# Specific 3D Printers, Scanners, & Hardware > RepRap Format Printer Forum > MakerFarm Forum >  Heated Bed wiring meltdown

## adamfilip

I was getting a max temp error on my printer
When I investigated the  cause I found out that the relay terminal block was slightly burnt and  the wires coming from the heated bed that go into the Terminal block  were melted together with the therm sensor.

The Therm sensor is dead and I need a new one. I tried splicing in a new wires but it will no longer produce a reading.
The relay switch also needs replacement as I can no longer open and close it

update : i ended up changing the terminal block with something similar from my local electronics store and replacing the thermal sensor with one from my 2nd hotend.
so im back down to one hotend.. for now

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## adamfilip

BTW these are the wires that come presoldered to the heated bed element. and not the wires i added myself from the power supply

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## beerdart

Loose the little ​Relay and get one from the auto parts store.

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## sniffle

i had the saem thing happen adam, i just soldered wires directly to the PCB on one side and resoldered the terminal on the other to make sure it was soldered well...

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## N5QM

So, what do we expect the root cause of this is, the little relay or?

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## sniffle

Slightly loose wiring?... Dunno really

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## Stigern

I had the same happen to me, short story long.

Changed wires to 12AWG and got me a beefy 30A car relay!




> So, what do we expect the root cause of this is, the little relay or?


I believe the contact surface is too small in those green screw terminals.

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## printbus

> So, what do we expect the root cause of this is, the little relay or?


If the relay was the problem, the relay itself would be undergoing the meltdown.  

For everyone's reference - from the markings on the terminals at least on my relay board, we know the terminals are FCI series 12-762 parts.  A two terminal version is likely FCI 20020705-M021B01LF.  The part is rated for 30 amps.  The datasheet is a bit confusing on wire capacity - first saying it is good for up to 10 gauge wire, but then specifying the allowable circular mils (basically the area of the copper surface viewed on end) for stranded wire at 4 mm^2.  #12 AWG stranded wire (3.31 mm^2) is the largest common stranded wire that matches that spec. 

As stigern suggests, contacts in some of these screw wire-to-board terminals leave a lot to be desired.  Some, for example, seem better suited for solid wire than stranded wire. The contacts in these actually don't seem too bad - at least the wire strands can't "squish" out from under the screw - an issue I often have on smaller terminals.  

Especially with the 12-inch printers, we're borderline on a number of things here, based on a worst case load of around 30 amps.  The relay is rated for 30amps, the terminals are rated for 30 amps, #12 wire is already arguably a bit undersized for 30amps.  Everything needs to be perfect.  Wire needs to be stripped so that the bare wire will fully insert into the contact with absolutely no insulation protruding into the contact.  If ANY wire strands are cut or nicked during the stripping process, the wire needs to be cut off and re-stripped.  It has been argued previously, but industry standard typically says NOT to tin the stripped end of stranded wire when the wire will be put under pressure such as here.  After wire insertion, verify that ALL strands are in the contact - that none have folded over or otherwise didn't get inserted.  Screw terminals need to be tight. I didn't decode the torque spec on the FCI data sheet, but I torque things like these as hard as I can with my small-handled screwdrivers.  Stranded wire can tend to shift with wire flex or with time, so screw terminals should always be retightened after a while. For these where we're borderline, I'd check them periodically, also inspecting the terminal block and the wire insulation for any sign of distortion or melting.

For those that are comfortable soldering heavy gauge wire, just soldering the wires to the relay board can bypass all these issues with the terminals.  You can't do it with #12 wire, but a standard practice I usually apply is folding over the stripped end of the wire so that twice as much wire is being clamped by the screw terminal.  That can be helpful when you're trying to use say a #24 wire on a terminal good for much larger wire. 

There's also nothing wrong with bypassing the relay board entirely and going with freestanding relay like an automotive type.

A final comment on wire flexing - flexing wire is generally not good for it.  Over time, it can lead to increased resistance and broken strands. Some flex is necessary - like in our wiring to the extruder and the y-bed. But don't allow that flex to occur at the wire connection points like these screw terminals.  Use wire ties some where, some how so that flex is limited to an open length of the wire.   I think there are Thingiverse designs for cable support brackets that extend rearward off the frame.  For the relay board, consider raising the board on standoffs so you can loop the wires under the board as a strain relief.  Anything to keep wire flex from reaching the screw terminals.

 Once a terminal gets a little loose or we start damaging the wire end from flex, things will degrade rapidly.

