Voltage regulator with no dropout voltage

Hi everyone,

I've been making plans for my first Arduino project, and am currently thinking about how I want to do the power supply. This project is going to be for home monitoring (temperature, humidity, leak, smoke, etc.) so it needs to be robust and not dependent on utility voltage being present. To that end, I'm planning on a setup where I have a 15v torroidal transformer (and appropriate rectifier, etc.) providing power to a picoUPS-120. This way I will be able to have the system maintain a 12v SLA battery, and switch over to it automatically should the power fail.

From the picoUPS (which provides 12v output,) I want to build a second regulator board to very tightly control the voltages going to the arduino, sensors, etc. since I don't want bad power to fry my investment. I want this board to provide regulated 12v, 5v, and 3v3 outputs. I had been planning on using like a TI LM2576 to do the power regulation, but found that it has a dropout voltage of around 1.5v, which would mean that the regulator wouldn't work for the 12v line.

My question is - is there any way that I could safely regulate the incoming 12v power, while using the power supply setup I've been designing? I know that I could use zeners, but I was hoping for something that was more sophisticated than just a siphon for higher voltages.

Thanks in advance!

Use switching regulators. They will maintain the desired output. http://www.pololu.com/category/131/step-down-voltage-regulators Couple bucks each, way less hassle than linear regulators.

Do you need to regulate the 12V supply? For what is it to be used?

You do not need regulation for motors, steppers, LEDs and the like.

Shunt regulators using Zeners are very much inappropriate for most uses - in particular as they are extremely inefficient.

Base on the block diagram, the output of the PicoUPS-120 was not fix at 12V, it can be 21V when using your 15v torroidal transformer or 10V to 14.5V using SLA battery.

Your 15v torroidal transformer which output 21V was way to high for the PicoUPS-120 which have a max of 18V

Use Buck-boost switching regulators. They will maintain the desired output. Step-Up/Step-Down Voltage Regulators Buck-boost and SEPIC converters work with input voltages that are higher than, equal to, or lower than the regulated output voltag

Thanks everyone :)

I probably don't actually need to regulate the 12v line since it's supposed to be regulated coming out of the picoUPS, but I want to do it anyhow just to be safe. I'm looking at using a UDOO because I want to be able to log readings to mySQL and display the results to a webpage, and need something relatively inexpensive with low power requirements but that still has decent processing capability.

One of the big reasons I'd been looking at the monolithic silicon stuff is because I need higher amperages; for example the UDOO says it wants to have 9-12v at 2A at its disposal, so I figured I should try for 3A or 5A just to be safe (I prefer not running things at 100% of their rated capacity ;) ) In reality, the 3v3 supply will probably be pretty minor amperage, and the 5v might be a bit more because I'll probably be running a small laptop hard drive off of it to store the data.

Looking at other monolithic silicon options on Digikey, it looks like a LM2678 might do the job? It looks more complex to set up than the 2576's or buying a pre-made setup like on Pololu, but I like the fact that it can handle 5A.

http://www.digikey.com/product-detail/en/LM2678T-12%2FNOPB/LM2678T-12%2FNOPB-ND/366923

Being a total noob to all this, I've been kind of relying on my Googling, so I may be totally off-base about the best direction to go with this; that's why I decided to ask the experts (aka, you all :) )

BillHo -

Sorry, somehow I missed your post until right now :/

I was looking at the VPT30-3330 transformer, which is supposed to regulate down to 9% between no load and full load (6.66A at 15V); from what I'd read, it sounds like that means that it will run 15V at 6.66A, and 9% higher at no load (which would be roughly 16.5v). Am I interpreting that correctly? The big reason I was looking at this particular transformer is because of its tighter regulation; most of the others I looked at were 25% at best.

I guess I'd mis-read the block diagram from the picoPSU; I'd thought that they were regulating the output to the load like they did to the battery, so it definitely sounds like I'll need separate regulation for things.

chrismyers81:
I probably don’t actually need to regulate the 12v line since it’s supposed to be regulated coming out of the picoUPS, but I want to do it anyhow just to be safe.

That is not a reason. “Just to be safe” makes no sense; it is not an engineering consideration here. Regulating something unnecessarily only wastes energy (turning it into heat).

chrismyers81:
for example the UDOO says it wants to have 9-12v at 2A at its disposal, so I figured I should try for 3A or 5A just to be safe (I prefer not running things at 100% of their rated capacity

But you do not need to regulate it, so that is simply not a problem. And if you did, and the 2A is the maximum specification, it means that it will not be drawing that current much of the time anyway, so 2.5A or 3A at the very maximum would be more than sufficient. The very fact that it specifies “9-12V” would seem to be telling you it does not need to be regulated. Your 12V battery in fact regulates the 12V supply extremely effectively anyway, and it will not fall to 9V.

chrismyers81:
Looking at other monolithic silicon options on Digikey, it looks like a LM2678 might do the job? It looks more complex to set up than the 2576’s or buying a pre-made setup like on Pololu, but I like the fact that it can handle 5A.

But it is inappropriate unless you need the 5A. Switchmode regulators operate at peak efficiency at the full design current, drawing half the current will (significantly) lower the efficiency. If you are concerned about the rating of the lesser regulator, you mount a heatsink on it, because that is where the limit to the current rating is.

Just trying to explain what engineering is all about.

chrismyers81:
The big reason I was looking at this particular transformer is because of its tighter regulation; most of the others I looked at were 25% at best.

It is true that larger transformers have less transfer losses by proportion, which means that they exhibit better regulation.

The reason why you do not “be generous” and grossly over-specify a transformer, is (in addition to cost,) that its (magnetic) leakage losses are similar across sizes, so a larger transformer has a greater fixed power loss simply from being powered up.

Cool, that all makes sense :)

I guess the reason why I was wanting to do 3A regulators for 3v3 and 5v is because I plan on the project growing over time; while it might just be monitoring a few sensors initially, eventually I want to throw in an LCD to make monitoring easier, add in some sort of SMS communication in case the internet connection goes down, and be able to interact with the house itself (eg., if it's cool and not humid outside and the AC wants to kick on, maybe just kick on the attic fan instead. Or, being able to control the furnace and AC based on sensors placed around the house. Stuff like that.)

I know that normally I shouldn't need to do a separate 3v3 supply, but since the UDOO/Ardunio Due on it only seems to support around 5ma on the outputs, I'm going to need to drive loads from an external source in most cases, and use the outputs on the board to drive a mosfet and power the sensors themselves from the external supply. (I know some of the outputs are higher than 5ma, but they're still not a huge supply.)

I was also planning on giving the 12v line more amperage because I know I'll need to probably power a fan or two for the enclosure, etc., so have been debating between a 3A and 5A regulator.

That's also why I was going to choose the 6A transformer versus like a 3A one; if the UDOO wants to have 2A at its disposal, and the battery charger wants 1.5A, and a small laptop hard drive wants about half an amp, I wanted to have enough breathing room to be able to add some extra stuff in the future without having to change out a bunch of parts.

I know that regulating a regulated input is overkill and probably very unnecessary, but in my case, I thought that the expense of a little bit of efficiency to be sure to protect what will end up being probably a $250 investment is worth it for me.

That's the reason behind my thinking anyhow? Again I'm a total noob to all of this, and it's entirely possible that with all of that, I'm still trying to over-engineer things. I will definitely keep all of that in mind though, and not try to provide a ton more than I need as I try to plan things out :) Thanks for your comments and calling me out on stuff :) I really want to learn the best way of doing stuff, and need that :)