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I'm starting a project to replace an incandescent lit house number sign with one lit by an LED strip. I'm using existing wires and plan to do AC to DC conversion in the basement (3.3 VDC or 24 VDC--TBD, but most likely will be 24 VDC). The total power is going to be low (3.3V for a microcontroller & phototransistor and 24V for the LED strip). My early estimates are on the order of 1 W total. During the day when the LEDs are off, the power consumption would be around 2 mW.

The existing circuit uses a 16 VAC transformer (shared with doorbell). I can use that voltage, or I can use 120 VAC. I don't feel the need to make a circuit--a module would best serve my needs. I don't know if there are any tradeoffs/advantages/disadvantages of using one input or the other. To avoid confusion: the tradeoffs I'm asking about are electrical/electronic in nature (for example, power conversion efficiency, ability to handle low currents). Can someone please enlighten me?

Thanks much!

Adirondack Jim
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  • To moderators: Would this go better in the DIY exchange? I am asking because I am not sure if this should be migrated. – user57037 Oct 14 '21 at 23:32
  • The OP is looking for a recommendation for some kind of "module" so it doesn't belong on EE.SE – Elliot Alderson Oct 15 '21 at 00:00
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    Using 3.3V instead of 24V means over 7 times the current and you'll need very thick cables to avoid losing most of that voltage on the way. I'd use the 16V AC you already have and not overcomplicate it. – Finbarr Oct 15 '21 at 00:07
  • Lower AC voltage avoids a lot of potential safety (and code) issues : if the 16V transformer is good, use it. Convert to DC with a bridge and capacitor, route that wherever you want, and step down to 3V at the point of use as Finbarr suggests. –  Oct 15 '21 at 11:53
  • Apparently I need to be clearer in my question. I think I confused people talking about some of the logistics of where the module would be. What I don't know is the efficiencies and any other Electrical Engineering tradeoffs using one voltage or the other. I'm not looking for someone to select a module for me, just give me some insight if I should tap off the transformer or 120 VAC. I'll edit my question shortly. – Adirondack Jim Oct 15 '21 at 12:38
  • Is your load is only 1 W, it would certainly be possible to design a SMPS to cover the entire range. If it's a one-off, it would be a good learning experience but but very economical. – winny Oct 15 '21 at 13:20
  • winny, yes, the load is estimated it to be 1W (mostly LED current). Yes, I could design my own ac/dc converter and learn a lot, but I'm already spending time figuring out the DC/DC conversion, particularly any heat issues. Since this part is in the basement, I'm not space constrained and can work with any heat dissipation (within reason--nice to be green, if possible), I figured a store-bought device would be appropriate so I can focus on the rest of the project. Thanks for your comment. – Adirondack Jim Oct 15 '21 at 20:11
  • Finbarr: sorry for the late reply (missed your comment). Thanks for the comments. Yes, I am concerned about I2R losses if I go with 3.3V, but the distance is pretty short (~ 15 ft.) and doorbell wire is reasonably thick. IF I decide on that voltage, I'll hook up a power supply to verify voltage loss. I want to do AC-DC conversion in the basement (versus the sign) to minimize heat buildup in the sign since the electronics will be encapsulated. – Adirondack Jim Nov 17 '21 at 14:37

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After giving this some thought, I believe after you've rectified the AC to DC, this comes down to a DC-DC converter question. So, the closer you are to the voltage, the more efficient you should be (and bucking is more efficient than boosting). And I'll have to make sure whatever I choose can handle low currents. I'll probably work off of the 16 VAC transformer. I say probably because there is a more logistic thing that can't be ignored: there's a LOT of 120 VAC to 24 VDC supplies out there specifically for LED strips. I haven't checked yet, but they probably are designed to handle low currents (because you can cut the strips to short lengths).

Adirondack Jim
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