Recommendations for heater fuse and holder

deckingman

Following on from this thread, https://forum.duet3d.com/topic/6194/hot-end-maximum-wattage-possible it seems that heater cartridges are as likely to fail with low resistance as they are to fail open circuit or shorted. So my appropriately sized 40 Watt heater could potentially become an 80Watt or 100Watt heater, and thus a fire hazard should the MOSFET fail. I'm told that I need to use an appropriately sized fuse to protect against this eventuality. As I have one of the very first Ethernet boards, AFAIK, there aren't any fuses on the board itself. So I guess what I need is some sort of in-line fuse holder? As I have a tendency to swap hot end assemblies, with potentially different heaters, I'd like to use something that I can fit close to the heater cartridge so that the fuse goes with the heater rather than being something that I have to remember to change when I swap hot ends.

Any suggestions? Also, there seem to be a plethora of different fuses. What type of fuse would be best in this situation - i.e. quick blow or some other?

dragonn

I think a quick blow can lead to false triggers. Last time when I bought a new heater I measured his resistance before powering it first time and it was 2-3 times lower then nominal but after powering it for 1-2s from a 12 supply it get up to the nominal value. In this scenario a quick blow could already trigger.
I would install something like that http://www.optifuse.com/LargePhotos/LPX-04B-I_lg.jpg search for in-line fuse holder. And I would use an automotive fuse.

wilriker

@dragonn Based on your recommendation I stopped at my local electronics dealer today and bought to fuse holders similar to the one you linked. They were dead cheap (I paid 1€/holder - although with a discount) but they seem to be a bit bulkier and have no LED compared to the one you linked but I can live with that.

One question I still have: in case of PWM being switched on the ground side - which seems to be standard as I learned yesterday from @dc42 - does it matter if I put the fuse into the Vin or GND wire? Is there any recommendation on how to do it? My naive approach would be to insert it into Vin.

dragonn

@wilriker it would be better to put it on Vin. If you put it on GND and somehow the heater shorts to something else the current could find another way to flow (for example through the printers frame).
And it is high unlike that you printer frame has Vin on it instead GND.

wilriker

@dragonn That makes total sense! Thanks again.

And, yeah, I hope my printer frame is never connected to Vin (but since it should be grounded to protective earth - reminds me that I need to verify that - it would blow another "fuse").

wilriker

@dragonn One more thing I am a little worried about: these fuse holders have fairly large wires going in and coming out of - much larger than those I use for the hot end heater cartridge. Will I not create a high contact resistance at the connection points? Or doesn't this matter bacause the wires to the hotend are already appropriately sized?

dragonn

@wilriker That holders are just designed for much higher currents then or hotend have, but this isn't a problem if you put the right fuse into it. And yes, wires for hotend are appropriately sized. So no worry about that.

wilriker

@dragonn Thanks for confirming.

wilriker

And another question: what would be the appropriate rating for the hotend fuse? I just took the same 7.5A fuse as is now used with Duet 1.04 but then I realized that this also covers the steppers so it will probably be too large for just the hotend.

I do have a 12V 40W heater cartridge which is run with 13-13.8V effectively making it a 52W heater.
This evaluates to 3.83A current.

I think a 5A fuse would be better, right? Or even just as "low" as 4A?

dragonn

5 or 4.5A would be right. 4A could be to close. It could happen that you hotend draws ~4A when it is cold and starts heating up (resistance is lower when temp is lower).

wilriker

@dragonn Thanks. AFAICT there are no 4.5A automotive fuses so 5A it is.

dragonn

@wilriker didn't check if 4.5A exist but yes, 5A will be fine too.

deckingman

5A at 12v is 60 Watt. So the question that needs answering is, in the event of a MOSFET failure, will a 60 Watt heater on your printer get hot enough to cause a fire? The only way I know is to tune a 60 Watt heater and see what the firmware reports. In the thread that I linked to, people were saying that an appropriate size fuse would be 10 to 15 percent above the heater wattage. That's why I started this thread. So it seems that inline automotive fuse holders are NOT the answer if appropriate size fuses are not available for that type of holder.

resam

https://www.youtube.com/watch?v=qVjWg2vuWzk

wilriker

@deckingman said in Recommendations for heater fuse and holder:

5A at 12v is 60 Watt. So the question that needs answering is, in the event of a MOSFET failure, will a 60 Watt heater on your printer get hot enough to cause a fire? The only way I know is to tune a 60 Watt heater and see what the firmware reports.

In my case with 13.8V this would already nearly be a 70W heater. Anyway when PID tuning my heater with 12.6V already gave me a warning that if left on continuously it could reach something around 750-800°C which is more than "adequate" to start a fire.

In the thread that I linked to, people were saying that an appropriate size fuse would be 10 to 15 percent above the heater wattage. That's why I started this thread. So it seems that inline automotive fuse holders are NOT the answer if appropriate size fuses are not available for that type of holder.

I could reduce my voltage a little bit (to 13V max) and use a 4A fuse in this case. But of course this would be rather a workaround than a real solution.

wilriker

@resam In this video he uses a 19V PSU to power a 12V 40W heater which then becomes a 100W heater - but I assume this only makes the process faster not worse.

resam

I'm wrong - ignore this post.

wilriker

@resam I was also wrong it is even 109W.