Notifications
Clear all

Melted 15A Fuse on Mini-RAMBO 1.3 Heat Bed  

  RSS
JS_SunnyFlorida
(@js_sunnyflorida)
Active Member
Melted 15A Fuse on Mini-RAMBO 1.3 Heat Bed

I have already been through the melted Molex heat bed connector problem, I desoldered the connector on the board and soldered the wires directly to the board. No problems for the last year of printing.  A couple of days ago I had a heat bed runaway error about half way through an ASA print (8 hour run time) that was running with a bed temperature of  105 degrees in an enclosure. Found no power to the heat bed, so I checked the mini-Rambo board and saw the 15A fuse had melted in half. I have not seen any topics on this, everything has to do with melting the Molex connector.

Looking for some corrective actions:

1. Would piping cooling air from outside the enclosure directly on the mini Rambo help? It is pretty hot in the enclosure, so I presume the board is hot and that is contributing to higher current being needed to maintain bed temperature. Melting the plastic case of the fuse takes a lot of heat.

2. Would adding a relay activated the mini Rambo heat bed power out help reduce the current/heat on the control circuit?  I could add another set of 12V wires from the power supply to run the heat bed through the relay. Watching the heat bed LED, it appears the mini Rambo driver is pulsing the bed heater as it reaches desired temperature, so a solid state relay might work better than a mechanical one.  I have already moved the power supply outside the enclosure to keep it cool. 

3. If the relay idea is feasible, would it work better if I used 24V to run the heat bed (while using mini-Rambo 12V for relay activation)? That would cut the current in half.

Any comments or other suggestions are welcome. 

 

Posted : 15/08/2021 1:23 am
JS_SunnyFlorida
(@js_sunnyflorida)
Active Member
Topic starter answered:
Melted Mini-RAMBO 15A Heat Bed Fuse

Photo of melted fuse, apparently it was hot enough to also melt some of the fuse holder. Board does not look damaged beyond the slight bulge in the fuse holder. I was able to clean up the fuse contacts.

Posted : 16/08/2021 12:33 am
JS_SunnyFlorida
(@js_sunnyflorida)
Active Member
Topic starter answered:
Melted 15A heat bed fuse

Posted : 16/08/2021 12:34 am
Diem
 Diem
(@diem)
Noble Member

STOP.

Something is pulling more than 15 amps.  Probably a lot more considering the damage.

Check resistance on the bed heater, check the heater power lines are not grounding through the chassis.

Check your thermistor, if it is faulty it may be calling for too much power.

That fuse is there to stop you setting fire to your workshop.

Cheerio,

Posted : 16/08/2021 4:07 pm
FlyRV
(@flyrv)
New Member
Melted 15A Fuse on Mini-RAMBO 1.3 Heat Bed

The fuse did not open from excess current (more than 15 amps), it got hot enough over a few hours to melt the plastic case, similar to the Molex connector melting on the same circuit. It appears only one leg of fuse heated up enough to melt the case, perhaps caused by resistance between the fuse blade and the socket contact surface. 

Those using enclosures are moving the power supply outside the enclosure for cooler operating temperatures and longer operating life. I have not seen the Mini-Rambo likewise being moved outside the enclosure for a cooler operating environment. I have not checked the operating temp of the Mini-Rambo, but running an 8 or 10 hour print with heatbed temps of 105 degrees in an enclosure is sure to be pushing the circuitry past it's design temperature rating (Typically 45-50C standard and 70-75C for industrial rated electronics).    

Posted : 17/08/2021 2:49 pm
FlyRV
(@flyrv)
New Member
Melted 15A Fuse on Mini-RAMBO 1.3 Heat Bed

The fuse did not open from excess current (more than 15 amps), it got hot enough over a few hours to melt the plastic case, similar to the Molex connector melting on the same circuit. It appears only one leg of fuse heated up enough to melt the case, perhaps caused by resistance between the fuse blade and the socket contact surface. 

