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This is a post to warn (new) sunhokey people about the hazard of your MKS catching fire !!

Recently i saw a few posts about MKS boards catching fire!!


Through the connectors of the MKS is running high current.
Any shortcircuit or to high resistance of the wires (because of wires not fully stranded) can cause a failure/fire.

First thing you have to do while assembling / or after printing a while, is make sure your power is disconnected from mains and also remove the usb cable.
Now check "all" wires for loose strands. All wires need to be fully stranded, so when you strip your wire make sure you don't cut any copperwires (if so cut of that piece off and start over, make sure you don't cut your wire to short).
Next, when you stripped your wire, turn the ends, after that you can fold the wire once.
Don't tin your wires as seen in the picture below (for detailed info about that, read the comment of Gruvin below)

Make sure all wires are long enough to fit inside the connectors and don't stick out. Double check !!
This will solve a lot of problems.

Because your bed is a moving part, make sure the wires don't pull themselves out of the connectors or strands will break, by simply mounting the wires to the acrylic with some sticky clamp.

User solutions/recommendations (i am in NO way responsible for these solutions, do at your own risk):
1) Replace original wires from the heatbed to 16AWG silicone cables.
2) Direct soldering the wires, desolder connectors from MKS and solder directly to printboard.
3) Placing relay between MKS motherboard, powersuply and heatbed.

As Gruvin stated (see comments) make sure you check your terminals from time to time.

The best and safest solution for this problem is using a relay.
So current is flown through the relay instead of your motherboard, saving all parts connected to your motherboard.

This picture is not for your MKS board but it shows how the relay could be connected between your bed, power suply and MKS board. (relay is the white box left from to the power suply).

When using a relay make sure you buy one with correct specifications (rated for 12V DC (direct current) and minimal 20 Amps).

Good relays are:
Solid state relay (INPUT 3-32V DC, OUTPUT 5-60V DC 25A): http://imgur.com/DJxWCD2
When using a Solid State Relay, make sure you disable PID for your bed in your firmware and attach a cooling fan on the relay.

Car Relay (12V DC 20A (picture shows 40A version)): http://imgur.com/XiDi4Ra
When using a car relay make sure it has a diode across the coil: http://imgur.com/ptlUfP5
Connecting car relay: http://imgur.com/ZJ2ndGB

As this may be true, the fires are not caused by a bad or loose thermistor.

The possibility of 3D printer catching fire also depends on the version of firmware that is installed ! More recent firmwares are more advanced and have the additional protection measures – like against a thermistor coming off place. For example, below you can find a commit message 43c298a (dated Jun 30 2014) from a Marlin Firmware repository. My cheap Chinese 3D printer had a slightly older firmware version installed, so I had to update its’ firmware to enable this “Thermal Runaway Protection”. Always update a firmware of your 3D printer!

/*================== Thermal Runaway Protection ==============================
This is a feature to protect your printer from burn up in flames if it has a thermistor coming off place (this happened to a friend of mine recently and motivated me writing this feature).

The issue: If a thermistor come off, it will read a lower temperature than actual. The system will turn the heater on forever, burning up the filament and anything else around.

After the temperature reaches the target for the first time, this feature will start measuring for how long the current temperature stays below the target

If it stays longer than _PERIOD, it means the thermistor temperature cannot catch up with the target, so something may be wrong. Then, to be on the safe side, the system will he halt.

Bear in mind the count down will just start AFTER the first time the thermistor temperature is over the target, so you will have no problem if your extruder heater takes 2 minutes to hit the target on heating.*/

same thing for me !!


now I have to change the mother board .. . it is a Little bit tricky ...

Hmmm. I'd like to clear up some potential misconceptions around this topic -- maybe.

Assuming one is running less than 15A, this problem is actually NOT about the terminal block not being able to handle the current.

What actually happens is that the connection becomes slightly loose, over time. This causes high resistance and often sparking, which in turn generates heat, which in turn loosens the connection more, which makes more heat/sparks, which can then go on to melt the PCB solder, which makes another high resistance point and more heat and, well ... can eventually get so hot that something catches on fire. The problem can slowly form over months, otherwise going unnoticed.

This is the case with ANY connection running sufficient current to get hot when resistance goes above a few ohms.

Now, this is by no means unique to the MKS boards or the connectors they use. Besides, the MKS board has a fuse to prevent fire damage from excessive current! So, why the fire, then? Why doesn't the fuse just blow? Well because in fact, sufficient heat to cause fire can occur well below the fuse rating OR the connector's capabilities. It's all about loose connections, as explained above. Even 2 Amps could cause fire, if the connection is poor enough and conditions just right. Current here is only a part of the problem.

(To be fair, those terminal blocks have an absolute maximum rating of 15A (or 10A, depending where you read.) However, properly used, they will handle that and more, safely.)

