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Light behavior (now I'm really confused)

Posted by AlanRoberts z7b NC (My Page) on
Fri, Feb 11, 05 at 8:25

In a previous post, I asked about substituting a hard-to-find tube in a light fixture. One of the replies suggested trying a T8.

Last night I purchased the closest T8 I could find along the drive home, a F17T8 soft white. Put the tube in and switched the light on, and the tube lit at what seemed full brightness. Let it run for a bit and didn't notice any undue heating of the tube or the lamp, so I put the plastic diffuser cover back onto the lamp.

After a few seconds, the tube flickered badly a few times and then dropped out of operation. The lamp, which normally makes a little humming/growling noise on startup and then is fairly quiet, switched back to the louder noise level, and the ends of the tube were glowing pinkish, almost like it was trying to preheat or something.

Switched it off, took the cover off, and repeated the experiment. It is repeatable ... Tube will start and seem happy with cover off, and then drop out of operation when the plastic cover goes on. Can anyone fill me in on whats going on? I'm beginning to wonder if whatever serves as the ballast/electronics on this lamp are failing or something.


Follow-Up Postings:

RE: Light behavior (now I'm really confused)

Sounds like it doesn't like being warm. Cheap ballasts "growl" when they are switched on, and I'd say yours is caput.

RE: Light behavior (now I'm really confused)

  • Posted by zink 6a (My Page) on
    Fri, Feb 11, 05 at 11:33

I don't know what part the plastic cover plays in this drama, but I am 98% sure that your ballast is not truly compatible with the T8 lamp.

I have seen the specs for the lamp/fixture at the LofA website, and I believe it has a magnetic ballast, designed for T10/T12 lamps. Although there are magnetic ballasts which can drive a T8 bulb, the T8's are primarily designed to use the newer electronic ballasts.

You will have to use a T10 or T12 bulb. The T8 will not work reliably.


RE: Light behavior (now I'm really confused)

Well, if you're really attached to that fixture, you could buy a new ballast or two to put in it.

One of the modern electronic ballasts like the SL-15. Ask Zink to be sure, but I think it'll work.

RE: Light behavior (now I'm really confused)

Shrubs ... I'm coming around to the bad ballast theory, even with a F20T12 tube in the unit, I'm getting more noise out of that fixture than from the other seven.

Zink ... I'm certainly getting the impression that it really wasn't driving the T8 correctly. In fact, I think something harmful has already happened to the tube just from experimenting. There already seems to be a "burnt spot" in the coating near one end (it isn't actually blackened yet, just a serious discontinuity in the color/brightness) which I don't remember being there earlier.

Jkirk ... I'm not attached to that particular fixture, but I'm footprint-limited on my seed-starting space, to a Home Depot style shelf unit that is 36" x 24" exterior and has four plastic tube corner posts, giving me an "interior" free space of 32" x 20". Thus the 4' shoplight solution didn't fit, so I did what I could with multiple "under cabinet" style fixtures.

After the fact I realized that those under cabinet units must work on the "razor blade" model ... The unit with included tube are inexpensive (although not compared to best-case shop lights), but then you get killed on replacement tubes. Seems like every 2' tube costs more than a 2-pack of 4' tubes, at least at my local Lowes & Home Depot (and neither currenly has any F20T10s :-(

I thought about hacking the existing fixture and switching to an electronic, T8-compatible ballast, but it is all glued/welded plastic. Perhaps with some careful Dremel work I can open it up without destroying any of the important bits.

Is it possible to just buy the tube end-mounts and "roll your own" fixture? I saw ballasts at the HD, but didn't see the tube mounts (was a quick trip for the test T8 tube, wasn't looking carefully). If that is possible, how far away from the tubes can the ballasts be? I think I could drop the weight of my light assemblies quite a bit if they were just end-mounts on a frame, reflector and tubes, with the ballast for each shelf's worth of lights mounted on the back edge of the shelf.

Jkirk's suggestion in my other post of cutting down a 4' light sort of has me pondering whether I could build from scratch to fit my shelf with less effort/wastage than remodelling an existing light.

Of course, that still leaves my in the 2' tube cost swamp. Should I be considering some other strategy (bent tubes, circles) to fit my limited space and drop my tube cost?

Thanks for all the help folks ... I can't imagine how much time it would take me to research my way through the knowledge you already have!

RE: Light behavior (now I'm really confused)

  • Posted by GaWd z9/10 NorCal (My Page) on
    Sat, Feb 12, 05 at 23:06

Unfortunately it sounds as if the T8s aren't compatible with your ballasts. It was worth a try though!


RE: Light behavior (now I'm really confused)


Want a good Dremel project? I have cut several 4ft shoplights down to 2ft lights, using Home Depot's Commercial Electric Shoplight #140-904, with the Sunpark SL-15 electronic ballast. You can then use a couple of tricks to get a great little light.

TRICK #1... You can take TWO 17wT8 lamps, wire them in series (end=end), and substitute them for a SINGLE 32wT8 lamp. Note - where the TWO 17w lamps connect end-to-end, wire both pins from one lamp to both pins on the lamp. [17w]=[17w]. This gives better current conductivity at the junction.

TRICK #2... You can take a 2-lamp shoplight ballast and wire it to overdrive a SINGLE 32w lamp. Then, you can substitute the TWO 17w lamps in series, and OVERDRIVE both of the 17w lamps, instead.

Now that you know this, you can shorten the 48" fixture to a 24" fixture, by cutting down both the reflector and the housing with your Dremel. When I cut the metal housing, I always cut it so as to leave some metal flaps intact that I can bend over, like a cardboard box, and re-create that end of the fixture. This means I couldnt use the leftovers for another 24" fixture, but since Ive only got one ballast anyhow, I dont mind.

Cut the reflector to 24", and cut out the rectangular notches to accommodate the sockets that youll slip back in there. I am now assuming you are going to overdrive here. So, the ballast will connect to the 2 sockets at one end of the fixture. At the other end, youll just wire the 2 sockets to each other, connecting BOTH pins together.

What you now have is a 24", 2-lamp fixture, with the lamps overdriven.

Has anyone else here made an OD'ed 24" 2-lamp fixture? I know someone else in this forum did it successfully.


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