The Nature of Fuses and the 12v Prusa Mk2s - Mk2.5s  

The Nature of Fuses and the 12v Prusa Mk2s - Mk2.5s

Warning! This post describes altering the circuit protection systems of Prusa 12v printers. This is posted entirely for information purposes only and I cannot recommend following my lead.  Do so entirely at your own risk, and of course it will void any warranty your machine might still be under.

Ah, our beloved Prusa MK2s - Mk2.5s (MK2X) printers! These are the printers that spread Prusa around the world by making high-quality, reliable and safe printers available at moderate cost.  And, as an exemplary company, Prusa made updating them to the latest configurations (within the machine's limits) not only possible, but easy and affordable.  

But alas, the Mk2X have an Achilles Heel, which was remedied on the subsequent Mk3X   and Mini models.... the relatively low voltage and high current 12v power system.  Although the Mk2X series printers typically run without issue for years, they are now all well, several years old and stuff is starting to fail.  Often, the issues occur in the heated bed system which represents the largest current draw at 12.5 Amps (A), but the 40w (3.3A)  hotend heater system is not immune. 

The usual issues are melted connector housing and terminals at the heated bed and power supply  connectors, often at the MiniRambo control board. It is very important to regularly inspect these connectors, especially the tightness of the small setscrews, which I have written a post about elsewhere in this forum.  

But today, I'd like to talk about the fuses. The MiniRambo has 2 fuses, a 5A fuse that runs the 3.3A hotend heater, the fans, the stepper motors and the MiniRambo itself. There is also a 15A fuse for the 12.5A heated bed.  In short, I feel that these fuse ratings are inadequate for the load.

I work in a field that includes rather a lot of electrical repair and have quite a lot of experience with fuses.  A fuse is intended to protect electrical circuits from overloads. It does this by having a small link made of a metal with a relatively low melting point.  As current increases to near the fuses' rating, the electrical resistance of the fuse causes it to become hotter and hotter until at some point over its rating, the link melts and interrupts the circuit, stopping current flow.  If the current flow is considerably over the rating, the fuse will melt very quickly, often with a "snap" or "pop" sound.  But if the current is at or near the rating the fuse gets extremely warm, even hot, just short of melting and with fairly high electrical resistance, which reduces the power available in the circuit. A 15A fuse, for example, might never blow at 16A or ever 17A depending on the rate at which heat is dissipated, and if it does, it might take a long time.

To avoid excess heating and large voltage drops, fuses are usually sized to about double the expected current load, that is if a 10A load is expected, a 20A fuse will usually be used.

A quick check of our  Prusa Mk2X printers reveals that the heated bed is expected to draw 12.5A and the hotend + everything else about 3.3 -4.5 A.  Normally for such loads, a 25A and a 10A fuse would be used.  But the fuses used are only 15A and 5A respectively.  A quick check with a FLIR device reveals that both fuses get quite hot, about 50C with the printer in preheat mode for 230C/80C.. at 25C ambient, and with the MinRambo cover open.

If the printer is working for many hours with the covers shut and high current draws as for ASA printing or in an enclosure with high internal temperatures, the fuses will get hotter and the heaters will receive lower voltages, impeding their performance. This also leads to the risk of melting the fuse body and the fuse connector housing. The fuses are undersized for such use.

The simple answer is merely to increase the fuses' ratings, and that is what I have done. My original 15A fuse was actually fairly discolored upon removal.  After installing a 20A and 7.5A fuse, their temperatures dropped from about 50C to around 32C under identical load. This appears to have reduced warmup times, especially for the heated bed.

But here is the problem: The engineers that designed the MiniRambo specified 15A and 5A fuses for good reasons.   A better choice would be to use a more robust control board or covert to 24v power (new 24v power supply, heated bed, hotend heater, fans) which would consume half the current and be well within the limits of 15A and 5A fuses.  As I did not want to under-protect the MiniRambo by too large a factor I chose 20A and 7.5A fuses instead of 25A and 10A ones.   

One final thing: Never, ever buy cheap fuses!  The cheap ones often fail to protect a circuit at all, even at double or triple the fuses' rating.

Good luck and Happy Printing!