Electrical Rules Checker - another who needs help

I cannot understand these errors or how to resolve them. Surely I am at fault, but not sure what I’m doing wrong.
Any help from those with superior KiCAD and electronics knowledge would be appreciated.

three parts of U2 have their outputs shorted. the ERC has a simple pin-pin check and in this instance an OUTPUT connected to an OUTPUT is not permitted and (almost) always this will cause problems

You appear to be paralleling the outputs to increase their sink/source capability to drive a FET harder/faster so the 1st question is… can this chip tolerate having its outputs shorted together?

If so the output pin type might need changing its type or another part (eg net-tie) placed to work around this ERC violation

Likewise, from EESCHEMA perspective, you have not provided power to U1 or U2. You have placed a PWR_FLAG at the connector, but where U1 and U2 tap their power is after R1. You need to put a PWR_FLAG at that node. also do you really want to use a 270R resistor in this position? A ferrite might be a better choice to mitigate supply line noise.

Also, what is to protect the FET from overvoltage when it is turned-off but L1 is charged?

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Now you understand connecting output together is not normally done. (ok in this case)
Normally the supply pins go to a supply voltage but you made a supply by R1 & C3. Checker can not see that.
These things are OK in your case.

What is the voltage, top left. If more than 12V what limits the supply voltage on the LM555 & U2?
I think the LM555 has good output drive. Do you need the 4099? Why not drive with all 6 gates in parallel? Normally R4 is on the Gate.

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Thank you for the informative replies.
Just for reference, here is the original source of the schematic.
I thought I had reproduced it in KiCAD faithfully, but I may have made mistakes.
Are the issues/mistakes pointed out also visible in the original source schematic?
Did I correctly layout the schematic in KiCAD ?

Hi @RolliePollie,

The 555 has a maximum output of about 200mA. Each output of a 4049 has about 3.5mA.

IF my memory serves me correctly, the dodge of paralleling cmos outputs only worked for the original (circa 1970s) A series chips. Doesn’t work for the B series and all subsequent types.

There was never a 4049 A series. It was a 4009 and it was a failure.

Another problem with this circuit is cmos outputs need current limiting protection when connected to real world devices such as transistors.

I hope these comments help.

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In this case, what type of pin would be best for this to suppress this error?
Also, if I edit the pin type, it wants to make this change for the device in the Library, not just the compnent in my schematic?


You can set the outputs to Bi or Passive to stop this error. I think you should just leave it alone. You don’t have to “fix the error”. I think you can not save to the library you will have to make a library and save it to your library.

On the “original source”;
Again the 4049 is not necessary.
It looks like you have a 12V 2A power supply which will not charge a car battery to 14.5V.
There is no current limit.
There is no over voltage limit. If you remove the battery the voltage will go up until it kills the IRF3205. You must turn off the 12V supply before unplugging the battery.
I do not like the circuit and most likely David Baker never tried it.
L1 must be large with a current rating well above the 2.5A of the fuse.

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Thanks for the insight Ron. Much appreciated.

Here is a link to a boost power supply. I have not used it but it might work. I think it will take your 12V or as little as 10V and boost it to another higher voltage. It has adjustable voltage output that should be set to 14.5 volts assuming a car type battery. It also has adjustable current limit that could be set to 2A or there about.

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Thanks Ron.
For battery desulfation I think the key is a “pulse” that rings through the battery to dissolve sulfation off the plates So I need more than just a Boost converter

As I browsed the internet, I stumbled across this which is another write up on the same circuit…
Here, it is in use by an aviation company.


This description looks being not based on 100% true.
And the construction (buffering 555 with weaker (then 555) buffer) looks being designed by someone having little electronic experience.
For me subject of dissolving sulfation by pulse loading was common in 1970+ years. Later I didn’t noticed it.

Regarding the output conflict, I had a very similar issue in a board I designed, and what I did was put a zero-ohm resistor on the output pin and the error went away.

The net tie suggested by another poster will have the same effect.

I also remember this. Unfortunately the only way to diagnose the problem correctly with the battery meant destroying the case, thereby rendering the battery useless.

There is an amazing amount of useful information on the internet.
There is also an awful lot of rubbish.

But the zero-ohm resistor is real element needing space at PCB and being assembled.
My solution is different - I never run ERC and have no problem with its errors and need not PWR_Flags at my schematics.

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