Neon Discharge Glow Tube Old Technology Cold Cathode Ionized Vacuum Tube
A special group of very early vacuum devices survived to this very day. This group is the cold-cathode gas-discharge glow lamps. Within the family is most prominent is the neon bulb. Currently they are often called glow lamps. Glow lamps are super-simple two terminal devices. In spite of their extreme simplicity these group of devices are exceedingly useful. Many amazing applications possible with this super simple device.
|Figure 1. Simple Cold-cathode Gas-discharge Glow Lamps|
The glow lamp is made of a minimum of two electrodes, spaced a short distance apart and enclosed (usually) in an evacuated glass envelope. The electrodes made of iron wire coated with some rare metal from the alkali group. The envelope is in partial vacuum and filled with small amounts of gas. Most often glow lamps use neon, argon, helium, other gas at 20-50 torr pressure. Mixture of noble gases as filler, 75% neon and 25% helium is also common.
A simple glow lamp is depicted on Figure 1. A more complicated arrangement is shown on Figure 2. Some neon filled glow display devices were made in 1970 by Burroughs Corporation ECD Division that where used in a standard television set in place of the CRT tube (Self Scan). It was a forerunner of the flat screen plasma TV.
|Figure 2. Complex Cold-cathode Gas-discharge Glow Lamps|
The basic circuit to operate a neon glow tube is simplicity itself. As the voltage is increased across the two electrodes of the neon bulb, the internal gas reaches an ionization potential - Figure 3. At the ionization voltage the device conduct and current flows. Unopposed, this current would build to such high level that if not limited by some ways, it destroys the device. For this reason, gas discharge tubes always operated with a current limiter. For limiting the tube's conduction current most often a simple resistor is used. In some small lamps the resistor is built into the envelope internally.
|Figure 3. Basic Cold-cathode Gas-discharge Glow Lamp Circuit|
Often used symbol for the neon tube is the one seen on Figure 3. Resistor R1 is required to prevent the tube from distraction due to excessive current. The value of that resistor is selected to limit the tube current to a few mA [miliamperes]. The smaller the tube current, the longer the tube life will be. A small amount of mercury added extends the tube life many folds. Unfortunately the brightness of the tube will also be smaller. A battery or some other voltage source would work equally well if its voltage exceed the ionization voltage of the tube.
Interestingly, in a completely dark place, the bulb may not ignite at all, because of a complete lack of ions. To overcome that start difficulty often the tube producer mixes the glow gas with a slightly radioactive isotope to help the turn on. Burroughs Corporation in some of their more complicated display tubes that had hundreds of elementary glow cells (Self Scan), used a permanently illuminated electrode in an invisible corner location that they named a "keep-alive".