Extruder polyfuse getting very hot and stopping print

I've been using my 3d printer for about 6 months, with the extruder heating up perfectly fine. Suddenly, it has stopped heating in the middle of a print, i checked the 5amp polyfuse and it was extremely hot. When it cooled down again, it was fine. Now it has been triggering very fast, which means heating up much faster than it ever did. I don't know what to do next, because everywhere, people are having trouble with the 11 amp fuse. Im using a 12v 15a power supply.

  1. Would it be wise to remove the fuse completely, and bridge the gap?
  2. If removing it is not a great option, then with what do i replace it? 5amp Polyfuses aren't available.
How can i fix this?


  • -1 bridging is opposte of wise
    -2 check your heaters and motor drivers, usually fuse is not the problem

    does heater voltage match the power supply voltage?

    (usually 12 V heaters are around 6 OHms , 24V Heaters are around 12..13 Ohms)
  • Hi,
    yes my heater voltage is 12v which matches with my power supply. I read that the more the fuse is triggered, the more susceptible it becomes to being triggered again faster. Is it the fuse? Do i need to replace it with an automative fuse? If so, would a 5A one be fine?
    Or what should i check to see whats going wrong?
    thanks in advance:)
  • do you have the possibility to measure the input current ?

  • The input from the power supply is 12 V 15A. Or did you mean the current drawn from the loads?
  • yes, you Need to measure the drawn current , otherwise you cannot see if the error is located at the Fuse or Heater

    or any other component.

    just straight forward toubleshooting

  • edited August 2017
    I'm sorry, I'm a complete beginner, could you tell me how i measure that? I'm assuming with my multimeter. Do i need to measure the resistance first, and by using V=IR, calculate the amperage based on the amount of voltage im supplying?
    Also, I need to measure the current drawn from the motors etc. because if I've understood correctly, the fuse controls the flow of current to all of those loads. Could you please tell me how I measure that?
    Sorry again. 
  • first of all measure the inrush current with your multimeter (select DC Amps).


    Fuse is a safety element, it just disconnects when current is too high.

  • My heater draws 2.88 amps in the beginning which slowly drops. So the inrush current is 2.88. Feels kind of high.
    But the fuse has other loads attached to it, like stepper motors. How do I measure the draw from those loads? 
  • measure inrush current at RAMPS , the input of the 5 Amps Polyfuse..... we want to know the complete current,
    not only heater
  • So basically, i switch on the main power to the RAMPS board, and then measure the current passing through the 5A fuse with nothing switched on? That means hooking each multimeter lead to each of the ends of the fuse? Sorry, again, im a complete beginner.
  • You would have to remove the fuse to do it that way.

    remove the +12 volt wire from the connector, then with meter set to dc AMPS range, connect the meter red wire to the 12v wire from power supply and the meter black wire to the +12 i/p of the connector
  • So i connected the red lead of the multimeter to the +12v on the power supply, and then connected the black lead of the multimeter to the main +12v power input on ramps. 
    The current draw (with no load turned on) is 0.16 amps. I got this on my multimeter when i switched on power.
  • Does this mean there's a short? Or there's a problem with the fuse?
  • I tried disconnecting all the loads, and found that the 0.16 amps were being drawn by the RAMPS board.
    Can someone please tell me if that's normal?
  • hook it up as martinh wrote including all loads and measure again, then you can see whole inrush current as i wrote before.
    if current is below the 5 amps your fuse might be bad , if current is more than 5 amps the problem is located some
    where else
  • I did that, I hooked it up like that with all the loads, (none of them switched on of course). Only the main power was on, and my multimeter read that it was drawing 0.16 amps. Do i need to measure this with the loads switched on also?
  • YES, that´s the point, all loads ON
  • Ok, did that, the maximum current draw thats happening is 4.3 amps.
  • so fuse should not cut off...
    but one question. did you change anything before this issue started?
    f.e. changed housing of electronics, removed blower etc.?
    btw do you have a board fan?
  • No i changed nothing, and i never had a board fan. Would it be a good idea to set one up? Now that i know its drawing a safe amount of current below 5 amps.
  • edited August 2017
    think solution for you is to replace the fuse.
    if you cannot get such a polyfuse ,replacement by fuse type as used in cars would be possible.
    its done also in some RAMPS Variations.

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