uCurrent Tiny Trace

[steve1515]

I was looking at Dave's uCurrent PCB and noticed the small trace pictured below. It's located in the middle of a much larger trace coming from the negative input. I'm sure there must be a good reason, but I don't know why. 

Can anyone explain the reason why this was done? 

[TheHolyHorse]

I've never looked at the schematic but it could just be a little trace to link two ground planes together . Without the schematic it could be pretty much anything.

[steve1515]

Here's the schematic: http://www.eevblog.com/files/uCurrentRev5schematic.pdf

I don't see any reason for the tiny trace to be there though. It's puzzling.

[thm_w]

Here's the schematic: http://www.eevblog.com/files/uCurrentRev5schematic.pdf

I don't see any reason for the tiny trace to be there though. It's puzzling.

Its a PCB trace fuse. The idea is if you have it on mA range, and put 20A through it, that trace will blow up and the current sense resistor or other parts of the board will not be damaged.

https://electronics.stackexchange.com/questions/200350/can-thin-sections-of-copper-traces-be-used-as-fuses

[steve1515]

Ah! That makes perfect sense!

Thanks! 

[TheUnnamedNewbie]

I think it is also simply a form of net-tie. Simply looking at the schematic, you can see that there is a net label -IN at the negative input, and it is then connected to VGND. The only way you can do that is with some form of net tie in between.

On second thought, it might not have to be a net tie. I think the 'net tie' is hidden in the (in my opinion) poorly drawn footprint of R1, which as 4-wire sense terminals.

[thm_w]

I think it is also simply a form of net-tie. Simply looking at the schematic, you can see that there is a net label -IN at the negative input, and it is then connected to VGND. The only way you can do that is with some form of net tie in between.

On second thought, it might not have to be a net tie. I think the 'net tie' is hidden in the (in my opinion) poorly drawn footprint of R1, which as 4-wire sense terminals.

Yeah since R1 is four pins they don't have to have the same net name, even if they are internally connected.
Unless I'm missing something, its not poorly drawn, its fairly typical to do it like this.

[Kleinstein]

I see 3 possible uses for this tiny thin spot:
1) as a testing point for pogo-pins
2) as possible point for an optional cut
3) to get a better defined current distribution to the near by shunt resistor. So moving contact at the terminal would not effect the shunt value.
    Normally I would consider the normal length of the trace sufficient - so kind of overkill (but cheap) for this purpose.

For a fuse the trace is still quite thick, and there are no protective diodes to really help the larger shunts to survive a high current.