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I'm having some issues with a PCB in an alarm unit. I think the unit shorted. I'm now trying to identify the broken component to replace it.

The board is mains powered, but also has a battery - attached in the pictures red and black wires - and in the reverse picture positive terminal with the square box.

There is a rectangular component between the two terminals - in the picture this is two vertical silver strips with a black middle section running between the bat + and bat- terminal. No conductance between top and bottom on multimeter. If I touch this - the unit sounds making me think this may previously have had a length of wire between the terminals that has burnt out.

I tried putting a length of wire between the two terminals pictured and circled - and the unit come back to life.

Should I solder a bypass or is this potentially dangerous on a 6V battery supply and mains?

I'm confident nothing else is wrong downstream - I know what caused the short and this has been fixed.

  1. Is this a fuse or something else?
  2. Can it be replaced?

PCB front side:

enter image description here

PCB back side:

enter image description here

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  • \$\begingroup\$ You lost me on "there is a track...". Between what terminals? What silver strips? What middle section? Mark everything on the pictures please. And if you touch it and it makes sound, it means it's powered. If there were a short, it wouldn't be able to beep regardless of where you touch it. ( ͡° ͜ʖ ͡°) \$\endgroup\$
    – Ilya
    Mar 14, 2022 at 15:21
  • \$\begingroup\$ It would help a lot if the thing you are talking about is in focus. Right now it's hard to tell what it is or was. Sometimes PCB traces are used as a cheap replacement for a fuse - but for those you would not need the silver strips - it could be a component unsoldered itself - was there something loose flying around when you took it apart? Usually the fuse blowing is a sign that something downstream is broken. \$\endgroup\$
    – Arsenal
    Mar 14, 2022 at 15:30
  • \$\begingroup\$ I have edited the original with a clearer circled image of the component i cannot identify. I've added notes to the original post \$\endgroup\$
    – RichHalliwell
    Mar 14, 2022 at 15:37

1 Answer 1

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It does look like that trace connected to the battery (+) (I think) burned up and lifted/separated. The solder mask is discolored (and blurry) and if you pick at it with a pair of tweezers (battery disconnected, of course) it probably is no longer laminated to the substrate.

Edit: with the better photo it definitely looks like that.

You can replace it with a real fuse soldered between the two vias. Deciding the required fuse rating is an issue, but anything in the 1A range is probably about right and at least is better than just shorting the vias. Assuming it's a low voltage (6, 12, 24V) battery, the voltage rating is not important and 100A interrupt capacity should be sufficient.

There's also the issue of why it burned up in the first place. If you have no idea how that happened, that's an issue, but perhaps something known happened to it such as a wandering conductive tool in the course of troubleshooting a dead/dying battery or something of that ilk.

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  • \$\begingroup\$ Hi - thank you for the answer. I've noticed in some other places people replace with single strands of wire or similar - is this an option? Can I bypass this fuse all together and just solder a direct connection. The battery hooked up is 6V1.2Ah and the circuit seems to draw almost no amps \$\endgroup\$
    – RichHalliwell
    Mar 14, 2022 at 16:00
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    \$\begingroup\$ Yes, you can, but it would be a very thin wire strand to fuse at 1A, so you might lose some protection and something else will break first. \$\endgroup\$
    – Spehro Pefhany
    Mar 14, 2022 at 16:29

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