I'm planning an installation of twelve 12VDC deck post cap lights, each drawing 1.6 watts. The manufacturer says the lights must be driven by a DC supply. They offer a system that uses a 50 watt supply with proprietary patch cords and splitters to connect the lights. I prefer to use less expensive standard 12/2 wire and a standard low voltage landscape transformer. What I'm finding is that most "standard low voltage transformers" output AC, not DC. Those that do output 12VDC have no built-in timers, dimmers or photocell functions. They're just power transformers that are on 24/7. What I want to do is install the power supply in the basement and turn on the lights at dust and turn them off 6 or 8 hours later. Dusk-to-dawn would be a reasonable alternative. My question is, what would a professional lighting installer use? I suppose one could use an external timer and photocell or convert 12VAC to 12VDC after the transformer but surely there's a proper power supply for 12V DC landscape lights.
Lighting – DC Landscape Lighting Transformer
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Related Solutions
You can buy LED fixtures off the shelf, which are basically ready to go. I've seen these as puck-shaped lights that are basically designed for under-cabinet or shelf lights.
There are other pretty simple off-the-shelf solutions as well, such as using rope light.
If you are talking about using raw LEDs and literally doing everything yourself, it's pretty simple. You need to get some high output LEDs (pay attention to color temperature, beam angle, etc to make sure it is suitable). You can get LEDs in different packaging: clear/diffused, large/small, forward/side-emitting. Likely your best bet will be a large frosted one. How many you need will just depend on how much area you're covering, and how bright you want it.
You'll need a DC power supply. The exact values are all dependent on what you use, so it's very hard to give an answer here. There are online LED array calculators available, which can give you something like this:
One of the major disadvantages of serial wiring (shown here) is that if one LED is disconnected or burns out, they all go out. Finding which one is broken can be difficult, and if you've ever done this with x-mas lights you'll understand what I mean.
Another possible disadvantage of the DIY solution is that you are either going to have to come up with a nice way to package this all underneath, or you'll have a bunch of wires and bare LEDs hanging under your cabinets (maybe hidden by some moulding, but still). This may turn off future potential buyers. Off the top of my head, using some split-loom tubing may be a way to hide the wires and make it look at least half-decent.
The halogen bulbs are 12 volts at 20 watts. Almost certainly the "High" setting is 12 VAC, and the "Low" setting is the same 12 VAC but through a half wave rectifier (just a diode). So, if you want to use the current switch and processor board, you'd need something that could be driven by 12VAC, both with and without a half wave rectifier.
The problem is that half-wave-rectified AC is a lot different from pure AC; it has a large DC component to it. Some LED drivers could handle it, others couldn't. I doubt you'll find a manufacturer that will sell their product as compatible with this, as it's too rare an application for them to worry about. So, you can try various products, but you may find they won't work, and you may even find that the "Low" setting damages the bulb/circuit.
An alternative would be to install your own switch, but getting it to look good would be tough.
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Best Answer
I'm not sure why you're going with DC. I've put in many systems with 12 volt AC transformers with built in timers and or photo cells. The lights I've used are all LED or incandescent and they run on AC. I've got 15 fixtures in my backyard using LED bulbs hooked up to a 12 volt AC transformer. The bulbs will take AC or DC so I went with the AC. Check with some different manufacturers or do some checking at your home stores or lighting stores. They have plenty of fixtures, bulbs, etc that run on AC.