Yes, with the right ballast, T5HO's can be dimmed! Even if you're not interested in that, there's some general construction techniques here that may still interest you.
Off we go...
Here it is on my 46G bowfront, at 100% high light. I usually run it at 50%-85%. It's dimmable to any light level, from lowest to highest useful levels in a planted tank, so there's no need to raise it to adjust light. Normally it rests directly on top, wasting no light or electricity to light the room instead of the tank. But it is still useful to raise it for maintenance. I'm using Sunlift spring tensioners. Takes literally a second to raise or lower it, you simply push it up or down and it stays put. Light is still pointed directly into the tank so you can see what you're doing. And I just had to have it after seeing something similar in the old movie, "The Incredible Mr. Limpet".
The front and top. This is the first time I've ever attempted to get a nice, glossy wood finish; and was actually the most scary and difficult part of the whole project. I utterly botched my first attempt. I found that my sander had somehow left gouges in the wood which I didn't notice until I applied the stain, which made it look like someone had taken a black crayon and scribbled all over it. Frustrated me so badly I put it on a shelf for months. I eventually came back and got it right. Sealed with 16 coats or so (I lost count) of spar polyurethane, converted to wipe-on with 50% mineral spirits.
Oh, and there's an exhaust fan there too, as well as the hanging mounts.
The back. I use a standard computer AC power connection. Cutouts for the connector and fan were started with a drill, and finished with a scroll saw.
The side. Slots for ventilation intake were started with a drill, and extended with a router, each using an identical diameter bit; you can see at the right edge where I didn't set the router guide up quite right. The insides of the slots were painted with black acrylic paint prior to sanding, which helps block stray light and meant I didn't have to worry about extending the stain inside.
Notice also that the sides extend down. They straddle the outer edges of the rim in it's normal resting position, so the hood can slide forwards and backwards; but not sideways, which would allow the hood to accidentally fall in.
Now that you've seen all the outer surfaces, you may be asking "where are the controls?" There are none
. It's designed to be centrally computer controlled via X10 home automation, like all my tanks. Should the computer fail, I can still manually control it and other tanks from a wired remote plugged into an AC outlet, which currently resides underneath this tank's stand. X10 is a whole topic by itself, and I'm not going into much detail here. This hood could easily have been designed with traditional manual controls, with the same dimming ballast.
The underside, at an angle. It's not as pretty from the bottom, nor does it have to be; since it rests on the tank, this is normally not seen.
Holes are drilled into the wood to accept screws mounting the acrylic splash shield. Since these screws will be removed and replaced a few times in this hood's lifetime, I put a plastic screw anchor in each one. More resilient to stripping, and if they ever wear out, they can be replaced with no further wear on the wood.
Where the screws go straight into the acrylic, none of holes/screws are countersunk. That could cause cracking if the screw were overtightened.
On the sides, I used 1"x1/8" aluminum strips on top of the acrylic, with countersunk holes/screws. This provides a flat surface for the hood to rest on the rim; and elevates it just enough so the other, non-countersunk screws can't dig into the rim if you go nuts sliding it around.
I thought I had an adequately sized scrap acrylic panel that would span the entire width, but I didn't. And I couldn't justify buying another whole 4'x8' panel for such a small but long piece. So I went with a split splash shield, in two halves; since again it doesn't matter how it looks, just how it functions. And it functions fine.
You might notice the ruler. It's not for size reference, it's another ugly functional piece.
Looking inside, with the splash shield removed. 2x 39W DD Giesemanns, the much-lauded Midday/Aquaflora combo. The Aquaflora is towards the front, which makes reds pop more since that's the light most reflected to the viewer. And Icecap reflectors. They're clipped onto the bulbs, and can be rotated to tweak light distribution.
With bulbs and reflectors removed, the guts are fully exposed.
I wanted color-coded wiring, but didn't want to buy spools of all the different colors. So I just used black, and color-coded the ends using colored heat shrink. Where I didn't have a particular color in my assortment, I used white, then painted it with a colored permanent marker.
AC comes in at the middle bottom, and goes to the left terminal block, which is reserved for AC connections only. From there, it goes several places.
First, to a 12VDC switching adapter, the black box to the right of the fan, which is securely mounted to the case simply with heavy-duty self adhesive velcro. The output goes to the right terminal block, reserved for DC only; kept separate from AC to reduce the possibility of a damaging or dangerous mishap while adjusting things. 12VDC goes to the fan through a resistor mounted directly to and just below the terminal block, which reduces unnecessary fan RPM and noise. And it goes to the LED moonlighting, which consists of blue T1-3/4 LEDs superglued to the previously aforementioned ruler, each with its own limiting resistor.
