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OT: Electronic hobbyists--wiring a florescent light
Old 02-02-2008, 08:34 AM   #1
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OT: Electronic hobbyists--wiring a florescent light

I've installed a UV-light antimicrobial unit in my drinking water system. It works great (I think--we're not sick yet), but the UV light needs to be replaced every year. The UV bulb itself is just a flourescent bulb without the phosphor coating. The replacement unit from the company costs $29 and includes a plastic endcap, wires soldered to the UV bulb, and a small incandescent "power-on" indicator bulb. The incandescent bulb is soldered between one of the 4 power leads and a cathode lead on the UV bulb.

The actual UV bulbs only cost $5 each, so I'm obviously interested in modifying this setup so I can just replace the UV lamp. (ER math: saving $24 per year at 4% withdrawal rate= this fix is worth $600 in the bank). I'm thinking of just de-soldering the leads from the UV lamp and replacing them with appropriate connectors, leaving out the little incandescent bulb.

Does anyone here know if there will be a downside to leaving out the incandescent indicator bulb? The incandescent bulb doesn't serve a function as far as I'm concerned, and it actually introduces a failure mode (since it is wired in series with the UV lamp, if the little indicator bulb burns out my UV lamp loses power and my family is drinking microbes). But, electronically is it possibly serving some function (current limiting?), or will my UV lamp work just fine without it? (The output from the ballast/transformer that feeds the unit is 44VAC, 6watts, if that makes any difference).

Thanks for any input.

samclem
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Old 02-02-2008, 09:44 AM   #2
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Yeah, the incandescent will pull down a little current but it depends on how large it is as to whether it'll affect the UV bulb. The UV bulb will burn a little bit hotter without the incandescent in the series. Not sure if that improves or reduces its abilities to kill any organisms, or would reduce the life of the bulb. My guess is if its a tiny incandescent then no; if its a big one, maybe.
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Old 02-02-2008, 10:06 AM   #3
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Couldn't you just wire in an appropriately sized resistor if you were worried about the current?
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Old 02-02-2008, 10:22 AM   #4
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Originally Posted by Gumby View Post
Couldn't you just wire in an appropriately sized resistor if you were worried about the current?
An incandescent bulb is not a linear resistance. It is a low resistance when cold, and increases in resistance when hot. They are sometimes used as current limiters.

Since this incandescent bulb is in series with the UV, it might be part of the design to provide a high in-rush current for the first fraction of a second to start the UV bulb and then limit the current once the UV 'fires'.

It's hard to say w/o more info, sometimes the wiring in series is done so you know if there is a fault or not ( which makes sense for this application ), but it could also be a current limiter for the UV bulb, if the ballast transformer isn't already providing all the current limiting.

Could you put the connectors on in such a way that you replace the UV bulb, and leave the rest of the circuit in place? I'm guessing that they are running the incandescent bulb at below it's rated current - it could have a life-span grater than your portfolio....

-ERD50
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Old 02-02-2008, 03:07 PM   #5
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Originally Posted by ERD50 View Post
It's hard to say w/o more info, sometimes the wiring in series is done so you know if there is a fault or not ( which makes sense for this application ), but it could also be a current limiter for the UV bulb, if the ballast transformer isn't already providing all the current limiting.

Could you put the connectors on in such a way that you replace the UV bulb, and leave the rest of the circuit in place? I'm guessing that they are running the incandescent bulb at below it's rated current - it could have a life-span grater than your portfolio....

-ERD50
Yep, I would know more if the little incandescent bulb had legible markings on it. It might be a bulb designed for 100 volts being run at 44 VAC and will last 100 years.

I can include the little bulb in the circuit, it just feels like I'd be deliberately including a part that serves no purpose (to me) and which increases the chance of failure.
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