Vacuum fluorescent displays (VFDs)
by Chris Woodford. Last updated: September 10, 2016.
Do you ever wake in the middle of the night wondering whether
you need to get up yet? By the time you've scrabbled around for the
alarm clock, chances are you've knocked over a glass of water, swept
the contents of your beside table to the floor, pulled a muscle
stretching too far, and woken up the rest of your household as well.
Unless, of course, you have an electronic alarm clock with a VFD.
Typically colored green, these digital displays are bright and clear enough
to see in darkness (or full sunlight) without straying an inch from your
pillow. Although VFDs have been widely replaced by LCDs in
applications such as pocket calculators and handheld games, they're
still used in microwave ovens, audio equipment, and car
dashboard displays. Ever wondered how they work? Let's take a closer look!
Photo: A VFD displays the time on this electric stove. VFDs are widely used on stoves and microwaves, partly because they're easy to read from a distance, but also because they work at much higher temperatures than LCDs (typically up to at least 85°C or 185°F)—so there's less chance of the stove's heat affecting them.
What is a VFD?
A few decades ago, electrical appliances were blissfully simple:
you could switch them on and off—and that was about it. Now things
are more complex and even relatively simple things like
clothes washing machines,
radios have digital
displays that tell you what they're doing. Some can show numbers,
some display numbers and letters, and others can show animated
Broadly, digital displays come in three different kinds that work
in three quite different ways:
- LEDs: These incorporate brightly lit segments
that work in the same way as the little light-emitting diodes (LEDs) used on instrument panels. Typically, LED displays show only numbers (digits), each one
made up of seven illuminated bars (or segments). They work equally
well in darkness or light, but consume relatively large amounts of
- LCDs: More versatile than LEDs and also more compact, LCDs
can show numbers, letters, words, or pretty much anything else. They're
used in everything from cellphones and laptops to pocket
calculators and LCD televisions. Although they use relatively little
power, they need a backlight (a light shining from behind them from
the back of the display out through the front) so you can see them
properly in low light levels.
- VFDs: Combine the advantages of LEDs and LCDs. They're as
bright as LEDs but easier to read, and they can be used to display
numbers, words, or letters. They're easy to read in darkness or light.
Photo: It takes seven individually controllable segments to make each digit of a numeric display like this.
How does a VFD work?
If you know how an old-style cathode-ray tube (CRT) television
works, understanding a VFD is simple. In a TV like this, there's a
hot piece of metal called the cathode whose job is to fire
electrons down a glass tube to the screen at the front where the picture is formed. The screen is coated with chemicals called phosphors that
glow (or fluoresce) when the electrons crash into them. By steering
and focusing the electron beam using magnets and a metal grid with holes in
it, we can paint an ever-changing pattern of glowing dots on the
screen—and that makes the moving picture you see when you watch TV.
How it works in theory
A VFD works in a similar way to this using three electrical
terminals (or electrodes) sealed inside a large glass bulb from which
the air has been removed:
- There's a heated filament (the negatively charged cathode)
made from tungsten metal whose job is to produce electrons.
It's the red bar in our diagram.
- Each segment of the display (which is a positively charged
anode) is coated with phosphor, like the screen of a TV.
These phosphor-coated segments glow with light (often a ghostly green color) when electrons hit them.
- In between the cathode and the anode there's a thin mesh of
metal called the grid that can be switched on or off,
controlling the flow of electrons from the cathode to the anode.
The cathode is negatively
charged (−ve) and each anode is positively charged (+ve), so electrons (yellow arrowed line) tend to flow
naturally from one to the other. When electrons strike the phosphor coated anode, it
glows with green light. We can stop this happening by making the grid negatively charged,
which repels electrons away from the anode. Changing the grid voltage is
thus a simple way of switching a segment of the display on or off.
It's also a way of controlling the brightness of the display: making the grid
more positively charged accelerates the electrons so they
rush past and hit the anode with more energy, giving off more light. The higher the positive
grid voltage, the brighter the display. (Brightness controls on VFDs are effectively just voltage controls on the grid.)
(Note that this is pretty similar to how a triode vacuum tube works.)
How it works in practice
And here's an illustration of a typical, practical VFD using similar color coding. The cathode (red) consists of two very fine wires running down the center. The seven anode segments (shown in green) are coated with fluorescent material and positioned beneath the cathode wires, with seven external leads to control them (shown in blue at the bottom). You can see a section of the grid at top left (orange). The cross-hatched "envelope" around the edge is the vacuum compartment in which the whole thing is sealed, typically made from glass because you have to be able to see the segments lighting up!
Lower artwork: A typical, generic, triode-style VFD illustrated as part of the prior art referred to in
US Patent #4,004,186: Vacuum fluorescent display having a grid plate coplanar with the anode by Ernest Gerald Bylander, Texas Instruments Inc., January 18, 1977.
Artwork courtesy of US Patent and Trademark Office.
How do you control a VFD?
Typical VFDs show several digits or letters—and maybe preformed
words too (like Manual, Auto, AM, PM, Defrost, or whatever). Apart
from the display itself, VFDs also need a way of being "driven"
(controlled electronically from whatever circuit they're attached
to)—and there are two ways of doing this. One is to use separate pin
connections to each segment of the display and a single grid.
This method is called static drive. Another method uses lots of different
grids (one for each segment) that are turned on and off in rapid
succession (typically over 100 times a second) by a microprocessor
(single-chip computer). Each segment of the display that appears to be permanently turned on is actually flashing
on and off very quickly (too quick for our eyes to notice) in sequence. This approach is
called multiplexing (or dynamic drive) and has the advantage
of needing simpler drive circuitry (fewer chips and pin connections)
than static drive.
Advantages and disadvantages of VFDs
Invented by Japanese company Noritake in the late 1960s, VFDs
became popular in the mid-1970s as a more versatile and attractive
alternative to LEDs (which typically consumed more power). In their
turn, VFDs were superseded by LCDs, which were more compact, cheaper
to make, more versatile (capable of displaying complex,
high-resolution color pictures of absolutely anything—which neither
LEDs nor VFDs can manage), and used even less power. But VFDs remain a popular choice for
simple, robust displays in electronic instruments and appliances
that need to work in a wide range of lighting or temperature conditions. According
to Noritake, VFDs can be far superior to LEDs and LCDs in certain
applications: they're easier to see in very bright light and
darkness, and they can be dimmed very easily for use at night
(simply by reducing the amount of time for which the segments are
switched on in a multiplexed display); they often look more
attractive than LCD displays; they can be seen from further away and
from a wide range of angles (a notorious problem with LCDs, which darken and
change color when you look at them from an angle); and
they work at a wider range of temperatures (an important factor in
low-temperature, outdoor equipment and for high-temperature
appliances such as microwave ovens and electric stoves).
The disadvantages of VFDs? Unlike an LCD, they can display only
predefined combinations of letters, numbers, or words. Compared to
LCDs, they have relatively high power consumption (so they're less
suitable for battery-powered gadgets such as pocket calculators and
digital watches). Other problems include some segments of a VFD
display gradually becoming brighter or dimmer than others (caused by
the phosphors glowing less brightly as they get older) and flickering (a
problem caused by using the wrong frequency for multiplexing).
Photo: In the 1970s, VFDs were widely used in pocket calculators like this before low-power LCDs started to replace them.