Overview over voltage regulators?

Other Hardware General Electronics
1

Hi, am curretnly kickstarting towards Arduino on a breadboard and then soon onmy own pcb. What I am still struggling with is the choice of components for power supply.

MIkrocontroller.net is big about 7805, LM317, LM723 ... The Arduino boards use completely different components. I do of course understand that there are different input and output voltages, fixed and regulatable, different loads, different voltage drops, different efficiencies (switching or linear) ... still I am completely missing an overview - starting with a table where the most common elements can be directly compared.
Especially concerning efficiency, I have problems getting those from a quick glance at the datasheet:

Especially, I have the following questions:
Arduino goes [Input]->5V->3.3V, as far as I understand. Would I want to use the same components? Checking Ebay, they don't seem to be common.
What would I use if I only wanted 3.3V? What would be the best choice for battery power (5V or 3.3V)?

2

I'm a fan of the xx1117's as 3.3v regulators (xx is some set of letters like LM or what not). They are a good choice for taking 5v down to 3.3v in my mind.

As for the 5v regulator, 7805's are always the ones I jump to. Widely available, fairly cheap.

In terms of efficiency, its never really something I have needed to worry about in my projects so can't be much help there.

ElCaron:
Arduino goes [Input]->5V->3.3V, as far as I understand. Would I want to use the same components? Checking Ebay, they don't seem to be common.

The ones Arduino use seem a little weird and uncommon as they are surface mount components, and really in part I imagine they are chosen for reliability, size, and more importantly cost.
I see no reason why you should hunt around for them when you can get others that do the same job.

ElCaron:
What would I use if I only wanted 3.3V? What would be the best choice for battery power (5V or 3.3V)?

If you only want 3.3v, you can use the 3.3v version of the xx1117. It has a fairly wide range (I think 15v is the max input voltage).
If you are looking at battery powered, it really depends on the battery. If you have a 9v battery, then go with 5v, as there is less wasted in the regultation. Alternatively you could look at switching regulators which are much more efficient as they convert current to voltage (or vice versa) which means you are not dissipating the excess voltage as heat. If you look around, there are a number of breakout boards for step-up/step-down.

3

Some people will say it only makes sense to use a dc-dc converter, since standard
v.regs tend to waste power. But I'll only mention some v.reg considerations.

  1. most people use a 1-AMP v.reg, so try to go with that.

  2. you want a low-dropout [LDO] v.reg, so the usual 7805 and LM317 are not good
    choices. For TO-220, the LM2940 is a good choice. Most or a lot of smt v.regs are
    LDO.

  3. in regards packaging, I personally dislike the tiny SOT223 v.regs found on many
    boards. After LDO, the next most important aspect is Pd = power dissipation.
    The little SOT223 v.regs will overheat when dissipating more than 0.3W or so,
    and that's not much of a load, after all. For a 9V power source,
    Iload = 0.3W/(9V-5V) = only 75mA. That's really a tiny load current.

I prefer using a TO-220 for the main v.reg, since it can dissipate a lot more power
without overheating. Or at least a DPAK, which is inbetween TO-220 and SOT223.

  1. in regards having both 5V and 3.3V v.regs, I was EXTREMELY disappointed with my
    Duemilanove board when I discovered the 3.3V power went away when I disconnected
    from USB and ran off a battery. Nuf said.
4

I've useed LOTS of LM78xx & LM79xx parts. But, any "3-terminal" linear regulator is just as easy.

I've never built a switching regulator. A switching regulator requires more components (including an inductor) and it's more complicated to design. But where I work, most of our boards have switching regulators. It's not a big deal to add a component or two to a board, and you only have to go through the design process once. Some of our boards have a ~15V to 5V switching regulator, plus a 3.3V linear regulator.

Regulator chips can be fixed or ajdustable/variable. An adjustable regulator takes a couple of resistors to set the output voltage. Of course you can add a pot (and maybe some other circuitry) so that the user can adjust it.

As far as specs, of course the 1st thing to consider is the output voltage and output (load) current.

Then, you can check the input voltage range. It needs to handle whatever voltage you are putting-in. For example, depending on the exact part number you are using, the LM78xx chips can take up to 25 or ~40V. If you go over the rated input voltage, the part might die.

