How do I fix a power problem?

How do I create a 3v3 output without causing errors? I have a 3v3 power flag. I also have an output from a regulator.
If I dont have the 3v3 connection to the output of the regulator, the track does not identify as a supply track and wont connect to 3v3 pins on the pcb.
If I have the 3v3 as shown, I have a 3v3 track and a number of errors.
Is this a simple fix?

And what are those errors? Also post version of KiCad please.

Power flag means the symbol precisely named PWR_FLAG. It has no voltage. Add this to all nets where there is is/are Power Input pin(s) but there isn’t a Power Output pin on it. The output of the regulator should be a Power Output pin, unless the symbol is defective, perhaps because not from the KiCad library.

The 3V3 is just a power symbol and used to connect all points that are in the 3V3 net.

Meaning: its pin type is Power Input.

One other small tip when using voltage rails. Instead of having a wire running all over the place for GND, 3V3 and +5V, use multiple power symbols. It makes it a lot easier to read and less prone to errors especially when your schematic becomes more complicated

1 Like

You could start by fixing the schematic. It’s a horrible mess of wires and I’m not even going to attempt to look at that maze. Regular conventions that work quite nicely are: signals go from left to right, and voltages go from top to bottom. Make voltage rails stand out in some way, so they don’t get confused with signal wires.

Its my first attempt…ever. I think its not too bad for a first outing. I am sure with a bit of experience I will improve.


Your regulator is not shown directly connected. Is that my issue? Should I change my diagram to conform to the image?

It’s connected via all the common points. So all the GND point’s are common and KiCad connects them together. Same with any other power symbol. All the 5V will be connected, and 3V3 connected etc.

This is also true for any global labels.

My example is electronically the same as your example but as you can see easier to read.

I think the issue you are having is missing PWR_FLAG

Can you post a screenshot of the errors you are seeing.

The same?
Your pin 2 is 5V and 3 is 3V3 while in original pin 2 was 3V3 and pin 3 was 5V :slight_smile:

The 3v3 power flag is there along with the 5v and the GND.
Is my problem the power flag denotes a power input. The regulator is also an input.
In effect, there are two inputs, one as the power flag the other from the regulator which, as far as I can tell is undefined.
I removed the checkbox on ERC for power input pin/power output pin and the error has gone.
This still leaves me scratching my head. Should the regulator output be defined somewhere?

Can you post the full schematic. It’s not clear where the 5V is coming from? Is it being provided by a external power source?

With the microntroller the 3.3V pin is defined as a power input. (This is setup in the symbol) If you have a 3.3V regulator the output pin of the regulator will be defined as a power out. So when the 3.3V from the regulator is connected to the microcontroller the ERC will pass. (The uC is powered from the regulator)

Now if for example you have a 2 pin header. One side GND and the other 3.3V (Say you were going to connect a external battery to this to supply the 3.3V power instead of the regulator) - if its a 2 pin header symbol you are using these pins will be defined as passive. So connecting a wire from 3.3V on the microntroller to the header and then running a ERC will result in the error you were seeing.

KiCad does not know this is a power pin, so to keep the ERC happy you need to add a PWR_FLAG to the 3.3V line.

This is not the case with your 3.3V line as the power is coming from the regulator. However it might be an issue on the 5V line as I’m not sure where the power is coming from.

LD1086 is apparently a “Low Drop” regulator, but a voltage of -1700mV is still a bit steep for it I guess…

1 Like

You have to understand that:

  • 3V3 is not power flag,
  • 3V3 is more like a label at wire so all wires with 3V3 are connected together (they are the one net),
  • ERC don’t understand circuit,
  • ERC only checks if at net that contains power input pins there is also a power output pin,
  • if there is no power output pin at such net you can tell KiCad to ‘not worry about it’ by placing PWR_FLAG anywhere at that net.
1 Like

First point in datasheet: - Typical dropout: 1.3V at 1.5A.
0.4V is enough margin if you are sure that 5V is 5V. Specially if your circuit consumes less than 1.5A.

It was directed at gmc. In the schematic he posted the voltage regulator has 3.3V input and 5V ouput, therefore negative 1.3V.

I wasn’t smart.
I assumed that someone can think of dropout as a subtracted voltage and thus with ‘-’ and din’t noticed at whom it was directed.
If you used emoticon…

Piotr has explained this well, but I have to stress this: There is only one PWR_FLAG The others are mere power symbols. Accept no substitutes!

A physicist friend of mine used to mock the “There is only one Jeep” slogan, saying: is that like Wheeler’s tongue in cheek theory that there is actually only one electron, one proton, and one neutron in the universe, and they zip back and forth in time, turning into their anti-particles on reversal, conjuring up the entire universe? So all those Jeeps you see are just manifestations of one Jeep. :crazy_face:

The 5v is from an external supply.

First you should have to do is to delete all GND, 3V3, 5V wires with just placing many GND 3V3 and 5V power symbols.
Second - may be use some buses to simplify the schematic:

1 Like