Q & A: germicidal bulbs

Hello, I am doing a project that involves purifying water using germicidal Ultraviolet radiation. The problem I am having is that I cannot get my UV light to run. The setup of the project is so, a 3W 10.5 Plusrite 5003 - Germicidal Lamp from 1000bulbs.com, is connected into a Satco 92-322 Medium to Intermediate Reducer Socket (also from 1000bulbs), this screws into a regular flick switch socket from Home Depot, to which I connected wires. The first time I tried to use the lamp, I connected it to a solar panel rated at 2V 500mA, that I got in a Horizon FuelCell Technologies kit. Unfortunately, nothing happened. I try flicking the switch, and changing how I connected the wires to the panel. Looking back the reason it did not work was probably because the voltage was not high enough. I then tried to run the bulb off a 9V battery; I tried connecting the battery to the socket and directly to the bulb. After that still did not work, my teacher suggested I try using the bulb in a desk lamp; the bulb worked for about a split-second before it died, it gave off an extremely bright blue-white light, and when I removed the bulb, it was a smoky grey on the inside, and gave off a peculiar smell. While screwing the bulb into a desk lamp probably was not the best idea, it did confirm that the bulb was properly functioning. But I have very few ideas about why the bulb would run with the solar panel or the battery. One thought was that the bulb would not run on the battery because the bulb was AC while the battery was DC, unfortunately, I do not know what kind of current the bulb had because the information was not provided. Do you have any thoughts on why the setup was not working, or what I did wrong? I am sorry if I have been unspecific or unclear.
- Emily Hagge (age 17)
NJ

You're very clear and specific, frighteningly so. Your teacher told you to screw a 10.5V mercury arc bulb into a 120V socket! You're lucky that it didn't explode, releasing mercury vapor. Don't ever, ever, do that again and tell your teacher never to do anything that stupid again.

These sorts of bulbs won't work except on the proper voltage. They aren't like incandescent bulbs which just get dim and orange-ish if they get slightly too little voltage. There's no chance a 10.5 V arc bulb could work at 2 V and little chance it would work at 9 V. 

You might start preparing for this project by reading this nice article on germicidal bulbs: http://en.wikipedia.org/wiki/Germicidal_lamp. It describes the principles of operation and some of the major safety issues.

Your guess that using dc voltage is a problem is correct. Although these mercury arc bulbs can run on dc, there are significant problems both in maintaining a steady current and in avoiding damage to the bulb. You can read a discussion on this site:http://en.wikipedia.org/wiki/Fluorescent_lamp#Electrical_aspects_of_operation. Although it is about fluorescent bulbs, they're just modified mercury arc bulbs with very similar electrical properties.

As far as the electrical operation goes, you probably would want to get one of the bulbs (about the same price as the one that was destroyed) that work in the same plug-in fixtures as some small fluorescent lamps. Getting a fluorescent lamp and swapping bulbs would then take care of the electrical issues. However, there major safety concerns with the UV light itself. Risks include blindness and melanoma, a deadly skin cancer. I wouldn't trust that particular teacher to guide you through those risks.

Mike W.

(published on 02/15/2012)