Getting power through diode bridge

Many similar questions have been posted, but I have not been able to find the answer.

When you do an electrical rules check, KiCAD checks to see if all devices that need power are getting one. So power sources must be declared in the schematics with power flags.

The problem occurs when the power supply is AC and there is a diode bridge. If you define a special diode bridge symbol and declare power input and power output pins (which is unfortunately not done with many predefined diode bridges), it works.

But what if you use four diodes in your schematic? How can you pass the power through them?

I know the workaround would be to define power flags after the diode bridge, but that’s kind of stupid. Is there any other way to let the program know that power is being transferred through diodes?

No, there is not.
See for some discussion.

It’s not a workaround, it’s how KiCad works. Otherwise there should be intelligence which would guess how the circuit works (i.e. simulation). After all, without explicit information the schematic doesn’t know what’s the voltage, diode threshold etc.

In v5.99 (the unstable development version) it’s possible to modify each symbol instance in the schematic, so you can change the pin type for one diode. But is it worth it and unproblematic? No. Just add the power flag, it tells explicitly what you want.

On the other hand there may be room for improvement, but it’s not clear to me what kind of solution would be easy to use and without problems in all cases.

In case of power diodes I would propose some kind of “bi-directional power” pins. It will let know KiCAD that power can go “through” the element.

But how would that be easier or clearer than just adding a power flag? You can’t just change the pin of the generic symbol in the library, so you should either change the pin type of that one instance or have a specific symbol in the library which you can use in that situation. Would it really be simpler and better than a normal power flag?

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From a practical point of view, my question is a non-issue.

But from the principle point of view, one should flag the power where the power really comes from.

You can have a situation like the reversed diode commonly placed between Vin and Vout of a 7812 regulator, to prevent device destruction if the input gets shorted. The simulator does not “know” that in normal operation, the diode never conducts and is not the source of power

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Yes you are right, diodes could be used for other purposes apart from bridge rectifiers. But if you can individually change pin types, as it will be possible in future versions of KiCAD, or if you define your own diode (I have defined CRS08 in my example myself), then it makes sense to pass the power, at least in my opinion.

On the other hand, I did not want to start a discussion, as I am really a very rare user of KiCAD. What I asked is not possible, and not likely to be possible in future, so … thanks for the answer.

I would place a Vin power component at the input of the regulator ( say +12 ), then a +5 power component on the output of the regulator. This is to reduce the number of common wires on the schematic so that it is easier to read. The voltage values also remind me of the design intention and suitable capacitor ratings.

Did you follow the link Radix posted to gitlab?

It has a more general discussion for this, which also handles ferrite cores, Fuses, inductors and other components that should act as a “short” for ERC.

I had a look. I do not know enough of KiCAD to benefit from it though.

Power flags are a way to tell DRC where power comes INTO the schematic. A flag indicates an external source of power.

A flag on the output of the diode bridge would be inappropriate.

That would be in an ideal world.

In a practical world, compromises happen. Currentlly DRC of KiCad can not “see through” diodes, fuses, inductors chokes, etc and you need to place the PWR_FLAG symbols wherever needed to make DRC happy.

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