Is my 5A polyfuse on my RAMPS board bad?

  • My printer stopped printing during a few prints, and i found that the extruder had stopped heating, and the motors had stopped running. I checked the code, and nothing was wrong. My 5A fuse though, was extremely hot. I wanted to verify whether it was my fuse that had turned bad or there was some kind of short in my circuitry.
    With the power switched on, none of my appliances drew any current. However, the RAMPS board drew about 0.16 amps.
    Is that normal?
    If that is normal, does it mean that my fuse needs replacement? Because none of my loads seemed to draw unnecessary current.
    Thanks in advance.

    RAMPS should draw some current as it has Arduino, drivers and LEDs attached to it. If the fuse gets too hot, most likely it's broken. Better replace it with normal replaceable fuse kit.

  • As @Mikhail Z commented, it does sound like the fuse may be bad.

    The first thing to do is put an ohmmeter across the fuse (with power off!) -- if you get high resistance the fuse is definitely bad. However, if you get low resistance that does not prove the fuse is good -- see @Tom's comments below re. polyfuses in particular, and how to disconnect from the rest of the circuit.

    If you don't get lucky testing a fuse in-line, remove it and put the ohmmeter on it in isolation. Whether good or bad, it's good to put in a fuse-holder or socket, so you never have to de-solder the fuse again.

    Some boards use auto-resetting fuses or circuit breakers, which might have more complicated ways of failing (you can always replace the part to be sure). I personally avoid auto-resetting for anything that supplies heaters; if there's a problem I want to intervene rather than letting it try again endlessly.

    Since the heaters and the motors are both down, it's a good bet it's the fuse or something very early (that is, "near" the power supply). If it were a single motor or single heater, then the output control (typically a solid-state relay, or perhaps the logic controlling it) would be a better bet. Though unlikely, it's possible for two or more such controls to fail at once, so don't rule that out completely.

    Let us know what you discover.

    Instead of taking the fuse out of circuit (which requires desoldering it), you can simply unplug the terminal with the 4 power wires. The fuse just connects the power input to the rest of the circuit, so if you unplug the power supply the fuse is effectively out of circuit.

    Also, odds are he'll get a low resistance reading on the fuse anyway. It's a PTC fuse. They increase their resistance as they get hotter, and as they increase their resistance they get hotter. They're designed to stay cool when operated at under their rated current, and thermally run away when you try to draw too much current. If you measure the fuse when it's cold, it will have low resistance. It's possible either the fuse is faulty (and trips below its rated current) or the motors/heaters are drawing too much current. You can't tell by measuring the resistance because it's a resettable fuse.

  • Its very likely the polyfuse is bad. In an ideal printer, given the heater cores and great deal of power, polyfuses are some of the worst things you can use. Polyfuses have a tendency to fail in "interesting" ways, especially around their trigger amperage. Fire is one of those failure modes.

    Not only that, but if this is a noname chinese RAMPS 1.4, then you're also looking at 1/2 oz pour (it should be 1 oz or more for power traces) for the circuit board and cheaping on everything possible. I've a few boards like this that are a firetrap, along with a badly poured PCB heated bed. In that case, I would consider getting a better controller. If that's outside of your budget there is another way to do this.

    Unsolder the polyfuses and put wire between them. Normally, this is unprotecting yourself. But we're going to fix that. Head to the local automotive shop and get yourself inline fuses (think of 2 wires with a fuse in a pill shaped device). You want a 5A and a 11A fuse. Or if you did the math and know better, get the fuses you calculated for. Now, make sure they're inline with the + side of the power.

    You now have external fuses that you know are rated for the appropriate amperage, unlike polyfuses.

    It is not entirely clear to me how zou want to use the 5A AND the 11A fuse. I think it is rather dangerous to give advice about fuses on a controller board without being totally clear how things should be done, especially because most of the questions in this field already show that the knowledge about the matter is not too deep. It creates a similar kind of firetraps you've been talking about without people realizing it. My downvote will be removed if the (in principle good answer) is a safe answer.

    Right now, doing nothing, a RAMPS with the polyfuses is **ALREADY** a fire hazard. I think I was also quite clear. You unsolder the polyfuses, and put heavier wires in their place. Then you go to the automotive store and get inline fuses to hook on the power inputs. So it looks like 12v->wire->fuse->wire->screw terminal for each 12v line going in.

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Content dated before 7/24/2021 11:53 AM

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