You have walked in the streets of a city, commercial buildings, and busy highways and lately even in small towns and seen advertisement signs that are lit. When darkness encroaches on the earth and man needs light, electric lighting has been the reliable solution since the late 1880s. Thanks to electric lighting businesses can go on in town centers at night. The advert boards that light up at night utilize neon gas-filled tubes to produce background light. It looks elegant and helps street folks to find their way in their business. Today we will look at one neon light transformer, a device that enables the operation of the tube to light up the signboard.
The electric circuitry inside the signboard constitutes a transformer, neon/argon/mercury vapour filled tube and of course the wiring. The transformer is specifically designed to power the tube and hence harness its advantageous properties. The tubes are designed to operate at high voltages typically 1kV to 15kV and small currents of 15-70mA typical range. Given that utility supply is about 120V or 240V single-phase supply, there is a need to step up the voltage. The high voltage ionizes the neon gas under pressure making it conductive and the current pass-through lighting up the neon vapour inside. Once the gas becomes conductive its impedance significantly drops and only a small current flows.
Construction
This is a step-up transformer with the secondary voltage being up to 15kV. The secondary winding has many turns compared to the primary winding. The turn ratio is quite high. The turns are wound on ferromagnetic iron core or toroidal-shaped material. The primary and secondary terminals allow connection of power supply input and neon tube respectively. One additional component is shunt ferromagnetic material in parallel to the secondary winding. The shunt electromagnets are equivalent to a large inductor in series with the secondary winding. The secondary terminals are connected to the bulbs with conductors as short as are achievable.
Operation
The device operates on the principle of electromagnetic induction. Magnetic flux in primary winding induces a high voltage in the secondary winding proportional to the turns ratio of secondary and primary turns. The high voltage provides enough energy to cause the ionization of neon gas inside the tube. This provides free electrons and the tube becomes conductive and neon gas particles produce light. This conduction lowers the impedance and a high current may be drawn; this is however prevented by shunt ferromagnetic parts. Like large inductors, they prevent the change of current by impeding, in this case, an increase of current. This makes the transformer work as a current source device. Due to the high inductance of the secondary side, the short circuit current is quite small, it is about 120% of the normal load current.
Selecting the right transformer for your signboard
Should you be shopping for this device, the most important factor to consider is your load rating. Different neon tubes require different voltage and current. The following tube properties will guide you in choosing the device to buy;
- The diameter
- The length
- Type of gas
- The pressure of the gas fill
The larger the diameter and the longer the tube the high the voltage and current it requires. On the other hand, the higher the pressure, the smaller the voltage it requires to ionize. The type of gas affects the ionization voltage and current on load. The good thing is that you don’t have to check this and then calculate the transformer to purchase guided by some chart. Simply check the tube rating namely voltage and current rating. Depending on its operating environment, ensure its IP protection class is appropriate.
Draw back
There is the stray capacitance between the high voltage terminal or conductors and the ground. This capacitance increases with the voltage and the length of the conductor. This is undesirable as it lowers the inductance of the secondary side thereby raising short circuit current in case of a fault. On top of this, it will reduce the current and voltage regulation of the transformer. Constant current is a requirement for the tube to function properly. The high voltage and capacitance can cause oscillation on the secondary side which causing flickering of the light emitted by the tube and buzzing of the transformer.
Conclusion
One cannot underestimate the importance of neon transformers when all adverts in commercial streets and town centers are powered by this device. The revenue attributed to adverts in business operations at night is transforming lives in great ways. Other than street lighting, neon lights are making the 24hour economy a reality in this 21st century. A word of caution is hereby rendered on the lethal nature of electricity; the high secondary voltage should never be measured with an ordinary voltmeter, makes sure your measuring device is rated above the transformer ratings. Finally, always engage a qualified and licensed electrician to work on a neon light circuit.