Following are the two most common causes for such poor connections, not including soldering or part faults on the board itself. In other words, "user error" causes ...

  1. Over time, after the first time the screw is tightened down on the wire, the wire strands compress and reorganise slightly. This loosens the connection a little and increases its resistance, thus causing the junction to heat. You should always re-tighten screw based terminal connections periodically, until you are confident all such compression has done its thing. Once you are completely certain of that, wait a month and tighten them again!

  2. This one is very common in marine related devices and probably not related to the MKS board. But could be. It is that some well meaning fellow made the situation in 1. above much worse, by soldering the wire ends! (Just the wires.) Solder is soft! It puts lead/silver barriers between each wire strands and therefore tends to compress a LOT over time. And not much time, at that. NEVER SOLDER WIRES THAT GO INTO COMPRESSION / SCREW TERMINAL BLOCKS. If you are ever concerned about corrosion etc, use a crimp shroud or grease or other liquid barrier to protect the wires. Never solder. Even if there's no real current involved, the connection WILL fail, if the wire is soldered then screwed down.

That all spoken like a dictating know it all, I guess the point I'm really trying to make here is that just about ANY screw/compression based connection can get very hot if it comes just a little bit loose -- even at currents lower than 5A. And by the way, those high current solid state relays are NO EXCEPTION -- though they are better, over all, for other reasons. Look are their current rating though ... 10A? 20A for the really expensive ones? At the end of the day, 10-15A is simply no problem at all for a soundly connected terminal block, such as used on the MKS.

Play safe ... try not to assume and remember to ask your local know-it-all before making any assumptions! :-)

Hi Gruvin,

Thank you for enlightening that, good info and lesson learned.

As i don't want to start a discussion and you probably now more about this part than i do, i want to mention that the reason i started this topic was to warn people about the danger of their printer catching fire.
There is a similar topic that warned me when i just got my printer about the danger of putting to long screws inside the power supply. Which saved me a lot of trouble.

Maybe the assumption made that the terminals where faulty is incorrect (thank you for pointing that out, i will change that in the main text). But isn't that what all people do, making assumptions based on (other peoples) experience/opinion?
And immediately asking all the know-it-all folks to inspect and analyse the problem, seems a bit off.

I just wanted to warn people as soon as possible as i saw more and more people showing pictures of their burned motherboard/printer. And i have to say, after this warning i haven't seen or heard anymore people having problems.

Also I feel a lot more secure using a SSR which has much bigger terminals which are placed further apart than the terminals on the motherboard etc. (even though that assumption maybe faulty too ;-))

Thanks again for your support and giving such good info.

BTW, i just read the main text again but nowhere do i make the assumption that the terminals are bad.

Hi there. Thanks for the response.

I couldn't agree more with all your comments. SAFETY FIRST and shoot for the lowest common dominator. I mean, if beefing up the terminals or adding a good sized relays make anyone feel safer, then by all means please do it, right? ;-)

My response was not targeted specifically at you. Rather at the general vibe surrounding the topic on the interwebs ... and in fact in the real world.

No, you did not mention the terminals being bad. They are operating on the edge of their specifications however and many others hold the (rightly conservative) belief that they are underrated for the task. But again, I just wanted to dive into more of the real, "whys". Knowledge is power and I didn't learn it myself until some time in my thirties. ;-)

So my hope, like yours I am sure, is merely that people who stumble on our words here may find themselves better equipped to keep their printers safe and enjoyable.

So, well done! :-)

Well said !!

I indeed hope that people who stumble upon our words will take the danger in advantage and see the danger of letting their printer run and leave the room/house for example.

Because large prints can take a while, people tend to leave their printer running while going out the room/door.
Having seen the risks this printer brings, i never let my printer unattended.
I had a instance where my thermistor went broke/rogue while printing. It caused the heater to heat up without any limit (except meltdown). I first smelled some burned plastic, after that things started smoking.... Sitting close to the printer, it let me cut the power before things escalated.

So yeah, having a 3d printer is fun ..... have fun and be safe.

Thanks mig2007 i will add this to the main post.

i have an old 2014 version of sunhokey i3.I found that cable from PSU to driver board is not fully exposed ( only few copper wire connected), thus causes the cable overheat. I would recommend that check all the connector and cable connectivity to ensure good conductivity to reduce wire overheat.

My MKS went up in smoke, too. Same part of the board, and the failure somehow killed the power supply as well. It's a real problem.

I replaced it with arduino/RAMPS and up to date Marlin. Now running a TAZ 5, and it's as solid as can be.

I had this problem too.
Manifested by stripping wires from the bed.
Wires changed to pure copper l=500mm, S=2.5mm^2.
I bought at a price per 1m 17RUR
The problem disappeared.
The solution with a solid state relay is very good, but it requires additional investment.

People please be AWARE and be SAFE.

Thanks for the heads up hiker!

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