And while we are at it, why do they make that obnoxious buzzing sound?
My understanding of flourescent lights is that they function by sending an electric charge through a gas vapor (I think mercury vapor). It takes a second or two for all of the gas to react, I guess.
This is only my limited understanding; maybe some flourescent light expert can come along and provide more info.
An old fluorescent lamp takes longer to light because there is a starter, which is a time delay switch, that allows electrons to flow through the lamp (between the filaments at each end) and ionize the vapor inside, producing light. A high voltage is needed to start the lamp, but much lower voltages to maintain the ionization.
Most current fluorescents are “rapid-start”, but it still takes a short time period for the filaments to heat up
The buzzing you hear is the ballast, which acts as a current limiter to the lamp. Buzzing is usualy a sign of a cheaply made ballast, i.e. the ones most companies buy for their offices.
While we’re talking about flourescents . . . There’s a flourescent lamp in our bathroom–the usuall two-long-tube variety. Sometimes it doesn’t light up, particularly when it’s humid (summertime, or if someone recently took a shower). At first I thought that the tube was just not seated well in the socket, because touching the tube would usually get it to light. (I touch the glass part, not the metal–no Darwin Award for me!) After a while, though, that stopped being effective. After months of experimentation, I developed a technique that works every time: stroking a tube. If I run my finger down one of the tubes, it always takes right off.
Can anyone explain why that might be?
SWAG here, Podkayne, but here’s what I think is happening. You’re ballast or starter is nearing the end of it’s life and can’t work up quite enough “kick” to start the lamp.
If you go into a dark room and rub a fluorescent tube with a piece of silk, you can make it glow due to the static electricity.
I’m guessing that your static charge is just enough of a boost to kick start the lamp.
There are differant types of flourescant lamps henceforth to be called FL’S.
The glow starter types have a bi-metallic strip inside them and a heater.As the heater warms the strip up it bends due to differant expansion rates in the two metals until it touches a contact. This gives the high voltage discharge components to charge up, the discharge voltage goes through the contacts and an arc jumps through the gas medium.
Once the arc has been struck the impedance of the gas medium drops hugely and since it is in parallel with the glow heater, whose resistance is much higher than the arc pathway, the current is diverted away from it and the bi-metallic strip cools returning to its original position(near enough). The current through the arc, a gas plasma, is limited by the choke device in series with it.
Other FL’s do not have glow starters and the choke or ballast unit is of a differant construction, essentially it is a two output tranformer, both outputs are fed across the lamp. A high voltage but also very high resistance output which produces the high strike voltage across the lamp, and a lower voltage but much lower impedance output which maintains the current flow.
Not easy to understand is this but imagine if the low impedance output failed, the high voltage output would strike the plasma arc but once this arc had been established and the plasma impedance fell the current would not be enough to maintain the arc, hence the need for the second transformer output.
It takes time for current to make its way through ballast units due to them working on the principle of rate of change of magnetic fields, however in the reverse it also takes time for the current to fall to zero for the same reason.
Circuits with delays caused by capacitors and inductors(inductors are choke units, transformers, and ballsts units)
are said to have charge up times called time constants.
You will also find round metal cased components inside FL’s but these are there to counter the effects of having inductive components which can adversely affect the mains power supply.
There are other types of FL but these are industrial or specialist(like mercury bath arc, carbon rod arc, and some types of ultra high output halogen arc)
Slight hijack: My understanding is that the ionized mercury vapor has strong UV emission. And that what puts the fluorescence in fluorescent lighting is the interaction of the UV radiation with a fluorescent coating on the inside of the bulb. So is an uncoated “fluorescent” bulb a good UV source?
A mercury vapor lamp does emit high levels of short-wavelength UV light and nearly all of it is absorbed by the phosphor coating on the inside of the tube. I suppose if you removed the phosphor coating, you would have a good source of UV radiation.
I think I’ll add a little to this and explain why the ballast unit can give such a high voltage kick in the glow start FL.
When you try to put a voltage across an inductive component like a coil of wire(which is what a ballst unit is) there is a counter voltage produced which tends to oppose the incoming voltage.
This is caused by the expansion of a magnetic field, the quicker the magnetic field increases the greater is the opposing voltage( more correctly called - back e.m,f) the back e.m.f is directly related to the rate of change of the incoming voltage.
If you then try to discharge this ballast unit by putting a short circuit across it you are trying to change circuit conditions instantaneously ie very fast, so the back e.m.f can be very large indeed, many many times the incoming voltage(which is the pinciple that those stun guns work on)
This voltage has the opposite polarity to the incoming voltage.
In the glow start when the bi-metallic strip touches that contact it is short circuiting the ballst unit with only the lamp gas in the way and this is why you get an arc that can jump such a distance.
The non-glow start lights use transformer action to work.
Under these extreme conditions of plasma arc and high voltage starting spikes the electrodes within the tube break down and form compounds with the gas, this takes a long time and many startup cycles but eventually the cpacity of the gas to carry an arc is compromised and higher and higher voltages are needed to strike the arc, eventually tube fails to start up.
When you hold the tube part way along its length an absolutely miniscule of current passes into you and a fragile pathway forms to the point you are touching, this reduces the gap that the arc has to jump and makes it easier to make a full strike.
I have seen some very surprising things happne with FL tubes.
If you insulate yourself from ground and charge yourself up with an extremely high static charge, you can then hold a FL tube in one hand and it will glow yet there is no obvious circuit path.
Fluorescent lamps are widely used as a source of UV light. An uncoated fluorescent lamp emits only short-wavelength UV-C light, though, which is useful for killing germs but not that great if you want to get a tan. Tanning lamps, blacklights, bug-zapper lights, etc. have special phosphor coatings that emit longer wavelength UV-B and UV-A light when struck by UV-C radiation.