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Why put two fuses of the same rating in series? Or am I mistaken?

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    #1

    Why put two fuses of the same rating in series? Or am I mistaken?

    Hello,

    This Razer Blade 15 2022 motherboard does not have a schematic. Razer also does not give any schematics or boardviews. Around the GPU VRM circuits I found these two fuses rated for 1.5A (from the letter K).

    From the PCB trace I can see that the two are connected. These two are not connected directly to ground. I believe therefore they are placed in series. I wonder why is this? Also judging from the size of the PCB trace, this carries 19V power.

    Thank you in advance

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  • Answer selected by gregory112 at 03-27-2025, 02:47 AM.
    Most likely this is a multilayer PCB and this common junction is feeding a hidden power plane. Yes, replace the fuse and test again. Perhaps you are lucky and the fuse saved you from a disaster.
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      #2
      No, trace each fuse back to a local ic. That IC is a regulator. So you are viewing 2 separate power rails and each has a protection fuse at the output and each output is tied together to drive a hefty current sucking load. Most likely these are multiphase power supply rails due to the high current demand by the load. Each rail will take turns turning on/off.

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        #3
        Thanks for the reply.

        So the fuses end connected to one with a PCB trace is the output right? I have a hard time understanding the trace as it does not lead anywhere, not even at the back of the board.

        The IC on the left as far as I know is MPS MP2940A VRM, and if I'm not mistaken, it is for the CPU. The top fuse seems to trace to the right side, the GPU circuits (judging by the number of VRAM chips around it).

        Nonetheless, the top fuse has blown. The big E7 capacitors near are not shorted. Because the PCB trace ends there without any connection to the back of the board, I am confused tracing it. I get that fuses don't get blown for no reason, well, I forgot to remove the battery and accidentally touched something with a screwdriver and I am trying to find if there are other shorted components near it. I am trying to find scenarios of what happened back then when I touched something and caused a spark, perhaps I touched something and it get shorted to ground, draws a lot of current, causing the fuse to blow. I intend to replace the fuse and try to plug it to power to test the voltage. Do you have any more ideas to test these?

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          #4
          Most likely this is a multilayer PCB and this common junction is feeding a hidden power plane. Yes, replace the fuse and test again. Perhaps you are lucky and the fuse saved you from a disaster.
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            #5
            Sure, I will replace it. I'll get back to it soon after I have done it.

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              #6
              Just want to update everyone on this forum just in case anyone is looking at this thread.

              Previously I thought because I sparked the motherboard with a screwdriver somewhere at the top, touching the heatsink, I suspect a huge current suddenly burst from the battery to the ground, killing one of the fuse. So far, I have only confirmed this fuse to be fauly, and other components nearby are normal (caps, ICs, and so on).

              I ended up just replacing the fuse and it works.

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