How to use the GTL3 Bulb: A Simple and Inexpensive UVC Source

WARNING: UVC, or short wavelength ultraviolet light, is dangerous. It causes sunburn, eye damage including cataracts, and skin cancer. Even relatively low power UVC sources, like the GTL3 bulb, can cause injury at close range. DO NOT directly expose your skin or eyes to an operating GTL3 bulb. Window glass and most plastics block UVC fairly effectively, but they may not block longer wavelength UV that the GTL3 also produces.

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…