Author Topic: uCurrent Tiny Trace  (Read 369 times)

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Offline steve1515

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uCurrent Tiny Trace
« on: September 19, 2019, 06:23:18 pm »
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?  :)

 

Offline TheHolyHorse

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Re: uCurrent Tiny Trace
« Reply #1 on: September 19, 2019, 06:26:19 pm »
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.
 

Offline steve1515

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Re: uCurrent Tiny Trace
« Reply #2 on: September 19, 2019, 06:30:44 pm »
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.
 

Offline thm_w

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Re: uCurrent Tiny Trace
« Reply #3 on: September 19, 2019, 11:20:44 pm »
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
 
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Offline steve1515

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Re: uCurrent Tiny Trace
« Reply #4 on: September 19, 2019, 11:35:19 pm »
Ah! That makes perfect sense!

Thanks!  :)
 

Offline TheUnnamedNewbie

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Re: uCurrent Tiny Trace
« Reply #5 on: September 20, 2019, 05:54:56 am »
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.
The best part about magic is when it stops being magic and becomes science instead
 

Offline thm_w

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Re: uCurrent Tiny Trace
« Reply #6 on: September 20, 2019, 09:58:32 pm »
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.
 

Online Kleinstein

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Re: uCurrent Tiny Trace
« Reply #7 on: September 21, 2019, 09:55:10 am »
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.
 


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