DC ballast for germicidal (UVC) bulbs

[dennise]

Hello,

I'm new here and I'm not sure if this post belongs here or in a physics forum, but here goes.

I'm working on a project to power a 6W (bulb input power) UVC bulb from a low voltage DC battery source (4.5 VDC). This of course requires a high efficiency ballast between the DC supply and the 6W UVC tube itself. Commercially available bulbs for producing UVC (254 nm wavelength) are produced as germicidal fluorescent tubes e.g. a G6T5 bulb. Germicidal bulbs are designed using using clear quartz glass tubes that have below atmospheric pressure. They use a small amount of mercury to produce the UVC wavelength.

I am running into ballast difficulty that seems to have something to do with these germicidal bulbs which are different in that they use clear quartz glass with no phosphor coating as in other UVA and UVB bulbs, and perhaps the lower pressure used in germicidal bulbs. DC inverter ballast suppliers and others have stated that their DC inverter ballasts designed for equivalent UVA and UVB bulbs will not reliably work for germicidal bulbs.

The issue is somewhat complicated by the fact that these ballasts do NOT supply filament current and instead operate such (hot cathode) bulbs in instant start cold cathode mode. My question is simply ... why do such ballasts work with UVA (i.e. so called 'black lights') and UVB bulbs, but not with the UVC germicidal bulbs.

It has to be something to do with the lower pressure used with germicidal bulbs together with their clear quartz glass construction as these are the only differences. Might it have something to do with the reduced opacity of the quartz glass allowing internal discharge heat to escape making the bulb run cooler compared with opaque UVA and UVB bulbs?

I'm at a loss for an explanation, understanding and some kind of circuit design solution.

Any ideas? Thanks in advance.

 

[NascentOxygen]

Hi dennise. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Who is the manufacturer of your bulb? Can you provide a link to data about your bulb?

 

[dlgoff]

According to Wikipeda's Ultraviolet germicidal irradiation page, required power densities are as follows:

Low-pressure UV lamps
These offer high efficiencies (approx 35% UVC) but lower power, typically 1 W/cm power density (power per unit of arc length).
Amalgam UV lamps
A high-power version of low-pressure lamps. They operate at higher temperatures and have a lifetime of up to 16,000 hours. Their efficiency is slightly lower than that of traditional low-pressure lamps (approx 33% UVC output) and power density is approx 2–3 W/cm.
Medium-pressure UV
These lamps have a broad and pronounced peak-line spectrum and a high radiation output but lower UVC efficiency of 10% or less. Typical power density is 30 W/cm³ or greater.

bolding by me

But what you are probably having trouble with, is your ballast doesn't have the oomph (Surge Voltage Required to fire the gasses in a new UVC tube.

... an electronic ballast may have the correct normal operating voltage, but cannot correctly pre-heat a hot cathode UV Bulb

http://www.americanaquariumproducts.com/uvbulbtroubleshooting.html

 

[dennise]

Hi dennise. http://img96.imageshack.us/img96/5725/red5e5etimes5e5e45e5e25.gif

Who is the manufacturer of your bulb? Can you provide a link to data about your bulb?

There are several manufacturers of the G6T5 germicidal bulb, but their specs are pretty skimpy e.g. I have not seen a spec for 'strike' (aka 'arc') voltage or filament current.

Here is a spec for Eiko's G6T5 bulb: https://www.divinelighting.com/eiko-g6t5-6w-germicidal-uvc-t5-g5-base-ozoneless-lamp-bulb-p-29477.html?zenid=rlqcgdlcr3ppvhk8m7aa2767l6

Click on "larger image" and you can see the clear glass wall (quartz I'm certain), together with the 2-filament pins on each end. In most low voltage DC inverter ballasts I have seen, the pins on each end are shorted together and the bulb is operated in cold cathode mode. This is done to extend battery life in portable hand held applications by eliminating filament current. This of course requires a greater strike voltage, but the bulb length is only about 9".

 

[dlgoff]

In most low voltage DC inverter ballasts I have seen, the pins on each end are shorted together and the bulb is operated in cold cathode mode. This is done to extend battery life in portable hand held applications by eliminating filament current. This of course requires a greater strike voltage, but the bulb length is only about 9".

Since this tube is only 9" long, this should be noted:

The increased fall voltage results in more power dissipation at tube ends, which does not contribute to light output. However, this is less significant with longer tubes.

Cold cathode lamps.

NascentOxygen asked you about the tube specs but I would like to know the specs of the ballast you're using.

 

[dennise]

Since this tube is only 9" long, this should be noted:




Cold cathode lamps.

NascentOxygen asked you about the tube specs but I would like to know the specs of the ballast you're using.

Ballast specs show little more than just bulb compatibility. Here is one ballast supplier that retracted compatibility with the G6T5 germicidal bulb and now only shows standard fluorescent bulb compatibility i.e. their 2D12-1-9 ballast for the regular F6T5 bulb. http://www.iotaengineering.com/emlib/dcguide.pdf

Again, what is the unique ballast difficulty associated with driving low pressure clear quartz glass germicidal UVC bulbs?

 

[Drayton Rorah]

Hi, Dennis
Was a solution to your ballast problem ever solved? I too am trying to power a g6t5 bulb with a dc ballast and I cannot find one that is compatible.

 

[Tom.G]

Try contacting the bulb manufacturer and asking where you can get a DC ballast to drive it, or what the specs are so you can build one.
Fortunately there are many mfg's that make equivalent lamps. One of them is bound to let something slip.