Troubleshooting short circuit in this design?

Thread Starter

bmbouter

Joined May 29, 2021
55
I have a board version of the attached schematic, and things are not working as expected. This builds on a previous design which worked well, which had the same subcircuits for the TP4056, the ESP12F, and the RT9013. This board has new circuits for the DTR/RTS MOSFET network (for 8266 programming), an ADC voltage divider (R11+R12), a voltage on/off switch at Q3, and the subcircuit on the bottom right which is meant to drive the enable pin of RT9013.

When I connect my FTDI to J1 the FTDI resets continuously, which occurs if the 3.3V and ground lines are the only ones connected to the board. I've tested the FTDI with other boards and it's working well. I don't even have to to program anything, when I touch the Gnd and 3.3V lines while the computer logs it says the FTDI disconnected (it's rebooting). In measuring connectivity between G and 3.3V from J1 I can see there is some kind of short circuit.

So these boards are toast, the only point of them now is to learn why they aren't working. I can desolder components to try to isolate the problem. What component would you recommend I desolder to learn more? Also do you see an issue with this schematic that would cause a short circuit of some kind?

Thanks!
 
Last edited:

WBahn

Joined Mar 31, 2012
27,509
Are these fabbed boards? Do you have a bare board? If so, check with an ohmmeter to see if the power and ground nets are shorted on the board. If so, there's the problem. If not, it's still possible that the particular board you populated has a flaw.

Have you carefully examined every component mounting to ensure that they are correct and that there aren't any solder bridges anywhere that could be causing the problem?
 

Thread Starter

bmbouter

Joined May 29, 2021
55
Thanks for the reply. To answer your question they are fabbed boards. The issue though wasn't the board, it actually was one of the wires between the FTDI and the flashing circuit. I think it was lightly connected and was basically debouncing the FTDI causing the laptop to effectively see it rebooting continously and very fast. When I did the FTDI swapping originally I didn't swap the cables!
 

MisterBill2

Joined Jan 23, 2018
13,206
A circuit shown as a bunch of snippets is really hard to follow, at least for some of us.
And a poor connection can cause a whole lot of problems. That is why resoldering every connection is sometimes the best fix.
How did you locate the "lightly connected" wire???
 

Thread Starter

bmbouter

Joined May 29, 2021
55
Regarding how I figured it out ... I reduced the number of connections between the FTDI and the board to determine which connections were the issue. At that point it was clear it occurred with as few as just ground and 3.3V connected. I then swapped with another FTDI but this time I (randomly I think) decided to use new wires with the second board and I couldn't reproduce the problem. After that swapping parts determined it was the wire. It's those female-to-female jumper kind so I think they can be loose at times. In this case it's a fabbed board so I couldn't really resolder anything (it's super tiny components).

I'd like to share my schematics in the most readable way possible. Any links to an example or advice of something that would be more readable?
 

MisterBill2

Joined Jan 23, 2018
13,206
There are a few schematic capture programs available, some much better than others. Some simulators produce a drawing in a format that demands the same simulator program to view it. Please avoid those.
 

Tonyr1084

Joined Sep 24, 2015
7,109
If in one cable you found a shorting incident then it's possible that the cable may have a wire not crimped inside the crimp. That stray wire strand could be touching another connector pin causing the short. I've seen that before, rare as it is.
 

MisterBill2

Joined Jan 23, 2018
13,206
Poor crimps and non-repairable connections are why I mostly solder the crimp terminals on, especially forthe MS type connectors, where the pin is attached to the wire and then inserted. That allows doing a quick repair when a cable bundle has one broken wire. And yes, I can solder that well so that the pins insert and remove as smoothly as properly crimped ones. But I do use a magnifier.
 
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