Replacing the PTC Polyfuses

I haven't read many good things about the RAMPS polyfuses and people replace them with regular fuses so I'd like to do that. I think its simple but since I'm new to electronics I want to make sure.

Should I buy some of these automotive in-line fuses? In-line Car Truck Automotive Standard Blade Fuse Holder Fuseholder. Desolder the polyfuses, and solder the new fuses in? Anything to watch out for?

Thanks in advance.

Well I know it doesn't answer your question, but I have never had an issue with my polyfuses in over 2 years of operation. You can certainly replace with the fuses you are talking about, but if they blow, you will need to replace the fuse. The polyfuse, on the other hand, will reset.

Make sure your fuses are for the proper amperage - 5 and 11.

I agree. If you haven't even used the board yet there is no reason to change them - though you can if you want of course. The important thing, is to have a fan constantly blowing across the polyfuses and mosfets when using the printer.

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Waitaki 3D Printer

As someone who has endless issues with Polyfuses, I would recommend replacing them from the start like you're planning.

Using a fuse-holder like that will work fine.

The issues I constantly get with the Polyfuses are:

1. They don't really protect anything, since they take ~20 secs to trip. If your PSU doesn't shut down, other things are more likely to be damaged in that 20 secs. If you have a fairly capable PSU and a short, this can actually lead to the Polyfuse catching fire as there is too much energy to dissipate, even if they've got a fan pointing at them.

2. The 11A fuse for the heatbed is only rated to 16V (you can't get them at higher voltage ratings). This affects you a lot if you run 24V for the heatbed (or all your electronics).

3. The 11A fuse is VERY CLOSE to the limit for many heatbeds, and can unnecessarily trip (IMO this is a design flaw with RAMPS in general caused by using Polyfuses).

4. Now that we're getting new designs and people running multiple hot ends, the 5A Polyfuse is also very close to the limit, leading to people having weird issues

I would replace the 5A Polyfuse with a 7.5A automotive fuse, and the 11A Polyfuse with a 15A automotive fuse.

BTW: A lot of newer electronics have ditched Polyfuses in favour of real fuses at least for the heatbed. A perfect example of this is RAMBO (designed by the same person who created RAMPS), which has ditched the Polyfuse for the heatbed in place of a small SMD fuse. I really wish RUMBA would follow suit, plus some of the other electronics out there.

PS: RAMPS isn't really getting any more development. Some German ppl have released a version with real fuses in place of Polyfuses, but it's not made it as an official version yet.

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Cefiar
1. They don't really protect anything, since they take ~20 secs to trip. If your PSU doesn't shut down, other things are more likely to be damaged in that 20 secs. If you have a fairly capable PSU and a short, this can actually lead to the Polyfuse catching fire as there is too much energy to dissipate, even if they've got a fan pointing at them.

They do what they were designed to do. The amount of time it takes for them to trip is based on the amount of current being drawn through them at what voltage. If you are looking for overcurrent or complete short protection, then get a fast burning fuse. They are designed to protect from running excessive current than what the downstream circuit was designed to handle over time, much like a circuit breaker for a home would do. The 11 amp fuse for instance is designed to hold at 11 amps, meaning you can run 11 amps through it all day and it won't complain. When you run more than 11 amps, then the fuse will start to heat, increasing resistance and lowering current. As time progresses, if it continues to heat then resistance continues to increase until the circuit eventually opens, or current is sufficiently reduced to prevent overheating. But just like many other safety devices, there's limits to what it was designed to protect.

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2. The 11A fuse for the heatbed is only rated to 16V (you can't get them at higher voltage ratings). This affects you a lot if you run 24V for the heatbed (or all your electronics).

Sure they are available at higher voltages, just usually with lower current ratings. Polyfuses operate by the power (voltage * current) being drawn through them so you can't really keep them the same size while increasing the voltage without lowering the current rating. But there are options. If you're going to run 24v, your current needs are halved for the same power, so this one should be more than suitable.

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3. The 11A fuse is VERY CLOSE to the limit for many heatbeds, and can unnecessarily trip (IMO this is a design flaw with RAMPS in general caused by using Polyfuses).

It's not a design flaw. It's operating as design. My Honda can't tow a semi trailer. That doesn't make my car a flawed design, it makes it not designed for what you're asking it to do.

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4. Now that we're getting new designs and people running multiple hot ends, the 5A Polyfuse is also very close to the limit, leading to people having weird issues

Again, you're asking more than what it was designed for.

This is how I replaced the 11A polyfuse: [www.thingiverse.com]

crdu: While I appreciate what you're saying about it not being designed to do these things, it's immaterial if people ARE using it to do these things. What I am trying to point out is that they SHOULDN'T be using it to do these things, and that given that a lot of designs now almost rely on these extra features, that either it should be replaced, or updated.

IMO the polyfuse was a BAD choice for the RAMPS design (at the very least for the heatbed). I'm not saying that polyfuses don't have their place, but I personally believe that the choice of using them on RAMPS was a bad design choice for RAMPS, as in this application they don't provide suitable protection, and in some cases can be downright dangerous.