Recently I needed a low power UVC source, and a quick web
search turned up the GTL3 bulb. The GTL3 looks like a standard incandescent
bulb but contains a small drop of mercury to create a mercury arc. In the picture above the mercury is the dot at the right end of the bulb. GTL3s are
cheap if you look around a bit; I bought three for about $20 including
shipping. They were made by Ushio, a Japanese bulb company.
The Ushio spec sheet shows that they operate at 10.5V and 300mA,
and produce 160mW of 254nm UVC. Their efficiency is about 5%, which is better
than most UVC CCFL bulbs, but lower than UVC fluorescent bulbs. They have an
E17 screw base, which is a somewhat unusual size between regular (E26/E27) and
candelabra (E12) bulbs. E17 sockets are hard to find, but E17 to E26 adapters
are readably available.
UVC bulbs are also called germicidal bulbs because UVC light
breaks down DNA, which is why it kills bacteria, viruses, and just about
everything else. See warning above. Most UVC bulbs are modified fluorescent
bulbs with no phosphor and a quartz envelope to pass UVC. Like other
fluorescent bulbs they operate at high voltage and require a transformer or inductive
ballast.
The GTL3 seemed like a simple alternative to fluorescent UVC bulbs. The spec sheet implies that it operates like an incandescent, just connect a 10V supply and go. Strangely, when I did a search for GTL3 wiring and circuits I found very little. So I applied 10V to one of my brand new GTL3s and… nothing happened. It drew about 100mA, well below its rated current, and there was no visible filament glow or purple mercury arc. What was going on?
One clue is that, according to the spec sheet, the GTL3 should
draw 300mA at 10V when operating normally, meaning with the mercury arc
running. My initial tests did not ignite the arc, so I was measuring the
filament current alone.
This implies that the mercury arc, not the filament, conducts
most of the current in the GTL3. Given this, you would expect the GTL3 to
behave electrically more like a low-voltage arc tube than an incandescent bulb,
and therefore that it might need a higher voltage to start.
So I added a 10 ohm resistor (for current limiting) in
series with my GTL3, and slowly turned up the voltage. At 15V the filament
began to glow slightly, and at 16V the arc ignited and the current jumped to
500mA. Lots of pretty blue mercury glow that will fry your eyes in a hurry. I
quickly turned the supply down to 14V, the current dropped to 300mA, and the
GTL3 was running just like it should.
The picture to the right shows a GTL3 with the arc turned down as low as possible. I was using a DC supply, so the arc forms around just one of the electrodes
Before the arc starts the GTL3 looks electrically like a regular
incandescent bulb with a 100 ohm cold resistance. Once the arc ignites the GTL3
has roughly a ten volt drop with a dynamic resistance of about an ohm. In other
words, it behaves like back-to-back 10V zener diodes in series with a 1 ohm
resistor.
Unfortunately, this means that the GTL3 requires a ballast to
run properly. However, because it ignites and operates at such low voltages, it’s
still much easier to use than a fluorescent UVC bulb. A 33 ohm 10W series resistor works
well as a simple ballast with a 24V AC or DC supply. The GTL3 will operate from
supplies as low as 17 or 18V, although under these conditions it’s difficult to
properly regulate the operating current with a resistive ballast, so I
recommend using an active current limiting circuit if you want to go below 24V.
Running on DC, rather than AC, will probably shorten the GTL3’s life, although
I don’t know how severe this effect will be.
Resistive ballasts become impractical at higher supply voltages
because they dissipate so much power. Capacitive ballasts are a better choice
if you’re running from AC. A 6.8uF 400V film capacitor works well for 120VAC
60Hz, and a 3.3UF 600V capacitor should be good for 220VAC 50Hz. Be sure to
include a bleeder resistor in parallel with the capacitor to discharge it when
the power is turned off. Capacitors this size can be dangerous! A 100K 1W
resistor will work for both 120VAC and 220VAC. Below is a picture of a GTL3 sterilizing my workbench. The capacitor in front is the ballast, and you can see the discharge resistor connected across the capacitor leads. There is so little filament glow the the light from the arc completely washes it out.
What can you do with a GTL3? I needed UVC to test a small
titanium dioxide photocatalytic oxidation (PCO) reactor designed to remove
ethylene from a plant growth chamber. PCO can be used to break down almost any volatile
organic compounds, so it eliminates most smells as well. Lots of info on the
web. Of course, the classic use for UVC is as a microbial sterilizer. It kills
everything! It would be fun to see what kind of effective range a GTL3 bulb has. Easy enough
to expose some agar plates at different distances and see how far away you have
to get before anything grows. Could be a simple science fair experiment…
