I need a battery balance circuit, that I will either buy or design. I have seen serpentine routing between the load resistor and switching component in these photos (from china). In addition, it is not on the gate pin.

Could you please give some information about that?

I checked this question:Purpose of "wave shaped" PCB traces

I don't think I need "delay" and "Equalisation of length of pairs of traces". What could be the purpose of this routing?

Thanks a lot.

Battery protection

Battery protection 2

  • 3
    \$\begingroup\$ I agree that it seems rather strange. \$\endgroup\$
    – user57037
    Dec 6, 2017 at 6:08
  • 5
    \$\begingroup\$ looks like an inductor or a really cheap fuse \$\endgroup\$
    – jsotola
    Dec 6, 2017 at 6:50
  • 1
    \$\begingroup\$ This is not a protection circuit, it's a balancer. \$\endgroup\$ Dec 6, 2017 at 7:55
  • 6
    \$\begingroup\$ Gah! That flipped resistor! \$\endgroup\$
    – pipe
    Dec 6, 2017 at 8:47
  • 1
    \$\begingroup\$ @pipe, one of the 100 Ohm resistors is flipped, too. \$\endgroup\$
    – user57037
    Dec 6, 2017 at 8:53

1 Answer 1


It is thermal decoupling of the relatively small transistor from the power resistor. Such that they can be physically close without risking the power resistor heating up the transistor and causing it's characteristics to change.

Guessing this is a MOSFET (the top mark A2xxx is too vague to be sure) The source-drain resistance increases with temperature, leading to a rise in power dissipation and hence thermal runnaway.

Preventing the initial external heating would help prevent the possibility.

  • \$\begingroup\$ I wonder how much the "meander" helps, considering that the source trace is thicker, straight and passes right by the power resistor.. \$\endgroup\$
    – Wesley Lee
    Dec 22, 2017 at 16:56
  • \$\begingroup\$ I could not think of a better reason (other than mechanical decoupling which would not make sense given other things on the board). As I don't know the parameters or the circuit design there is no way for me to model it. I think this one would be down to the experience of the original engineer. My guess is they got it wrong first time and then added the meander to fix the issue. \$\endgroup\$
    – Jay M
    Dec 22, 2017 at 17:16
  • 2
    \$\begingroup\$ Btw the fuse idea is daft. The sot23 is good for at least 100mA, whereas that size of copper could probably handle 30-40A for a short while or 5A+ indefinately. Much longer than the sot23 would last. \$\endgroup\$
    – Jay M
    Dec 22, 2017 at 17:33
  • \$\begingroup\$ Yeah, I'm not saying I disagree with you, just that it looks like a poor implementation of the idea. \$\endgroup\$
    – Wesley Lee
    Dec 23, 2017 at 2:03

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