Can someone please help me in the analysis of this arduino board power supply works

enter image description here

To my understanding,

  1. In the presence of DC power through the DC jac, Dc voltage is fed to the MC33269 regulator to generate +5V. This same voltage is fed through R10/R11 divider to +terminal of the comparator, while the 3.3v OUTPUT of the FT232RL is fed to the -terminal. the positive differential result drives the output high. However, i'm not sure the effect on the second comparator and the FET
  2. When USB connector is used to power the board, I'm not sure what happens.

Larger version:

enter image description here


2 Answers 2


I am not impressed with the quality of this schematic. Someone was too lazy to export this thing in Eagle without colors, which don't mean anything to people outside of Eagle. Then there are the two mystery blocks at left. The top one shows 5V and GND with a cap accross it, but without any hint what the thing being connected to power is. The bottom one is connected to PWRIN and GND, but again no hint of what it actually is. I wouldn't trust much from this person or organization since they can't even get the little obvious things right and clearly lack the pride in their work that should have made it too embarassing to show this mess in public. I guess this is one more confirmation that Arduinos are not just microcontrollers for dummies, but also microcontrollers by dummies.

Anyway, back to your question. It looks like the point is to switch actively between USB power and the PWRIN power line. When PWRIN is present, it will always be used whether USB power is available or not. For VIN to be useful, it has to be above VCC30 after being divided by two by R10 and R11. From the names, we can guess that would be 6V, which could be the minimum IC4 requires to make reliable 5V out (I don't recognize the IC4 part number and didn't check). You are right, there is no purpose to IC5B. It is a unity gain buffer, but the output of IC5A should have the same impedance and drive capability.

Note that the way T1 is oriented, the FET body diode always lets the USB power voltage onto the 5V net. This lets the system bootstrap up and eventually turn on the FET fully if the board is only powered by USB. If external power is used, the FET will be off and the diode drop will prevent any substantial current to be drawn from the USB power.

  • \$\begingroup\$ a little harsh on the Arduino team wouldn't you say? and by the way I failed to export the image without colour, sorry about that, I don't use eagle so I just did the first thing I saw possible. And thanks a lot, that was very helpful indeed \$\endgroup\$
    – TiOLUWA
    Commented Feb 12, 2012 at 0:15
  • \$\begingroup\$ @TiOLUWA: That doesn't make sense. If you didn't originate the schematic, then why were you running Eagle on it in the first place? People looking at a schematic shouldn't have to know nor care what software it was made with, which is why it's usually exported as a PDF. In any case, the other stupidities still stand. \$\endgroup\$ Commented Feb 12, 2012 at 0:21
  • 1
    \$\begingroup\$ @OlinLathrop There are usually PDFs and editable copies of the schematics made available. It helps to support the "open hardware" concept to allow people to modify it as they wish and create their own boards. And as far as the color, is there an issue having schematics in color? I can see a problem if the colors means something or if the colors are too light to see, but I don't think this is the case here. However, I do agree with all of your other points, including not trusting schematics from people who leave boxes with no label as to what it is. \$\endgroup\$
    – Kellenjb
    Commented Feb 12, 2012 at 0:58
  • 5
    \$\begingroup\$ I actually think the color helps present the information much better because it is easier to tell what is considered a wire versus a component. This is completely separate from what software it was generated in. \$\endgroup\$
    – Kellenjb
    Commented Feb 12, 2012 at 20:25
  • 8
    \$\begingroup\$ -1, Angry rants and ridicule about Arduino based solely on a schematic's color scheme are silly. Anyone confused by a schematic that uses a whooping 2 colors shouldn't be giving advice on circuit design. \$\endgroup\$
    – Cerin
    Commented Apr 19, 2015 at 2:44

This Arduino power supply is designed to "do the right thing" no matter which source of power is plugged in.

the right thing

"The right thing" is:

  • When a person plugs in only the USB cable, the CPU and everything else powered by the +5V line is powered from the +5V USB power.
  • When a person correctly plugs in only a 12 V wall-wart, the CPU and everything else powered by the +5V line is powered from a +5V voltage regulator powered by the wall-wart.
  • When a person correctly plugs in both the USB cable and the wall-wart are plugged in at the same time, all the power comes from the wall-wart, and no power "back-flows" to the USB host.
  • When a person keeps plugging and unplugging the cables, the power smoothly transitions from one to the other, so that as long as at least one is plugged in correctly at all times, the CPU continues to run uninterrupted.
  • When (not "If" !) a person plugs in a 12 V wall-wart incorrectly -- reverse polarity -- no current flows to or from the wall-wart, no damage is done, and the system acts exactly the same as if that wall-wart isn't plugged in at all.

wall-wart power

Many systems use 1 diode for each source of power to power the system from whichever input voltage is higher, which automatically handles the "smoothly transitions" requirement.

The diode works fine on the wall-wart power side.

USB power

Alas, a diode on the USB-power side wouldn't work for the Arduino. When running off USB power alone, a diode drop (typically about 0.6 V) would cause everything to run a diode drop lower than USB power -- so it would have been typically 4.4 V, which is apparently (?) inadequate.

mystery parts

Later versions of the Arduino schematic clearly label the 3-pin box "powersupply DC 21mm", indicating a 21 mm barrel plug.

The mysterious "4" and "8" pins in the top-left of the Arduino schematic are the power pins of an 8-pin dual op-amp. That op-amp is used here as a comparator.


I don't know why the designer didn't use a comparator IC, or why the designer used both op-amps in the package when just one op-amp is adequate -- but since it clearly works, I'm not going to say it is "wrong".

The op-amp and the pFET implement something very close to an "ideal diode": when only the USB cord is plugged in, the op-amp drives the pFET hard ON, giving a voltage drop across the pFET less than 0.1 V (so everything runs on something close enough to 5.0 V).

When a person plugs in a USB cord to an Arduino that previously had nothing plugged in, the body diode of the pFET "T1" lets power from the USB cable leak in enough to bootstrap the op-amp power supply voltage up to about 4.6 V, more than enough to power up the op-amp, which then turns that pFET hard ON, pulling the voltage the rest of the way up to more than 4.9 V.

When a person plugs the wall-wart into the Arduino power jack, the op-amps turn the pFET hard OFF. The pFET body diode prevents power from the voltage regulator back-washing to the USB host. In principle USB power could continue flowing through the pFET body diode into the Arduino, but that's going to be pretty insignificant since the USB power is close to the same voltage as the regulated voltage generated from the wall wart.

p.s.: When a tiny company sells 250 000 boards, I personally use the word "successful" rather than "dummies".

  • \$\begingroup\$ very helpful answer . Can you please tell me why two resistors RN3B,RN3C are not connected to anything? the schematic is not professional at all \$\endgroup\$
    – Amrith
    Commented Jun 10, 2022 at 7:00
  • 1
    \$\begingroup\$ @Amrith: RN3B and RN3C (as well as RN3A and RN3D) are all part of a single component, a resistor network (RN). Such a 4-resistor RN component has better resistance matching and costs less to place on the board and often takes less space on the board than replacing RN3A and RN3D with 2 electrically equivalent separate individual resistor components, even though the RN3B and RN3C sections of that component are unused. \$\endgroup\$
    – davidcary
    Commented Jun 13, 2022 at 0:43
  • \$\begingroup\$ thanks for the reply. I was designing my own arduino uno using schematic for pracise using KiCAD unfortunately I had placed normal resistors instead of RN. too late I guess. But cool to learn something new. Thanks \$\endgroup\$
    – Amrith
    Commented Jun 14, 2022 at 5:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.