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Hi, I'm in the process of making a custom board, but I ran into a question: what values should I use for voltage regulator's electrolytic caps?
I've looked through different Arduino board schematics (Mini, Pro, Nano etc.), and found different values: 4.7uF, 10uF, 47uF, 100uF...

My board will be hooked up to a car battery, so ~12V... I guess I could just use 47uF, like the ones on my Duemilanove (although the schematic says they should be 100uF), but  I just wanted to get some insight, first.
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Its really not critical. Generally the bigger the better, so the 100uF is probably the best choice.
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It's usually not critical, but the best answer is "read the datasheet for the regulator"...  Note that some low-dropout-voltage regulators are a bit fussy about decoupling capacitors.  1uF on input and 10--33uF on output usually works.  This is for low-current regulators - the higher the current rating the higher the capacitance needed.   The theory is that the capacitor does the high-speed regulation of output voltage and the regulator fills in at lower frequencies.
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As was mentioned you should read the datasheet. Some regulators are
very fussy about ESR (effective series resistance). In particular the MC33269
datasheets list a narrow range of ESR's for the output capacitors. A minimum
ESR is required for output stability.

Other regulators exclude low ESR capacitors like ceramics (unless a series
resistor are used). Others recommend tantalums.

(* jcl *)

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I know this is kind of off topic here, but speaking of C2 you might want to read this:
I don’t know if anyone notices but there is a problem with the Arduino schematics with their power supply. It seems that the power supplies have been rubber stamped from one design to the next Arduino (UNIO to the MEGA 2560). It seems that there might be a mix-up of part symbol with part name. I have the MEGA 2560 so I can personally speak from that model when I try to describe what I am seeing. (From here on out, I will refer to the Arduino MEGA 2560 as just MEGA- just for clarification.)
First, I think IC1 and IC2, for all the schematics that I have run across, the regulator power ICs are swapped with their symbol on the Arduino schematics. I am surprised that no one has picked up on this or caught the swap. I am talking about the MC33269x-x.x which you can see from the data sheet here >> http://www.onsemi.com/pub_link/Collateral/MC33269-D.PDF
Grumpy_Mike said in one of his post there is only one regulator on the board, I think there are both regulators on the board. I think who ever did the drafting for the power supplies were not paying attention to how they were drafting and got the symbol mixed with the name of the part.
Second, if you go looking for the MC33269ST-5.0T3, you will not find it at any part house. Both Mouser and DigiKey do not have that part in 5 Volts. YES, the ‘ST’ part is suppose to be in a SOT-233 package, but if you look at that data sheet the ‘ST’ part only comes in the 3 Volt flavor. That does not change the fact that if this is the correct Mfr. Part #, there is a SOT-233 package on your Arduino board (at least it is for the MEGA). Now here comes the part ‘B’ of my confusion- on any of the Arduino schematic they show this part as a part with more than 4 pins. That would make this part a SO-8 part not a SOT-223 part. And the MC33269ST-5.0T3 is clearly a SOT-223 part, which would mean that this is IC2 on all our schematics.
NOW where in the heck you ask is this SOT-233 part on your Arduino board you ask? Well for the MEGA board; if you hold the board like this >> http://www.hobbyengineering.com/pics/i4312-ProductFront-400x223.jpg (thanks hobbyengineering.com for your picture- Sparkfun at this time does not have their pictures updated- they still have the old FTDI chip picture showing) it is north of the power jack soldered to the right side of the silver square (which is being used as a heat sink- just north of the power jack for the regulator). Also, you might have not notice that’s missing from all our schematics is the really cool poly fuse that’s between the SOT-233 package and the USB connector. I really wish Arduino would update their schematics- because the use of the poly fuse is so cool to use for current protection. Yes, I have read about it, just not seen any datasheets on which one they are using. And just like a good bone, the Lab loves to collect datasheets on cool things.
Now you’re asking where in the heck does the Blacklab think he’s sniffing the second MC33269 chip? Well if you look at that JPG picture that I gave you, I will walk you were it is at. Now look at the USB connector, look right of it and you can see the NEW ATmega8U2 that replaces the old FTDI chip for USB communications. South of that ATmega8U2 chip you will see a silver oval which is the crystal for the ATmega8U2 chip. And south of that is an 8 pin SOT-233 chip, which if you get your magnifying jeweler glasses out you will see it connect as you see IC1 in the Arduino schematic. Therefore IC1 is really the MC33269D-5.0 (NOT IC2). If you look at the two silver cans right of the Power jack those are the 74u capacitors. And the block just a little up and to the left of the two capacitors is the D1 M7 diode. Now, what I just noticed C2 is on the 5 Volt side in the MEGA schematics and it’s on the supply side in the Uno schematic { http://arduino.cc/en/uploads/Main/arduino-uno-schematic.pdf }.
I hope I have proven that both power regulators are on board and it might give inspiration for you to copy their power design on the next board you make. Don’t forget to add that poly fuse to your next design too.
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Quote
I think there are both regulators on the board.
No, but there are two regulators on modern boards.  "U2" provides 3.3V.   Try these annotated diagrams: http://www.flickr.com/photos/58843278@N00/5602080472/in/set-72157622049680333#/
http://www.flickr.com/photos/58843278@N00/5402516093/in/set-72157622049680333/

Quote
got the symbol mixed with the name of the part
Possible.  As far as EAGLE is concerned, they're just names.  Making sure that exactly the right name gets associated with exactly the right package/etc is frequently a neglected topic.  And probably deservedly so, since I'd rather the part name not change just because I change packages.  (In more recent versions of EAGLE, there are "attributes" for a part where you can put things like orderable part numbers.)

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missing from all our schematics is the really cool poly fuse
That would be "F1", on the right side of the schematic between the USB connector and USBVCC signal.

I haven't noticed any out-of-date issues with the current set of schematics/etc.  Not since NGDM...
(ok, excepting ethernet, apparently.)

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