Weird Science

Neon lamp

Polish ver­sion is here

How to Make Gas Glow?

The neon lamp, also known as a neon tube or neon indi­ca­tor lamp, was inven­ted in the early 20th cen­tury by the French inven­tor Geor­ges Claude. It is the sim­plest type of gas discharge lamp. It con­si­sts of a glass bulb with embed­ded elec­tro­des, fil­led with low-pres­sure neon gas. When a high vol­tage is applied to the elec­tro­des, the gas emits light due to the phe­no­me­non of glow discharge.

Gases are gene­rally con­si­de­red good elec­tri­cal insu­la­tors, and this is mostly true. Howe­ver, under spe­ci­fic con­di­tions, elec­tric cur­rent can flow thro­ugh a gas. One type of elec­tri­cal discharge in gases is the glow discharge.

Let's recall what elec­tric cur­rent is. It is defi­ned as the orde­red move­ment of elec­tric char­ges in an elec­tric field. Howe­ver, an ordi­nary gas con­si­sts of neu­tral atoms that are elec­tri­cally neu­tral as a whole. So how is con­duc­tion pos­si­ble? Let's ana­lyze this fur­ther.

The gas inside a neon lamp con­si­sts of neu­tral atoms, but there is always a small num­ber of ions pre­sent. This is mainly due to ioni­zing effects from natu­ral radio­ac­ti­vity and cosmic radia­tion. When a vol­tage is applied to the elec­tro­des, an elec­tric field forms between them. Char­ged par­tic­les (ions) in the elec­tric field expe­rience a force known as the Lorentz force, which cau­ses them to move toward the elec­trode with an oppo­site charge.

Because the gas inside the neon tube is at low pres­sure, the par­tic­les can tra­vel gre­a­ter distan­ces between col­li­sions. If this distance is suf­fi­cien­tly large, the par­tic­les gain eno­ugh energy to ionize the atoms they col­lide with. This leads to a rapid incre­ase in the num­ber of charge car­riers, a pro­cess known as ava­lan­che ioni­za­tion.

Addi­tio­nally, nature tends to mini­mize energy sta­tes: par­tic­les quic­kly tran­s­i­tion from an exci­ted state to a sta­ble, lower-energy gro­und state. The excess energy is rele­a­sed in the form of a quan­tum of light (a pho­ton), which is why we obse­rve the gas glo­wing.

Each neon lamp has two key elec­tri­cal cha­rac­te­ri­stics: the bre­ak­down vol­tage (igni­tion vol­tage) and the lower extinc­tion vol­tage. This cau­ses neon lamps to exhi­bit nega­tive dif­fe­ren­tial resi­stance within a cer­tain range, simi­lar to tun­nel dio­des and negi­stors. Because of this pro­perty, neon lamps were histo­ri­cally used in oscil­la­tor cir­cu­its.

Let's Expe­ri­ment!

Cau­tion: Expe­ri­ments invo­lving high vol­tage sho­uld only be con­duc­ted under the supe­r­vi­sion of an expe­rien­ced instruc­tor if per­for­med by minors. The author assu­mes no respon­si­bi­lity for any inju­ries or dama­ges resul­ting from these expe­ri­ments. Pro­ceed at your own risk!

Neon lamps were widely used in the past. Today, they are most com­monly found in vol­tage testing screw­dri­vers (elec­tri­cal testers). Below are ima­ges of two neon lamps.

Ilustracja:

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Ilustracja:

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The lar­ger one has a built-in high-value resi­stor in its base, allo­wing it to be con­nec­ted direc­tly to the mains sup­ply. The smal­ler neon lamp has a very low power con­sump­tion and is used as an indi­ca­tor light in elec­tri­cal swit­ches or vol­tage testers.

If we want to con­duct expe­ri­ments with neon lamps, we need a high-vol­tage power source. For neon lamps, a high cur­rent is not requ­i­red (ZVS dri­vers are unsu­i­ta­ble because they have exces­sive power out­put). The sim­plest solu­tion is to use a small inver­ter desi­gned for powe­ring LCD moni­tor bac­kli­ghts. I use a power sup­ply salva­ged from an old scan­ner, which ori­gi­nally powe­red the scan­ning lamp. It looks like this:

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This cir­cuit gene­ra­tes an AC vol­tage high eno­ugh to ignite a neon lamp. When con­nec­ted, the gas inside the lamp glows visi­bly. The glow always appe­ars near the nega­ti­vely char­ged elec­trode, but when powe­red with AC vol­tage, the roles of the elec­tro­des switch at high fre­qu­ency. As a result, we obse­rve light emis­sion near both elec­tro­des.

Ilustracja:

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Ilustracja:

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The color of the emit­ted light depends stric­tly on the type of gas inside. In the case of neon, it is an orange-red hue.

Inte­re­sting Phe­no­me­non

Let’s assem­ble the cir­cuit shown below, con­si­sting of a neon lamp and the pre­vio­u­sly men­tio­ned inver­ter.

Ilustracja

At first glance, this setup might seem unu­sual. Only one power sup­ply ter­mi­nal is con­nec­ted to the neon lamp. The cir­cuit appe­ars to be open, sug­ge­sting that no cur­rent sho­uld flow. Howe­ver, if you bring your hand close to the bulb or touch its glass, the gas inside will start glo­wing. Why does this hap­pen?

This phe­no­me­non occurs because alter­na­ting cur­rent (AC) can pass thro­ugh an elec­tric capa­ci­tance. In this case, the capa­ci­tance between the power source, the expe­ri­men­ter’s body, and the sur­ro­un­ding envi­ron­ment is suf­fi­cient to allow a small cur­rent to flow. The cur­rent is very weak, but it is eno­ugh to ionize the gas and cause it to glow. Because of the low cur­rent invo­lved, this expe­ri­ment is com­ple­tely safe.

Enjoy expe­ri­men­ting! :)

Fur­ther rea­dings:

Marek Ples

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