You also need to consider the minumim input voltage. The LM78xx series has a dropout voltage of 2V. That means if you are using a 7805 (5V output), you need to feed-in at least 7V. If you go below 7V, the device will "drop out" of regulation and you'll get less than 5V out. If you are going from 5.V to 3.3V, you'll need a "low dropout" regulator, since you are only going to "drop" 1.7V across the regulator.

If you are using a linear regulator, you also need to calculate power dissipation. For example, a 7805 rated for 1.5 Amps, will probably burn-up if you run it at the full 1.5 Amps with 30V input voltage.

As you may know power (Watts) is calculated as Voltage x Current. In the above case, I've got 25V dropped across the regulator with the 1.5A current flowing through the regulator and the load. There is 37.5W (1.5 x 25) dissipated by the regulator and 7.5W (1.5 x 5) dissipated by the load. You can see this is very inefficient, with more power (and heat) "wasted" by the regulator than is being used by the load. And, the full 45W has to be supplied by the 30V supply. 5V linear supply running off 10V is (about) 50% efficient. (I say "about" because there is also a tiny amount of power required just to "run" the regulator.)

Usually, none of this is a problem if the current draw is low. Unless you are running from a battery and you need the battery to last as long as possible. If heat, power dissipation, & efficiency are important, you can use a lower input voltage, or go with a switching design.

Switching designs are nearly 100% efficient. It's usually not necessary to calculate power dissipation (and it might not be possible to calculate accurately) as long as you stay within the voltage & current ratings.

Since switching regulators are so efficient, nearly ALL of the power goes to the load. This means that the regulator reduces the voltage, but you generally get more current out than you put in.

The exception is at low currents. As an extreme example, with no load connected there is power consumed by the regulator and you have zero efficiency! In very-low current situations, a linear regulator might be more efficient than a switching design (depending on the design details, etc.) because the linear design might require less idol current, or less power to "run" the regulator.

There are variations of switching regulators, such as DC-to-DC converters. I bought (didn't build) a DC-DC convert to convert +12V to -12V for an automobile project that need both positive & negative supplies. Another DC-DC converter application is a "step-up" regulator. For example, a circuit that converts 5V to +12V, or -12V.

5

Thanks, that gave some clarification.

However, concerning the suggestion to choose according to my design: That's exactly my problem :slight_smile: If I don't know the possibilities, I cannot choose the right one. Unfortunately, there seem to be a lot of possibilities around. A table with the most common components would be neat. Until that, is it a good idea to just search "switching regulator 3.3v" on ebay and then check for the input voltage range and maximal load? (Ebay, because that is the only supply I know in Germany, where shipping is not 5 times higher than the component price.)

Regarding designs, I think there are not to many possibilities, at least for home use. There are 3.3V, 5V, and 3.3V/5V setups, and there is power from the wall and from a battery pack of AAA/AA batteries. (I would choose 9V block, as they are expensive, have less capacity and the rechargeables are not as good).

6

An appropriate starting point is a xx1117. These are under $.50 USD and are rated up to 800ma and 15V input as Tom mentioned; it's when your input voltage goes over that 15V that you need a LM317 or LM780x as they can handle up to ~40V inputs. When you go over currents of 1A but less than 3A or your input/output voltage difference is high then an LM2596 switching regulator is a popular choice; you can find prebuilt modules all over eBay for a couple USD each. Over 3A usually means you'll be buying a power supply tailored for your project or using a power supply from an old computer, etc.

DVDdoug:
Switching designs are nearly 100% efficient. It's usually not necessary to calculate power dissipation (and it might not be possible to calculate accurately) as long as you stay within the voltage & current ratings.

That efficiency claim of 100% is not accurate; 80% would be more correct but some can get a little over 90% under ideal input voltage and load.

7

Have you tried Farnell?
German Site:

Yeah the shipping is just under €8, but if you work out what components you need and then order them all it one go, I'm sure it will work out cheaper than all the individual eBay shipping costs. The components themselves are also generally a lot cheaper than eBay and there are many more to choose from. I get all my stuff from them (all be it from the UK branch).

8

Well, on EBay, there are a lot of sellers from Asia selling without shipping cost. If you have 2-3 week, that's not bad.
E.g. LM1117T is 1,99€ at Farnell, or 3,79$(!) for 5 on EBay, shipping included.