Couple questions about CCFLs

[caphair]

Hey guys, not sure if this is the right area to post this but I'm limited in what I know about electronics and because of this I tend to be very curious/inquisitive to help me get a better understanding of things.

I know the idea behind how a florescent lamp works. I'm wondering though:

1. Why is AC voltage used to get them going? In terms of Monitors and TVs. Couldn't the DC voltage be stepped up to do same thing? This question arises from how CRTs work and flyback transformer used to create electron beams and that high voltage is DC.

2. DO the CCFLs in Monitors/Tvs use the rapid or instant start method? I'm guessing instant start since there's only one lead on both ends as oppose to two.

These questions might be obvious to others but like I said I'm really on a journey of full understanding, thanks in advance!

[PlainBill]

Not all florescent lamps are created equal. Those used in lighting are hot cathode lamps and are driven by the AC line (typically 120 volts in the US, but higher voltages are common for industrial applications) at 60 Hz. A DC voltage could be stepped up, but is pointless. One other point - current limiting an AC source is trivial - that is one purpose of the ballast. Current limiting a DC source is non-trivial.

CCFLs are a cold cathode florescent lamp driven by 5 mA 700-1500 VAC at typically 40 Khz. As such, they don't have the start method used in hot cathode florescent lamps.

PlainBill

[caphair]

CCFLs are a cold cathode florescent lamp driven by 5 mA 700-1500 VAC at typically 40 Khz. As such, they don't have the start method used in hot cathode florescent lamps.

So the high voltage is what gets them going. Question now is does that mean one side is - and other + for this to happen? I know in a regular fluorescent lamp both ends have a filament so there's no real potential difference between both ends.

[PlainBill]

Originally Posted by caphair

So the high voltage is what gets them going. Question now is does that mean one side is - and other + for this to happen?

No. They are driven by AC. A DC potential across the ends can cause problems.

Originally Posted by caphair

I know in a regular fluorescent lamp both ends have a filament so there's no real potential difference between both ends.

Ok, WOW!!!! Does this mean you can touch a pin at each end of a florescent bulb while it is operating? Try it, have someone post a picture of your reaction.

PlainBill

[caphair]

Lol my temporarily confusion about ac/dc led me to that statement. So the potential difference in ccfls is created by ac. I was thinking dc in that both electrodes had to be a fixed + and - at both ends for there to be a voltage through the tube.