AC also goes to an extension cord, or at least the female part of one, at the left side. Both X10 modules needed to control the lights are plugged into that extension, and are heavy-duty velcro'ed to the case.
Now to the heart of it, the dimming ballast at the top. It's a Lutron ECO-T5H39-120-2, made specifically to drive and dim 2x 39W T5HO's. Dimming ballasts are rather expensive through traditional retail channels. But they frequently come up on Ebay for much less, since there is little demand. I paid $25 for this one. And I've bought three others, of a different type, for even less. No one has bid against me on any of them.
When selecting a dimming ballast, you have to pay attention to the control scheme. There are three standards, and you may have to look at the ballast's spec sheet to see which it uses.
"Digital" requires a specialized controller, and unless you can also lay hands on the controller cheap and know how to interface it, I don't recommend these.
"Control voltage" uses a DC voltage, from 0-10V, to control dimming. The 10V is supplied by the ballast itself, all you have to do is connect a potentiometer. These are great for DIY. But, since I wanted to control via X10 modules, which are AC, I went with the third type.
"Three wire" uses two wires as the AC power supply, and a third wire is connected to a specialized dimming controller which is similar, but not identical to, a standard light dimmer. Both produce identical AC signals (phase control dimming for the geeks). The problem arises in that unlike a light bulb, the ballast consumes no actual power from the dimmer. The specialized dimmer can handle that, a standard dimmer will not. Fortunately, it's a simple matter to make a standard dimmer work, by also attaching a small load that draws enough power.
I found that it only required 1.2W of load to make my X10 lamp module, which works just like a dimmer apart from the computer control, function normally with the ballast over nearly the entire dimming range; excluding a few percent on the low end which wasn't useful anyway. Standard dimmers from the hardware store will work too, though the minimum required load may vary according to model. I put together a 12K 2W equivalent resistor from parts available at Radio Shack, seen attached below the AC terminal block. If you're uncomfortable dealing with resistors, an incandescent nightlight bulb would probably work fine for any dimmer.
This particular ballast dims from 10% to 100%. It cannot be dimmed to zero, or turned off, via the third wire. That meant I had to add a second X10 module, an appliance module, to turn the ballast on and off by cutting power to it. A plain old AC switch would work the same for manual control.
Here you can see a little detail of my ugly yardstick moonlight.
But that's not what this picture is for. Underneath it, you can see I've mounted the ballast on a wooden standoff. Had this ballast been mounted on metal, flush mounting would have been preferred, as the metal would act as a heat sink. But wood doesn't conduct heat well, and it's better the back be air-cooled. In retrospect, this probably wasn't necessary. The ballast is incredibly efficient and generates little heat. But better safe than sorry.
That's right. The bulb sockets aren't screwed in. They too are mounted with self-adhesive velcro! And unlike the other components, this isn't even heavy-duty velcro, just the standard self-adhesive stuff.
Worried about it detaching unexpectedly? Don't be. That little wooden strip glued on the inside of the case is just thick enough, that between it, the bulb length, and the socket thickness; when the bulb is inserted, it exerts a bit of pressure on the velcro. So the only way it could come loose is through sliding, and velcro doesn't allow that. Remove the bulb, and the sockets can be easily detached and moved, allowing further fine tuning of light distribution. The "ruler of moonlight" can similarly be moved, removed, or more added without any drilling. By design, there's just enough room for a third T5HO and Icecap reflector; but I'll never need it.
The fixture has now been operating flawlessly since January of this year.
I think that about covers it, other than one thing.
Why didn't I go with LEDs? If at some point down the road I decide I want to change spectrum for different aesthetics, or to experiment with effects on plant growth, I wanted it to be easy. I originally intended this to be a LED fixture, where I could dial in a reasonable spectrum variation the same as dialing in the intensity. But the design progressed, it quickly got expensive and complicated. And there were still issues I wasn't sure how to solve. Multi-colored shadows. Spotlighting near the top when the hood sits directly on the tank, where I specifically wanted the hood. Excessive ripple effect, which I didn't really want at all in this tank.
In the end, I decided it was easier to make a dimmable T5HO, and change bulbs for different spectrums; than to make a color-adjustable LED fixture. Plus, as far as I know, no one else has a dimmable T5HO fixture. That alone makes it worth doing.
Questions? Comments? Can you see my underwear in the reflection from the Icecaps?
I hope not, but fire away.