120vac to dual 12vdc/5vdc rectifier design? (low-current)

[CCrowley]

First up, I'm fairly new to this aspect of things. Normally I'm the software designer, technician, etc, not the circuit designer. I'm quite eager to learn, and have done a good deal of reading and researching thus far. On the other hand, I have a healthy (fearful) respect for electricity, so I'm here to make sure I do it right. A cursory search didn't reveal anything here to me that I could be certain would work for my application.

Now, as I mentioned in the subject line, I'm looking to build a small rectifier for 120vac to both 12vdc and 5vdc. Whether that's one circuit or two that happen to be in the same enclosure is something I've yet to determine. I'm going for efficiency here, primarily for the sake of keeping heat generation as low as possible.

The goal:
Assuming this is feasible, the rectifier(s), microprocessor, etc listed below would all be contained within a single-width lightswitch receptacle. This particular receptacle is in an outside wall of a home, exchanging it with a larger receptacle isn't an option, and it is surrounded by blown fiberglass insulation. Heat control is key here. The space normally taken up by the control unit of a more complex light/dimmer switch is available here, as the dimmer switch being installed in place of it will be a simple trimmer potentiometer.

The 12vdc line would be used to power a yet-undetermined number of "rails" for lighting LEDs (1-7 rails total). These would originate in the receptacle, presumably attached via transistor to PWM pins on the microcontroller. At present these wires do not exist, do not have any specifications, and can be adjusted to fit any needs that may come up. One more "rail" would be present as a ground. This is in effect an indirect-exposure track lighting system for LEDs, with different wavelength LEDs available on separate tracks for independent control. The number of LEDs and exact power requirements of each are not yet known, I'd assume standard 20mA LEDs though.

The 5vdc line would be used to power a microprocessor to control the device. The microprocessor in question is a Teensy 2.0, which can technically function between ~2.4v-5.5v, but 5v would be optimum for CPU speed and general functionality. Under no circumstances would I expect this to draw more than 30mA, but I'd leave some room for error regardless. This is the biggest part that leaves me uncertain. I'm not sure exactly -how- stable the current has to be, how continuous, etc. If left to my own devices and research, I've proven to myself I'd overdesign this part, resulting in higher cost, danger to myself, and potential frustration. In the worst case, I'd just pick up a standard 120vac-5vdc USB charger, strip it down, and use it directly, but I don't learn anything from doing that, and I have a feeling there's a better option. By all means, correct me if I'm wrong here.

If I've left any pertinent detail out (if it's a known detail anyway), feel free to ask. There's no timeline I need to conform to here, no due date to speak of. Also, as it has come up before, I do have access to the circuit breaker and can run this on an isolated circuit initially (but only initially, not in final deployment) if needed as well.

As a final note, while I'd like to think it wouldn't be an issue here, if you're someone who is not willing to be patient with a novice of sorts, don't worry about responding. There are many other forums and mediums where I could listen to people instructing me to hire a professional or stay within my field.

Thanks in advance, everyone.

 

[MikeHalloran]

Why 12 volts?

Mike Halloran
Pembroke Pines, FL, USA

 

[IRstuff]

First of all, you want to run it through a transformer so that you're only dealing with something like 16VAC. There's no need to rectify 120VAC directly. So, rectify the 16VAC, add filter capacitor, and add a couple of switching regulators, like:

http://para.maxim-ic.com/en/search.mvp?fam=stepdn&270=Internal Pwr. FETs&tree=powersupplies

On the parametric selection, you can select for both 12V and 5V, like:

http://www.maxim-ic.com/datasheet/index.mvp/id/3991

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[CCrowley]

12vdc is what I've been pointed at continuously for LED power sources. I'd also heard some reference to 12vdc being easier to obtain than most alternatives (5v and 3.3v especially), whether that's accurate or not I'm not certain. I'd looked at an SMPS before, but from the looks of it, even if it'd do what I want, that's a bit more fire than I'm qualified to play with.

 

[IRstuff]

I think you've got it backwards. The chip regulators are basically by-the-numbers design. If you follow their reference designs, you should get working hardware. Rolling your own regulator, even a linear one, without the design experience can be a recipe for disaster.

TTFN

FAQ731-376
Chinese prisoner wins Nobel Peace Prize

 

[Skogsgurra]

It sounds like you are doing this as a learning project more than a circit design that needs to be optimized for cost and performance. Then, it will be difficult to get the "right" aswer from us practizing EE:s. We tend to use cost-efficient methods and material and that means switchers, dedicated IC:s and supplier's design support. As IR says.

If this is a design that you or your company will be selling, then it is even more important that you benefit from the experience and technology offered by National Semiconductor, Linear Technology, Maxim and other companies. The "fire" isn't that big. If you light a fire on your own, it may quickly get out of your hands.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[LionelHutz]

Your "specs" are confusing. This goes in a single gang box on the outside wall of a house. You then want to connect up to 7 lighting strings/cables to it with seemingly seperate control available for each. So, you should first be asking if you can make this thing with the necessary plugs or terminals plus the necessary control interface assuming it needs to be highly water resistant on the exposed front face.

Next, you should figure out what kind of LED light array you need to make useful landscaping lighting or outside "mood" lighting. There is no point designing a controller that can handle a 10 LED light array and then finding out that 10 of those LED's are basically useless due to producing so little light.

I expect you will want to use some higher voltage level under 50VDC and a fairly simple unregulated or poorly regulated supply. Then, use the LED's in series or series/parallel to allow the higher voltage and use the microcontroller to regulate the current to the LED's via PWM.

Overall, it's very wrong to be designing a power supply for something that doesn't exist. You can't build the power supply until you know what power requirements these LED strings will have. Typically, you come up with the circuit you want and then design a power supply for it. Sometimes, you will prototype a circuit with a lab power supply and then build the production supply later.

 

[VEBill]

within a single-width lightswitch receptacle

The space requirement is nearly impossible extremely challenging. The seven additional connectors is going to be an issue. Also, there are Code issues with AC and low voltage sharing a box.

The chassis size will have to be the same as a 'device', not the size of the box itself. There are a multitude of solid state gadgets that fit into electrical boxes like a switch. Many of these gadgets have microprocessors and so on. You could probably learn a lot by purchasing several examples and opening them up for your analysis.

 

[IRstuff]

"Code issues with AC and low voltage "

Not sure what they might be. There are roughly a dozen manufacturers of light and appliance switches that contain low voltage electronics, X10, Insteon/SmartHome, Intermatic, Wayne Davis, Leviton, etc., to name a few.

TTFN
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[VEBill]

...AC and low voltage WIRING sharing a box.

Corrected -^

Of course the gadget itself can and does contain low voltages sealed within it. But based only on the examples I've seen, there seems to be a trend of keeping the low voltage wiring out of the AC box.

(It's an assumption on my part that it's in the Code.)

 

[IRstuff]

OK, thanks for the clarification

TTFN
faq731-376

 

[CCrowley]

Fair enough, Skog. As you said, yes, this is more a learning project than anything else, though it will be going into use in a couple places eventually. Either way, between the lot of you, you've given me enough info to go on for now, a direction to aim, and most importantly a clearer idea of what might be a bad idea. For now I'm going to hit the books a bit more, check out some of the power supplies and such available, and see where this takes me.

I just want to say now, as a first experience here, clear, honest, concise answers as you've given are fairly rare in most mediums. I appreciate it greatly. I'll be back to ask more when the time comes that I need it. Thanks everyone.