Fluorescent Lamp Heater Filament Current

Hello Steve,

This might help you:

best regards,

Wim PA3DJS

martin

In preheat lamps, the starting current is a little more than the normal operating current. For those lamps, typically the filaments have current sent through them as opposed to having a voltage source applied to them.

In F40 rapid start lamps, the voltage applied to the filaments is about

Avoid applying near or over 10 volts to a fluorescent lamp filament unless current is limited. At about 11-12 volts, an arc usually forms across the filament. This often occurs in preheat lamps, but that is OK since current is limited.

There is a variation of "rapid start" known as "trigger start". That involves a ballast with filament windings that produce voltage that decreases once the lamp starts. Such ballasts are intended to run preheat lamps.

One source: Sam Goldwasser's F-Lamp FAQ. A recent version is somewhere in

That document mentions voltages from a common rapid start ballast.

- Don Klipstein ( snipped-for-privacy@misty.com)

through

G4-T5 ,

Thanks Wim, that helped a lot. In my reading so far, I had seen references to 8V and 4V as heater filament voltages. This link cleared up much of the mystery. 4V, (3.4V to 4.5V), seems to be the most common. ... Steve

through

G4-T5,

Hello Don, Thanks for all the info. I've already read f-lamp.html, (I have a local copy of most of Sam's FAQs), including your page on discharge lamp theory. It was very informative. So was the link provided by Wim.

I'm designing a couple of inverters to run a 4W and a 20W tube from 12VDC, ideally in modified 'rapid-start' mode. (The heater current is reduced or removed after the tube ignites.)

It appears that most tubes use 3.4 to 4.5V, while some need about 8V, as you mentioned. I have two tubes in front of me, a 20W coil type, taken from a compact fluoro and a 20W T10 black light tube. The compact fluoro coil tube filament has a cold resistance of 4 ohms and lights up dimly with 3.5 to 4V (DC) applied. The black light tube filament has a cold resistance of about 8 ohms and lights up dimly with 8V (DC) applied.

I'll do some hot filament current measurements next. First, though, a couple more questions, if you don't mind.

1. Do you know if it's better to run the filaments so that thay're visibly glowing, or just short of that point?
2. Most small inverters built for this purpose, especially 4W to 8W tubes, use a single-transistor oscillator with positive feedback derived from a feedback winding on the inverter transformer. This results in a pulsed output. Does this mean reduced brightness and if so, can it be compensated for by adding extra turns to the HV secondary, or is a 50% duty-cycle complementary drive better?

... Steve

through

Thanks, Martin. I'm reading my way through this one and picking up a few extra pointers.

... Steve

They should visibly glow.

There are a few factors here:

1. There is "current crest factor". That is ratio of peak current to RMS current. The less that is, the longer the lamp life (whether fluorescent or HID or low pressure sodium) and the better the efficiency (for fluorescent and low pressure sodium). I somewhat remember some sort of industry standard that this figure should not exceed 1.7 for most discharge lamps.

1. The current through a fluorescent lamp should have little or no net DC component. The voltage waveform across the lamp should have little or no net DC component. Voltage waveform, after weighting by magnitude of current waveform, should also have little or no net DC component. What a net DC component here does is cause positive mercury ions to drift towards the more-negative electrode. This can cause a mercury shortage at the positive end of the lamp, maybe even a majority of the lamp, after several hours or a couple weeks of operation. This gets worse with lamps that are longer or have greater ratio of length to cross sectional area or greater product of length and current density (amps per square centimeter of cross section of the discharge). The 4 watt F4T5/G4T5 is more tolerant, but that is a low efficiency lamp (due to high percentage of lamp voltage being in voltage drops at the electrode processes). If the current waveform has negative and positive half cycles roughly equal in duration and there is a capacitor in series with the lamp, I suspect you should be good even if the waveform lacks "half cycle symmetry". You should also be good if you have "half cycle symmetry".

My experience so far is that cheap solid state ballasts with a single transistor but also with a capacitor in series with the lamp tend to lack mercury drift problems, at least in shorter lamps. However, I expect improved lamp efficiency and improved lamp life with a complimentary drive scheme.

Another item from my experience: Most (but not all) fluorescent lamp products that run from batteries have the lamps being underpowered.

- Don Klipstein ( snipped-for-privacy@misty.com)

visibly

tubes,

compensated

Thanks Don, you're a wealth of information. You confirmed what I thought regarding duty-cycle and symmetry. (I was planning to use a 10nF

I'm not sure which way to go with regard to filament heating. I'm leaning towards heater windings on the transformer with a PTC thermistor in series with each. (Simple but probably effective.) Alternatively, I'm considering connecting the lamp power to one side of each heater, then connecting the other side of the heaters together with a small capacitor. I'd then need to be able to sweep the frequency from higher at startup to reduce the capacitor's reactance and increase current through the heaters, then lower for ignition and reduced heater current. (This is a bit more complicated, but should work fairly well.) I want to avoid tube blackening as much as possible, otherwise I wouldn't even bother with heater connections.

... Steve

I was just thinking. In the case of using a capacitor between the second side of the heaters to vary their current, I wouldn't need to manually sweep the frequency from high to low. It should happen automatically if I use an oscillator with feedback from the transformer. The frequency will drop as the transformer becomes loaded by the disharge current. ... Steve