RAMPS 1.4 fuse overheats within seconds. Do I have a bad fuse?

The problem arises when turning on the heated bed.

I took my multimeter and it shows that the two D8 screw terminals have a potential difference of 12.02 V, exactly as it should be. My heated bed has a resistance of 1.1 Ω (including the wires), again as it should be. This results in a current of about 11 amps, within the specification of the fuse. I'm also cooling the board with a fan.

Despite of this, the polyfuse overheats within 5 seconds of turning on the heated bed. It becomes far too hot to touch and it shuts off power to the heated bed. The three MOSFETs, however, remain cool.

I reduced the duty cycle to 33% by changing Marlin firmware settings. My multimeter now reads about 4 V and the current should be about 3.7 amps. Despite of this, the fuse STILL overheats, albeit rather slowly.

What is going on here? Has anyone ever had a similar problem? Should I replace my RAMPS 1.4 board (admittedly, it was the cheapest one I could find on eBay)?

I would very much appreciate your help

Edited 1 time(s). Last edit at 12/28/2014 03:39PM by Arjuna.

It sounds to me that either you have a short in the heated bed circuit that you haven't found yet, or your heated bed has a resistance lower than the value you measured, or the polyfuse is the wrong type. Here are some things to check:

1. What does your multimeter read when you short the two probes together? Subtract that reading from the 1.1 ohms you measured.

2. Follow the PCB traces on the RAMPS and locate which polyfuse feeds connector D8. On my RAMPS board it is the larger of the two fuses, and I guess this should generally be the case; but perhaps they were fitted the wrong way round on your board.

3. Can you read the part number on the polyfuse that feeds D8? If so, look up its datasheet and check its rating.

If you suspect the fuse and you can do basic soldering, you can replace it. I just found an 11A 16V polyfuse on Amazon UK for GBP3.81.

Edited 1 time(s). Last edit at 12/28/2014 05:08PM by dc42.

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Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Thanks for your reply!

It's indeed the bigger fuse of the two. The fuse is in the right spot, corresponding to what I've seen on the RepRap wiki. I Googled the part number and all I get is two Chinese websites. Not very useful.

However, I'm now 99% sure it's the fuse itself that's faulty.

I checked the current flowing through the bed and it's about 10 amps at 100% duty cycle, 5 amps at 50% duty cycle, 2 amps at 20% duty cycle, etc. Just as you would expect. The fuse horribly overheats, even at 50% duty cycle. It was literally the cheapest RAMPS 1.4 board I could find on eBay, so it should be no surprise that the manufacturer used cheap low-quality parts for it. Buy cheap, buy twice...

Hey Arjuna,
Just curious are you set up for PID or bang bang on the heat bed. I tried PID and my board acted the same way. Bang bang works fine.

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"To teach is to learn twice"

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maddane
Just curious are you set up for PID or bang bang on the heat bed. I tried PID and my board acted the same way. Bang bang works fine.

PID or bang bang shouldn't make a difference which initially turning it on. The difference would only come into play once the temperature comes near the target.

Polyfuses work by having a conductive polymer that changes resistance based on temperature. They are already imprecise components and getting very cheap just adds to quality issues. The 11 amp fuse on Ramps is right at the threshold for operating current so any flaw in the construction can be a big problem. I'd replace it with a new one or do as others have done and use an 15 amp automotive-style blade fuse.

Hi there,
i just found this thread and i need some help.

First i am new to this board (just read till now).

My Problem is that the F1 Fuse just broke in the middle.
Now i want to replace both with something better but dont know which one to use.
Maybe you can give me a link to Polyfuses that will fit on my RAMPS.

I have a 12V 29A power supply if that is necessary.

Nachbar

Edited 1 time(s). Last edit at 03/19/2015 03:51PM by Nachbar.

This is the 11 amp polyfuse: [www.digikey.com]
This is the 5 amp polyfuse: [www.digikey.com]

I can't give any recommendations for a fuse holder if you wanted to switch to automotive style blade fuses. Search fuseholder on digikey or ebay and see if you can find one that has the correct spacing to fit on the RAMPS board. Another option which you probably could find at a car parts store is inline fuses you can put on your supply wires and just bridge where the existing polyfuses are at.

Hello,

thank you for your answer.
I want to replace the existing Polyfuses with automotive fuses.

Wich fuses should i get? I have 32V 5A and 32V 10A here.
Can i use these two or should i get 12V 5A and 12V 10A?

Nachbar

For fuses, voltage doesn't matter as long as it's equal or greater than the voltage you expect to run at, although some margin of safety is nice. The 12V fuse probably is rated for more than 12V, but is marketed as an automotive fuse which typically is 12V, so to avoid confusing people, they underrate it. Since you likely are running right at 12V +/- some, I'd just play it safe and get the one that is explicitly rated at 32V.

I would also get a 15amp fuse instead of 10. A properly speced heated bed will draw 10+ amps. If you have a slightly out of spec bed, you'll draw more current than the fuse is rated for, blowing it, even though your RAMPS board is capable of handling it (up to about 15amps).

Hello cdru,

thanks for your answer.

I will go for 32V 5A and 15A ones.

Now i am looking for nice holders like on the RAMPS 1.4.2
Or i will just connect some cables or something like that.

But with the Fuses i was very unsure which ones i should get.
Now everything is clear thanks a lot.

Nachbar

Quote
Nachbar
Now i am looking for nice holders like on the RAMPS 1.4.2
Or i will just connect some cables or something like that.

Whatever you wish. Functionally, they will all do the same. You can get an inline fuse for your supply line, a solder in socket that mounts on the board, or a socket on leads that get soldered and the fuse kinda dangles.