Look at the nice color on this polyfuse. This one won’t be giving me problems anymore. I removed both fuses and shorted them out. I should have done this a year ago I save me time.
There is already an automotive fuse on the 12v line. How important is a fuse on the 5v line? The psu shuts down immediately if shorted.
The 12 volt fuse will not protect you 5 volt power converter. or psu (don’t know your controller board)
How much current will the PSU short-detection or main 12v fuse allow through the load that used to have a polyfuse? Will that much current melt wiring insulation and light something on fire?
It’s a ramps board. I am not sure how much current will force a psu shutdown, but I can tell you that shorting the D9 screw terminal to the heatsink on Q3 with a screw driver didn’t blow anything up…
It’s not a 5V line. It’s a 5A line. The 5A line on Ramps runs the motors and extruder(s)/fan outputs.
Really? I assumed that the 11a fuse was all the 12v and the 5a line was the 5v. If you are correct then the inline 12v fuse should be sufficient for my needs.
@Mark_Rehorst , I am trusting the psu and the automotive fuse on the 12v.
I did just learn that shorting 5v to ground on the servo ports doesn’t cause a psu reset. I does cause ramps to restart as expected.
PTC fuses are very unreliable for the use they are being put to on the RAMPS board. In fact they cause more problems than they fix. The guy behind RAMPS (who has stopped working on RAMPS) no longer uses PTC fuses in his controller board designs (which should give you a hint).
Just to fill in some background here:
PTC fuses take AGES to trip. As in, at high currents (like >22A in an 11A PTC) it can take well over 20 seconds to trip (ie: to cut off supply). If your wiring or board can’t handle the possible current you’re pulling via a short circuit, then it’s going to fail anyway.
PTC devices work on heat generated internal to the device, meaning that not only does it take time to heat up (to trip), but that the ambient temperature actually determines how long before. and at what current they trip at.
Once they have tripped, they start to cool down. Once they have cooled down sufficiently, they will restore the connection, leading to it heating again. The hysteresis on this is usually on the order of every 2-3 seconds, so if your system does survive the first short, you will then end up with that happening on and off every 2 seconds. This on/off cycling may actually cause further problems.
Should you use separate fuses on your 5A and 11A inputs? It’s probably best. If you blow the fuse on the 11A input while printing, you might be able to swap it before the bed cools too much. That said, as long as your bed and motors/extruder all run at the same voltage, as long as BOTH inputs to the board run off the same fuse, it shouldn’t be too much of a problem.
Note: The only reason they’re marked 5A and 11A is the ratings on those PTC fuses. The tracks on the 5A side will handle more than 5A, and the same goes for the tracks on the 11A side. That said, you may find that the terminals definitely will not go that high as many (but not all) cheaper boards skimp on quality connectors.
@Stuart_Young , great summary. I believe my problem arose some time ago when the fuses may have been touching each other. The threshold current just got lower and lower with each trip. I patched it with a fan for a while but that recently stopped working.
Lastly: If you put 55A+ through the 11A PTC, you can pretty much expect the thing to catch fire. The internal heat generated is so great that before the connection is broken, the device will burn.
This means if you’re using a high output supply (like many decent gaming PC PSU’s) that can supply this sort of current, that a short may indeed cause the PTC to catch fire. And god forbid if you’re using a large battery to run your printer, as then you have no current limiting at all.
@Matt_Harrington Yeah, since they’re heat driven, and the 11A one does generate more heat than the 5A one, if they’re touching, the heat from the 11A one can cause the 5A one to trip too early. Fans “help” (for some definition of “help” that I wasn’t previously aware of), but they don’t solve the problem.
Automotive fuses are very appealing as they are cheap and (usually) obvious if blown. As for the 5v and damage. I found out the hard way that a ceramic screwdriver is a very good idea for
tuning the pots on the drivers, when previously using a metal screwdriver it slipped of the pots shorted somewhere for all if 1/10 of a second and popped my Arduino. That I believe was on the 5v circuit. As for fire hazard the closest Ive had was when the thermistor came out of the heat block so heater went on and actually melted the PTFE liner in the barrel
So @Stuart_Young and @Matt_Harrington would you recommend replacing the PTC fuses with car or other board mounted fuses, and if so what rating would you suggest for each line? Thanks
@Richard_Earl I would use a 15A fuse for the heated bed (though it depends on how much current you bed pulls, but a lot of 12V 200mm x 200mm beds can pull up to 12-13A on average) and 5A for the motors/extruder (if you have more than one extruder, I would look at 7.5A).
@Stuart_Young , thanks for the info.
good painting
