About driving 36WT8 FL with 25% of its nominal power.

Hello.

I am developing electronic ballast for fluorescent lamps (standard lamps, mostly for 120cm 36W T8 lamp).

But the main problem here is that the ballast should power the lamp with 25-30 % of it is nominal power (e.g for 36WT8 it should be around 10 W) and that the lamp should have the same lifetime (as it has when it is driven with it is nominal power).

I have read some information from the internet concerning driving FLs and dimming them, but got more questions than answers.


Right now I am considering the fallowing simple schematic:
The power source for the lamp will be Half-bridge (IR2153) with frequency about 34 kHz.
This half-bridge will drive a simple resonance circuit (inductor in series with lamp paralleled with a capacitor).
I am planning to select (by calculating and experimentally) such an inductor and a capacitor, that the resonance frequency will be close to 34 kHz and that the power in running stage will be about 10 W.

For such schematic I would like to ask questions:
1) As I understand when driving the 36WT8 lamp in 10W mode, the filaments will be cooler than when it is driven with 36W (please correct me if I am wrong). How will this affect lifetime of the lamp (lifetime of its cathodes and other parts)? Will they loose its emission capability faster or will it only be better for cathodes to work in low power mode?

2) My first experiments revealed that the lamp ignites and runs normally. But, driving the lamp with about 10W by this method the plasma (output light) of the lamp looks very funny. I can see something like light-waves, which go from one end of the lamp to another with low speed (about 1 m/sec). Especially I can see more of these waves when I hold the lamp with my arms.
What can be this effect associated with? How can I get rid of it?

I am feeling that I should constantly heat the filaments, but I really would like not to do it, if it is possible to drive lamp with 10W and produce normal light without reducing the lamp lifetime.

I would really appreciate any help with these issues.

Thank you.

Re: About driving 36W T8 FL with 25% of its nominal power.

I am very sorry, but could anyone help me with this.

Thank you.

hello,

you bring back memories as i remeber the strange rippling thing inside the flourescent tube and i wondered what it was.

i used IR21593 and it has a dimmer input so you can dim flu's down to 10W no problem.....but it does it by increasing the frequency.

the IRF.COM website has ballast design software which should be useful.

if you use too low power.......maybe if the current gets too low....i think the flu tube can end up going off and on very quickly and this can damage it bad.

the datasheet should help you but to be honest, most datasheets of flu tubes say very little....there is detail on the ballast software of irf and i found they had good lamp detail there.

Added after 2 seconds:

hello,

you bring back memories as i remeber the strange rippling thing inside the flourescent tube and i wondered what it was.

i used IR21593 and it has a dimmer input so you can dim flu's down to 10W no problem.....but it does it by increasing the frequency.

the IRF.COM website has ballast design software which should be useful.

if you use too low power.......maybe if the current gets too low....i think the flu tube can end up going off and on very quickly and this can damage it bad.

the datasheet should help you but to be honest, most datasheets of flu tubes say very little....there is detail on the ballast software of irf and i found they had good lamp detail there.

Thank you for the reply.

IR21592(3) are good ICs, but the budget for our project don’t allow us to use this IC. But I think it will be useful for me to try and compare the dimming with this ballast.

As I see from the application notes and datasheets for IR2159x, they use the same resonance circuit around the lamp. They don’t use special heating, like for T5 lamps, when the ballast inductance has two more additional small windings and by means of voltage (from these windings connected to the filaments through a capacitor) you constantly heat the filaments.

So, do I understand right? that if to dim the lamp by means of IR2159x, say to 20%, the lamp life time will be the same as you power it with nominal 36W.

And also you mentioned “strange rippling thing inside the fluorescent tube”. What was it associated with and will it be the same with IR2159x. I know I should try it by myself and I will get more useful data. I will do it a bit later (few days from now) and then tell my own results.

But if you know it from your own experience could you please share this information with me in this forum.