amazing! A fast spin in a sleek form factor.
Incredibly, the same can be done with our trannies and filter components. If they are run faster, they can be shrunk, yet still deliver the same power! But as frequency increased, the characteristics of the core material and the physical arrangement of the windings (loosely known as the magnetics of the transformer) became exponentially more significant. This was the limiting factor.
Heavy iron was simply not good at hosting rapidly-fluctuating magnetic fields, but there was no better material available for widespread use. So, manufacturers of extremely high-end equipment that needed to be light-weight and small sometimes opted to use air-core transformers – which had no core at all! This allowed them to run at high frequencies, at the cost of power. Without any substance to maintain the induced field, the transformers delivered very limited current.
50 years later, we now have incredibly strong magnetic materials available for use in transformers. Modern HV transformers work at insane frequencies – into the MHz realm – and are nearly invisible if you have poor eyesight.
The Prototype – self wound
So! Four years ago, with all this highly scientific understanding about ice skating and component obesity under my belt, I set out to wind my own tiny high-frequency HV transformer.
In the next exciting post in this series, I go through all the practical steps taken – pain, frustration, but lots of fun!