FOLLOWUP COMMENT: When later removing the relay board from my printer as part of the migration to Smoothieboard, I noticed a new issue with the FCI terminal block that could explain the meltdowns.  What I observed is that the design of the terminal block pins does not allow the connector block to sit flush on the board. The connector block is off the board by a portion of a mm.  That may not be a lot and likely doesn't matter for most applications. In our case, wire pushing and pulling on the terminal block could rock the connector back and forth, especially for those who have used large gauge, typically low strand count wire.  Over thousands of bed movements, this rocking will stress the solder connections of the pins on the board, and eventually cause the resistance of the connection to go up.  That resistance will lead to heat.  Reflowing the connector didn't help in how my connector was seated on the board - there's apparently something about the pins that prevents them from inserting farther into the board.  The available options in improving your luck with the relay board work include...
 strain relief the wiring as mentioned earlier so that wire movement can't push and pull on the terminal blockuse high strand count wire like that intended for radio control hobby use to minimize strain on the connector (#12 R/C wire has something like 400 to 700 strands, and the silicone insulation helps keep it very flexible)add shims under the connector block body so that it can't be rocked back and forthglue or otherwise bond the connector body to the boardsolder the wires to the board instead of using the connector

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## ex-egll

Has anyone tried using a solid state relay for the heated bed?

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## adamfilip

What about using 2 lines of 12 guage for positive and negative. to help carry the load
I do notice that since i changed up the terminals.. the heated bed warms up slower.

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## Chadd

Looks like a loose connection caused arcing and melted the insulation on the wire.

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## gmay3

If this happened with all stock/makerfarm parts I would let Colin know about it. If he's supplying a kit that causes this he should know about it and supply some higher temp insulated wire.

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## sniffle

> If this happened with all stock/makerfarm parts I would let Colin know about it. If he's supplying a kit that causes this he should know about it and supply some higher temp insulated wire.



actually i think if he's going to put wire onto the heated bed it needs to be higher quality wire, that wire while stranded has very thick strands and almost bends like cable instead of quality wire that has very fine strands

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## gmay3

^ agree with ya

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## adamfilip

I emailed Colin about it before i posted on the forum. I am out of warrenty (30 days)
he suggested I try an automotive relay. if i didnt want to order a new relay

A

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## gmay3

I guess I would be interested in whether this is a relay issue, contact/connector issue, or a wire issue.

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## N5QM

I have been noticing the lack of flexibility of the cables from the bed heater to the relay, I have been considering replacing them with the same silicon cables that I use to feed power to the relay and RAMPS board sooner rather than later because of this.

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## MiniMadRyan

Just wondering how many hours Adam had printed with before this occurred. I usually check and re-tighten my connections every 3-4 prints, which is about every 10 or so hours, and haven't noticed any issues, mind you, I print primarily PLA, with the bed set at 70c...

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## adamfilip

My printer has probably run for 100 hrs before this issue came up

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## MiniMadRyan

Ahhh, thanks for that Adam. I'm around the same amount also, I'll keep an eye on it and see if it's an issue.

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## adamfilip

I have upgraded to a 40Amp Automotive Relay
works great.. hope it stays that way  :Smile:

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## BGN

Me too after having exactly the same problem as Adam. Only gets lukewarm even when working hard. No problem with the wires to the heat bed either.

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## sniffle

i will have to look into that... if nothing more than to get rid of the screw clamps and go to crimps

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## N5QM

I created this to support the bundle of cables coming from the PCB.  It attaches to the wooden part below the heater PCB.

http://www.thingiverse.com/thing:702335

I figure it might help keep those wires from breaking loose on the PCB side.

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## DaChank

Could someone post a wiring diagram for using one of those automotive relays?

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## beerdart

See attached. 


> Could someone post a wiring diagram for using one of those automotive relays?

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## sniffle

... Got a 30A auto relay... All I could find... It melted wires today... Gonna get a 40A to hopefully fix it

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## paintz2007

Being an rc guy we usually use the 5 second rule.. my bed wires are at about 1 second.. gonna replace with 14wag silicone wire this week

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## sniffle

> Being an rc guy we usually use the 5 second rule.. my bed wires are at about 1 second.. gonna replace with 14wag silicone wire this week



more detail please?

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## Chadd

The relays aren't what is causing these wires to melt. 

A poor connection is what is causing it. 

When you have a poor connection it creates resistance, resistance creates heat, heat causes the connection to become worse creating more resistance and more heat until you have a failure.




