Heatbed issue

So I just got a Prusi 3i from my nephew for my son..so we got it to print bunch of things since Xmas.. and then had issues with the filament
going in and coming out..

So I took apart the extruder made sure it was all cleaned out and put all back together the same.

We went to heat it all up and when it got to temp the wiring for the mk3 heatbead started smoking.. so we turned it off... looked at it
and re-stripped the wires and put back together.. now it wont heat the bed at all.. the extruder temp works fine.

So in reading some threads I tested out the volts to the system.. its 12 to the board, 12 to the heatbed when I use the negative
from the power leads, so I know it has power.. I assume that the board uses a negative switch to control the heating..?

so not sure if my board is toast now and cant or wont attempt heating.. ?

I did hard wire the heatbed with board power and when I put negative on, in like 5 seconds the negative got real
hot.. so is there a short in the heatbed ? does that happen often ?

just not sure what to check and what to replace at the moment..
any ideas ?

Thanks in advance.

My machine is different to yours so I can't be exact. My hotbed resistance is 1.2 Ohm so that at 12V the bed draws 10 Amps. It sounds to me that your bed is drawing far too much current. Have a look at the printed circuit side of the bed for obvious flash marks or shorts. This is an aluminium surfaced bed if I remember correctly. Have you clipped anything on top of the bed with metal clips? These can rub through the insulating varnish and partly short the bed via the edge of the aluminium surface. If there are springs between the bed carrier and bed and the carrier is metal they can also cause a similar problem.

I'm not sure what you mean when you said that you hard wired the bed with board current and when you put the negative on the wire got real hot. Where were you feeding the bed from? Also you shouldn't be trying to make connections carrying this sort of current with the power on. They don't like it.

Bed heater connections need the wire ends properly terminated and need to be securely connected. If not they can arc and melt the connections and wire insulation.

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I'm not sure what you mean when you said that you hard wired the bed with board current and when you put the negative on the wire got real hot. Where were you feeding the bed from? Also you shouldn't be trying to make connections carrying this sort of current with the power on. They don't like it.

So maybe I did a No no.. but I just hooked in the hot to the main board power and had the negative off.. I wanted to see if I gave 12v to the
heatbed if it would heat up.. but when I touched the negative to the main board power negative the wire got hot real quick.

So my bed could have a short in it I assume its the wire is getting hot when it gets full power.. unless the head bed
doesnt get full 12v when heating is required ?

so lets just say I need a new bed.. any idea why its not calling for power to the bed. When I had
heating on via the PC for the bed I never got power to the bed. So I assume that could be because something
on the board is shot now because of the over heat.. or I guess the thermostat is dead ? does the
thermostat control the heating at all ? or just give feedback to the board to know when to
turn heating on and off ? Just not sure what could be shot if the heaterbed is not getting
power to it when I tell it to heat..

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Bed heater connections need the wire ends properly terminated and need to be securely connected. If not they can arc and melt the connections and wire insulation.

the solder points look fine to me

It definitely sounds like your heatbed has a short circuit,can you measure any resistance(ohms) from the heatbed terminals to the alluminium?
If it is shorted then it is/has probably blown the fuse that feed power to the bed.
What board do you have ?

I'll try to check the resistance on it later today.. never did that before..

but on the board the only big lettering I can find says MPX 1 and BASE 1.3

I was looking for fuses before and didnt see them under the power bank at top.. so I
guess it doesnt have them ??

if I do negative to positive on the hotbed leads I get .8 or 0.9 on the 200 setting on my meter..

I read if its below 1 then its bad ?

It sounds as though you have a partial short on your heat-bed. The resistance for a12V 120W bed should be pretty close to 1.2 Ohms. At the figure that you are reading it will be taking 50% more current than it should. Heat beds are not expensive I suggest that you replace it.

sounds good.. I guess I'll try that first..

but on that note.. mine on the back is a MK3 Heatbed Dual power..
so I see lots on Amazon... but mine is a 5 pin... all these I see are
3 pin.. this some new style and mine is old ?

what do I look for to make sure its a match ?

There model numbers on these ?

can you post a picture of the back of your heatbed
It probably has 2 connections for a chip thermistor but would be best to check

since 2 and 3 are joined anyway.. I assume I can use a 3 pin..

I attached the image

Where is the wire connecting pin 3 to negative? I can't see one in the picture and the table shows that it is required for 12V. If you look at the table for 24V, connections 2 and 3 are shown for opposite polarity so this is not a built in connection. Pins 4 and 5 are only there so that you can solder the thermistor to the central pads and don't need to have the thermistor wires trailing underneath the bed.

Edited 2 time(s). Last edit at 01/10/2017 12:28PM by Supermec.

Negative is on 2, 2 and 3 have a bridge and positive is on 1

which is how the pin chart shows it should be for 12v

So YOU have fitted a wire between 2 and 3 ? I don't see how there can be a built in internal bridge if the connection table for 24V requires that 2 and 3 be of opposite polarity, an internal bridge in that case would represent a short circuit. Don't forget that we are dealing with very low resistances here.

This worked for months fine when my nephew had it.. and worked for me fine for like a month.. just stopped working the other day..

and the only thing I can think that maybe did it was when we hit return on the extruder and it started dragging the clip with it.. maybe
it scratched something ?

but back to the wiring.. its wired according to the chart.. it wants PIN 1 (+) and PIN 2 & 3 to be (-), wire is on 2 for Neg.. and tiny wire
is bridging 2 & 3...

what replacement bed do I use ? all the new ones just Neg and Pos now ? no double Neg anymore ?

I dont see any on any sites that look like this one..

any reprap mk3 heatbed is fine, pins 4 and 5 on your heatbed are if you want to fit a surface mount chip thermister,but most use the wired thermister like you have fitted atm

ok got my new heatbed in the mail.. smaller than the orig one.. but I hooked it up.. put the orig thermistor in it, because it had the
2 pin connector for the board on it.. the new one didnt..

still same issue.. no power to the bed... I hit set on Pronterface control but no power goes to the bed..

so if the Thermistor was bad what would it do ? make it not heat at all.. or just heat out of control..

what do I replace next ? the board ?

Do you have a lcd connected to the printer, if you do, does it show the ambient temperature for the bed

can you measure the resistance of the thermistor when unplugged from the controller, it should measure ~100k ohms at room temperature

Edited 1 time(s). Last edit at 01/14/2017 05:22AM by obewan.

thanks for the continued assistance !!

so Bed on LCD shows 16/0

Thermistor resistance on old and new show 130 ish

I never looked at this before.. maybe it always says this when not heated up at bottom of LCD .." Err MINTEMP "

Is that 130 ohms or 130 kilo ohms ?

I would try lashing up the new thermistor to try it.

If the old thermistor is open circuit then the firmware will prevent the bed from heating, but pronterface should allso indicate the error

130 ohms... so I put on the new thermistor.. still no heat power...

so I assume something is wrong with the board ? whats a replacement one ? they all interchangeable ?

on the board the only big lettering I can find says MPX 1 and BASE 1.3

one that I found looks to be the same layout.. but its MPX 3.. that a newer version ?