By Radu Motisan Posted on October 9th, 2011 , 976 Views (Rate 1.66)
Neon lamps are nothing more than two metal electrodes inside a sealed glass bulb, separated by the neon gas inside. At room temperatures and with no applied voltage, the lamp has nearly infinite resistance. However, once a certain threshold voltage is exceeded (this voltage depends on the gas pressure and geometry of the lamp), the neon gas will become ionized (turned into a plasma) and its resistance dramatically reduced. In effect, the neon lamp exhibits the same characteristics as air in a lightning storm, complete with the emission of light as a result of the discharge, albeit on a much smaller scale.
The capacitor in the relaxation oscillator circuit shown above charges at an inverse exponential rate determined by the size of the resistor. When its voltage reaches the threshold voltage of the lamp, the lamp suddenly "turns on" and quickly discharges the capacitor to a low voltage value. Once discharged, the lamp "turns off" and allows the capacitor to build up a charge once more. The result is a series of brief flashes of light from the lamp, the rate of which is dictated by battery voltage, resistor resistance, capacitor capacitance, and lamp threshold voltage.
In essence, thyratron tubes were controlled versions of neon lamps built specifically for switching current to a load. The dot inside the circle of the schematic symbol indicates a gas fill, as opposed to the hard vacuum normally seen in other electron tube designs. In the circuit shown above the thyratron tube allows current through the load in one direction (note the polarity across the load resistor) when triggered by the small DC control voltage connected between grid and cathode. Note that the load's power source is AC, which provides a clue about how the thyratron turns off after its been triggered on: since AC voltage periodically passes through a condition of 0 volts between half-cycles, the current through an AC-powered load must also periodically halt. This brief pause of current between half-cycles gives the tube's gas time to cool, letting it return to its normal "off" state. Conduction may resume only if enough voltage is applied by the AC power source (some other time in the wave's cycle) and if the DC control voltage allows it. (source)
The thyratron cool down time (also called recovery time) is a process in which the ionized gas inside looses electric charge and returns to neutral state. The time required is strictly dependent on the gas used inside the tube. To achieve short deionization times, hydrogen and deuterium are used. A short deionization time means that the tube is getting ready for another pulse faster, resulting in higher operation frequency and oscillator frequency. Here are some examples:
3C45 Hydrogen Thyratron: 0.6usec deionization time
8503 Hydrogen Thyratron: 5-40usec
309CE / FG17 Mercury Vapour Thyratron: 1000usec!
3C31 / 5664 / ELC1B Xenon gas Thyratron: 500usec
Turning OFF a Thyratron working with AC current is easy. If the thyratron conducts on one alternation, the next alternation will turn it off. It is a different story when the Thyratron uses DC High voltage, and this is the most common case. Instead of the simple capacitor in the relaxation oscillator diagram, a pulse forming network is used. PFN has inductive properties so the positive discharge voltage has a tendency to swing negative, turning the thyratron off and ready for the next discharge.
This kind of approach has been used in radars, and Radartutorial.eu has a very good article describing the functionality details : "
Radio frequency energy in radar is transmitted in short pulses with time durations that may vary from 1 to 50 microseconds or more. A special modulator is needed to produce this impulse of high voltage. The hydrogen thyratron modulator is the most common radar modulator. It employs a pulse-forming network that is charged up slowly to a high value of voltage. The network is discharged rapidly through a pulse transformer by the thyratron keyer tube to develop an output pulse, The shape and duration of the pulse are determined by the electrical characteristics of the pulse-forming network and of the pulse transformer.
The Charge Path
The charge path includes the primary of the pulse transformer, the dc power supply, and the charging impedance. The thyratron (as the modulator switching device) is an open circuit in the time between the trigger pulses. Therefore it is shown as an open switch in the Figure:
Once the power supply is switched on (look at the dark green voltage jump in the following diagram), the current flows through the charging diode and the charging impedance, charges the condensers of the pulse forming network (PFN). The coils of the PFN are not yet functional. However, the induction of the charging impedance offers a great inductive resistance to the current and builds up a strong magnetic field. The charging of the condensers follows an exponential function (line drawing green). The self- induction of the charging impedance overlaps for this.
The Discharging Path
When a positive trigger pulse is applied to the grid of the thyratron, the tube ionizes causing the pulse-forming network to discharge through the thyratron and the primary of the pulse transformer. (The tyratron is „fired”)
A DIY Relaxation oscillator using a Hydrogen Thyratron
To test the working principle, I have designed a relaxation oscillator using the following:
- high voltage DC supply (using a 100W flyback transformer, and a Royer push and pull oscillator), providing 8KV 10mA.
- rewound mot supply , for tube filament
- a ferrite choke for charging the PFN
- a pulse forming network, as presented here.
- a hydrogen thyratron, TGI2 400/16
- a 200V pulse generator, more details here.
For a test load I have used an air transformer (tesla coil). This would be my first Thyratron Tesla Coil (THYTC). And here are the results: