I took a 4' shoplight, used a Dremel tool and cut it in half to make a 24" shoplight, and rewired the ballast to overdrive the lamps. It works great. I learned how to do it from threads here ... look down about 20 topics to the one about overdriving WallyWorld shoplights.

I don't know anything about terrariums, but maybe a CFL with a good reflector would get light to the bottom of the terrarium better.

Here is a link that might be useful: CFL kits

Compacts are my second choice, partially because of cost, and partially because I want to use Gro-Lux tubes, which are only available at T-12.

The information isn't always presented very clearly but all you need to get a fluorescent light working is an electronic ballast and some wires going to each end of a fluorescent tube. Standard endcaps are generally used since they clip over the end of the tube and make it easy to connect the wires. Depending on yourenvironment you should also consider some form of reflector or enclosure to get the majority of the light onto the plants.

When considering whether a given ballast can be used for a given tube, remember that they are current control devices. They provide a constant current over a certain range of voltages. T8 ballasts (in the USA) are designed to provide less current than a T12 ballast so they will normally underdrive a T12 tube even if they are designed for a higher power. You can, if you like, double up T8 ballasts to overdrive a T12 tube, although it will be less overdriven than with two T12 ballasts. Always remember that these approximations only apply within the design range of the ballast, outside that range it may not work at all, may work for a short period before it blows, or may overdrive or underdrive depending on whether you are asking for too much or too little power.

So, to drive a T12 tube normally, you need a T12 ballast. A 40W T12 ballast will typically drive a 20W T12 tube normally. If you look on the ballast case, it will usually tell you all the tube powers that it is designed to operate.

For reasons of cost and convenience, I would suggest you look at compact fluorescents whenever you have a length shorter than 48". The short tubes and low-power ballasts cost as much or more as the longer higher power ones and tend to be less efficient. You may want to look at buying your own ballast and wiring it since prebuilt compact fluorescents tend to be expensive.

A second recommendation is to get yourself an electronic quad T8 ballast and wire it to four 36" 25W tubes. Should be enough light for you. Or if you can fit in six of the thinner tubes, even better.

Some thoughts on overdriving. I think overdriving T12s is a fools game unless there is absolutely no other way to get the required light intensity, you're just wasting money. Try T8 tubes first, you can get more in the same space. Then try T8 tubes running from a T12 ballast, which will be somewhat overdriven. No problems with reduced life here since T8s are designed to run properly on T12 current, in fact they do in Europe and they sell the same tubes in the states. Then look at T5s, thinner again but fittings are expensive unless you build your own. Don't bother overdriving most T5s either, the high output ones you should be using are already overdriven to the degree that makes sense.

I guess it comes down to quality or quantity. The discussions I've read in this forum have led me to believe that Gro-Lux+Chroma 50 or 75 is just about the best I can do in terms of light quality. On the other hand, T5 or compact fluorescents would be easier and cheaper in producing more light in less space, but the wavelengths that the plants use will be a lower percentage of the light produced.

If I go the compact or T5 route, which bulbs should I use to maximize the spectrum useful to plants?

which bulbs should I use to maximize the spectrum useful to plants?

If you ask three different people that question, you'll get at least four different opinions ;)

Fair enough. :)

Having thought furthur and read a little more,
I guess T5 or CF is a better bet. I only have a 24"x18" space to work with, and the more I think about it, the more I want to make sure that I have reflector space.

Having made that decision, I have a couple of questions regarding those formats:

1) Do manufacturer use the same phosphors for thier T5 and CF bulbs? ie, If I'm happy with the Philips Cool White and Warm White in T12, should I expect the same spectrum from Philip's T5 warm white and cool white bulbs?

2) In Zink's original post, he mentions using the Sunpark ballast to power T5 HO bulbs. Is anyone doing this? Is it safe?

3) Whats the lumen maintenance like on T5 and CF? I assume that it's better than the 6 months that is generally recommended for T8 and T12...

1. Almost all T5 bulbs and almost all compact fluorescents use the standard triphosphors, so they produce mostly light at the same red, green, and blue wavelengths. The relative amounts of red and blue light vary to produce a more orange light, warm white, or a more blue light, cool white. Even more blue and less red gives you the 6500K daylight colour too. Many cheap T8s and a lot of T12s still run halophosphates and they produce a radically different spectrum. You can identify them from coding such as 540 which is a halophosphate 4100K, as against 840 which is a triphosphor 4100K.

2. I can't find the post you refer to, but doubling up the Sunpark ballast on T5 tubes will overdrive them to some extent. Overdiving T5s is not necessarily the most productive way to go. A T5HO tube is already effectively overdriven by the standard ballast and they are designed to operate efficiently at warmer temperatures than other tubes. The downside of this is that overdriving them further produces more dramatic loss of efficiency and lifetime, and higher temperatures, than with T12s and especially US T8s. Using electricity always involves some element of risk, so don't overdrive unless you are comfortable that you are able to safely strip and connect wires, and know what the earth is for. Incidentally, you can overdrive a compact fluorescent if it has a separate ballast, but the same problems apply and to a greater extent, the compact shape and thin tubes mean they are already pretty much maxed out and overdiving them is asking for trouble.

3. Lumen maintenance on T5s is universally good or very good. Expect 90%-95% of initial lumens all the way to the end of life. Compact fluorescents are not so good. Results vary, partly depending on the ballast start type. Cheap small domestic bulbs may lose 40%, better ones only 20%. The high power versions which are effectively a single T4/5 tube bent in half or in four, perform closer to 90%.

Thanks for the info. I think I'm going to try to use 24W T5HO bulbs with the Sunpark ballasts. How would you recommend I wire things up? Since the bulbs are 24W, and the ballast is rated for 25W T8s, should I simply use 2 bulbs per ballast?