> ... Got a 30A auto relay... All I could find... It melted wires today... Gonna get a 40A to hopefully fix it

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## sniffle

right i get that, i meant about the 5 second rule and your wires being at 1 second

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## paintz2007

It is painful to hold the wires for longer than 1 sec

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## sniffle

ahhh gotcha, worked all weekend going to get a new relay tomorrow.  I'll check it and see after replacing it.  my current relay actually melted internally from the resistance... all i can think is that i didn't crimp hard enough... but who knows... I'll find out soon enough

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## eghm

My D8 stopped powering my heatbed tonight.  D9 works, I don't see any burns on the side of the board I can see, I assume it's the same terminal issue others have reported.

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## lakester

Since you're "in that neighborhood", perhaps it would be worth going with a relay as well.

I don't know what the numbers are for the 8", but a relay couldn't hurt..., especially if you've possibly burned out a mosfet being run close to a limit.

Out of curiousity, do you mostly print PLA or ABS?  (i.e., what are your typical bed temps?)

0.02USD.




> My D8 stopped powering my heatbed tonight.  D9 works, I don't see any burns on the side of the board I can see, I assume it's the same terminal issue others have reported.

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## eghm

> Since you're "in that neighborhood", perhaps it would be worth going with a relay as well.
> 
> I don't know what the numbers are for the 8", but a relay couldn't hurt..., especially if you've possibly burned out a mosfet being run close to a limit.
> 
> Out of curiousity, do you mostly print PLA or ABS?  (i.e., what are your typical bed temps?)
> 
> 0.02USD.


Thanks for the suggestion, I'll keep it in mind.  I've only printed PLA using a bed temp of 65.

Something odd might still be going on.  I haven't changed anything other than using D9 instead of D8 and it seems the bed will no longer heat when I go to print.  I have to explicitly preheat then print.

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## eghm

An update on my heating bed challenges.  Sometimes after preheating and starting the print the bed temp runs away rather than the bed not maintaining heat.  My current work around is disconnecting one of the bed power wires.  I'd be thankful for any ideas.

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## mccallum

According the the wiring diagram Colin has on his website for the relay board, J2 is just a jumper to connect power supply ground to heatbed gnd....you can tell by the traces on the board as well. 

Why not run a dedicated 12AWG ground wire (Silicon wire) from the power supply ground directly to the heater bed Gnd and bypass the relay board J2 terminal altogether?  Connect the +12V leads to the relay board as they were before.  While you are add it you might as well change the +12V wire to 12AWG Silicon wire from the heater bed to J1 'Out' Pin...to allow for more flexibility as the bed moves back and forth.

Thoughts?



hb_relay_diagram_1.jpg

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## printbus

> ...Why not run a dedicated 12AWG ground wire (Silicon wire) from the power supply ground directly to the heater bed Gnd and bypass the relay board J2 terminal altogether?  Connect the +12V leads to the relay board as they were before....


Yes, you could bypass use of the J2 terminals.

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## usarmyaircav

Sorry to revive an old thread, but I am looking for clarity.  I have the I3v 10" with Ramps, and looks like one of the connectors on the relay melted down, can someone point me to the correct 40A automotive relay, and also an accurate wiring schematic?

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## adamfilip

Better off getting a Solid State relay and then  you can control the temp via PID 
http://makerparts.ca/products/ssr25a...nt=13319734212

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## usarmyaircav

> Better off getting a Solid State relay and then  you can control the temp via PID 
> http://makerparts.ca/products/ssr25a...nt=13319734212


What does would this get me over a regular automotive relay, and where do I find how to control the temp via PID?  Probably order it off of Amazon to avoid the over the border possible delay's and costs

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## adamfilip

Sure order wherever you can get it. DC/AC

using this vs a automotive one.. you dont get the constant clicking of a relay. 
you can enable PID for the Heat bed Therm. 
SSR has a LED so you can see when its on and off. switches faster, and its silent.

with PID it switches it on and off faster to regulate the tempterture more evenly.. less highs and lows.

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## printbus

> Sure order wherever you can get it. DC/AC...


This might mislead people.  The SSR has to be one designed to switch DC loads.  An SSR for AC loads uses a different switching technology that simply won't work on DC.  There's been a few people that bought an AC SSR and learned that the hard way.  

Usarmyaircav, search the Makerfarm forum on "solid state relay". There's a thread that Chadd put together that likely includes the detail you're looking for. It's at least recent enough (i.e., newer than mid-April 2015) to not suffer from the 3DPrintBoard lost images & attachments problem like this thread does.

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## usarmyaircav

I watched the Thomas Salander video on SSR's and he mentioned to make sure you get the SSR that works with your power supply so good catch on that.  will do a search for that, thanks Kevin.

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## usarmyaircav

I ordered the SSR from the chadd thread, along with heatsink.  Looking forward to getting that in soon I hope!

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