More fun physics around CFL, UV or glow discharge lamps

We talked about CFL (fluorescent lights) in our previous article.


I would like to demonstrate a few more experiments here to give you better understanding about physics which going on inside lamps.

Glow discharge lamps consist of glass container (usually cylindrical shape). This tube has
low pressure gas inside. Pressure is lower then atmospheric or another words it is partial vacuum. Then manufacturer allow little amount of inert gases like neon or argon or can be some helium. Those one used for coloring light emissions during plasma glow. Also some small amount mercury added for getting better plasma stability and specific spectral properties.

A compact fluorescent has glow discharge plasma. That means plasma inside is not that energetic and in fact it is more like cold glow plasma. Electron densities in that plasma can be around ne≈1016m−3 and electrons running around that plasma has very low energy (around few electron volt). More so, Ions has much lower energies too. If you think plasma inside consume all gaseous material and every atoms there is involved in the plasma discharge, you are wrong. Only one atom gets to participate in that crazy dance. The rest of material stand cool.
But if you have a chance to rise current flow in to light bulb, you might see much more light out of it. This might not go too well for the life of the tube and bulb will go hot first and dies ether after few minutes or right after you turn it off. This is simple to understand. Plasma having inverse resistance over applied energy. So the Ohms law: I=U/R is very good to describe metals. Plasma can not be described by Ohms law. In plasma the more you give energy to it, the less you have resistance of plasma. So here is conundrum. If you have wire, it gives resistance to current and truing to increase current in metal will lead to increasing resistance of the wire. Plasma on opposite: more you give current less it has resistance. This is because with higher current you have more and more atoms participate in plasma and thus you have more ions and electrons generated.
So we learn that we should always control current in plasma type of devises or they go crazy and in that craziness they burn everything down. This is why ballast is needed for driving glow discharge lamps. Ballast is a starter and at the same time current limiting device.
Temperature is also playing important role. Those atoms receiving bare minimum energy with current limiting and thus if we have cold environment, CFL and other similar lamps won't start. Try to make an experiment: around -10C, most likely CFL wont work, but hold it for 10-15 minutes with your hand and it will get going. This trick can be used not only for low temperatures, but for some old or under powered lamps.
See here:
Normally lamp wouldn't start because I on purposely lowered input ballast voltage below bare minimum. And CFL wouldn't start. But touching and holding it even for a second will start them going.


Also if you use high powered drivers (for example even 20 Watt ballast we have is overkill for 4W bulb) and let the lamp glow on that driver. You will see extremely bright light and after 30 seconds lamp dies. So what happened you might ask. Very simple. Lamp will have over limit amount of plasma and not the cold plasma, but with over 10% atoms being involved in that and the whole bulb will get very hot (over boiling temperature). I registered 250C and more with pyrometer. This will expend glass and seems around wires. So the seems will get damaged. Simply because it was not built to hold that temperature.


Viewed: 61