Those using enclosures are moving the power supply outside the enclosure for cooler operating temperatures and longer operating life. I have not seen the Mini-Rambo likewise being moved outside the enclosure for a cooler operating environment. I have not checked the operating temp of the Mini-Rambo, but running an 8 or 10 hour print with heatbed temps of 105 degrees in an enclosure is sure to be pushing the circuitry past it's design temperature rating (Typically 45-50C standard and 70-75C for industrial rated electronics).    

Posted : 17/08/2021 2:49 pm
JS_SunnyFlorida
(@js_sunnyflorida)
Active Member
Topic starter answered:
Melted Mini-RAMBO 15A Heat Bed Fuse - Fixed

Just a follow up to my previous post. After cleaning the fuse socket contacts and inserting a new 15A fuse, I noted the left side did not provide much resistance to inserting the fuse blades. I was able to slightly bend the socket contacts closer together to provide a much improved blade contact tension on the fuse. After reinstalling, it appears the Mini-Rambo is functioning normally and the fuse is showing no sign of thermal problems with bed temps at 110 degrees for a 6 hour print. For good measure I replaced the heat bed wires with 14 ga wire and increased the power supply output voltage to 12.5v under full load. Also prompted me to buy a spare Mini-Rambo 1.3 while they are still available.     

Posted : 23/08/2021 10:56 pm
Chicago Keri
(@chicago-keri)
Trusted Member
The nature of fuses

Good idea to buy a spare MiniRambo board while they're available. The 12v. heated bed draws quite a bit of current for it to handle. You might want to consider an EinsyRambo instead, as I'll explain.

Fuses are thermal devices, that is they work by getting hot enough to melt the conductive link. Under normal circumstances, a 15A fuse will start getting very warm at or above 15 amps. Over an extended period, it will melt the link at around 15 amps. Because of this, a fuse is normally sized roughly about double of the expected continuous load so as to keep the temperature low. If considerably more current than the rating is passed, it will melt very quickly, or "blow".  If you were to run a 12v bed on 24v, it would double the current, not halve it. To halve the current you would need to install a 24v heated bed along with the 24v power supply, see below..

On a 12v Mk 52 heated bed as used on a Prusa Mk2.5/s, the normal current draw is about 12 amps. This is unusually close to the fuses' rated current and will cause the 15a fuse to heat up considerably over an extended period.  Put it in a heated enclosure and is is extremely likely to melt at some point. Any loose or dirty connection will also generate heat so the fuse contacts as well as the connectors must be checked regularly.

More usually, a 20a fuse would be used for a 12a load but this is probably more current than the MiniRambo was designed to handle. 

The best solution is to upgrade to a 24v system, where much smaller currents are used. Unfortunately, this means that the power supply, hotend heater and heated bed need to be replaced.  This is complicated, because the 24v heated bed calibration points are not compatible with Mk2s - Mk2.5S standard firmware, so you would have to mod the firmware or install an EinsyRambo and just make a Mk3s out of it.

¡no entiendo Español!
Nein! Nicht Versteh!
Я немного говорю по-русски но не очень хорошо, и...
I'm not very good at English either! Maybe someday I'll find a language I'm good at?...
Posted : 30/08/2021 12:31 am
JS_SunnyFlorida
(@js_sunnyflorida)
Active Member
Topic starter answered:
The nature of fuses

I considered the 24v route after seeing the Mini-Rambo 1.3A will take an input voltage from 12V to 35V, so that would only require changing the heatbed and nozzle heater to 24v versions. It appears the MOSFETs that drive the heatbed can handle far more current than the 15A fuse would allow, however I am not sure what the circuit board traces will support. It appears the heatbed MOLEX connector is a weak spot based on the number of melted connectors reported. 

The cost to upgrade to the EinsyRambo and other Mk3 parts is almost the cost of a new MK3, which is why PRUSA never offered the 24v upgrade to the MK2S that I started with. My current MK2.5S MMU2S was upgraded from a MK2S with the MMU1, so I think it is as good as it will ever be for a 4-year old printer.   

Posted : 30/08/2021 3:33 pm
Share: