Ramps Board Cooked?

He everyone. I've made ~ 20 prints with my Robo3d when today, in the middle of a large print, the heated bed died on me. I had to remove the first print run, as it failed, and after removal I cleaned the glass with acetone. Next time I tried to print the bed wouldn't heat past ~ 50c. I've played around with things and I think isolated the problem to a blown cap on the ramps board or the board itself (it shows a red LED instead of green when powered on) . The attached pic shows the cap in question, I suspect it's blown as 1) the bed won't heat at all and 2) when powered on this cap is VERY hot to the touch. Am I on the right track here? Anyone else had this issue? Or is the whole ramps board blown? I'm curious if I may have broken the bed itself. Measuring resistance between the positive and negative terminals of the bed = 1.2Ohms. Not a lot of resistance there, but I'm not sure what the correct value is either.

Thanks in advance for any advice.

 

those are fuses not caps. http://reprap.org/wiki/RAMPS_1.4

If those are getting hot then you may have a short somewhere in your heater wiring or heat bed.

 

Excellent. thanks! Any idea what the R value is from + to - on the heat bed?

 

let me go meter mine, i think i still have the original robo heatbed. I'll let you know.

edit: Well I found the heatbed but dont know where I put my multimeter. maybe someone else can read theirs out.

 

Thanks! So I just fiddled some more and if I disconnect the thermistor the red error LED goes away and things seem fine. The thermistor resistance = 84K Ohms unplugged @ ambient. I think that's WAY too high right? Wondering if I damaged the thermistor with excessive acetone cleaning of the table?

 

I disconnected the heatbed leads and measured from them. 1.4Ohms - i think it's fine.

 

well if its not heating up all the way then, yes it could be a bad thermistor.

still doesn't explain why your polyfuse is getting hot. How hot was it??

 

OK so now I think it's the bed thermistor having an intermittent lead or intermittent function. it will respond to temp fine, but if I squish it down under the heated bed, with the glass on top, it starts failing to read correctly.

Re the polyfuse - the temp is such that I can place my finger on it for ~4 seconds before it's uncomfortable. this only occurs when I attempt to heat the bed. Haven't tested if this also occurs when attempting to heat the hot end.

Anyone know where to obtain a spare thermistor with the correct specification for this unit?

 

Anyone know what the part # is for the thermistor used on the heatbed? Mine's definitely intermittent.

 

Check all the cables running up to the thermistor first. Could be just a loose connection and not the thermistor.

 

First - thanks to Mike for your quick suggestions and help! Second, it seems my excessive acetone cleaning may have caused the thermistor head shielding to melt a bit, causing it to ground when contacting the heatbed. I've insulated it with an extra layer of kapton and not it seems to be working OK. Go figure!

 

Probably not a great idea to clean your printer with acetone iso-propyl alcohol is plenty good

 

sounds like you found your short that was causing your polyfuse to get hot..

 

The problem is gone over here: http://forums.robo3dprinter.com/ind...ddition-to-prevent-heat-bed-cutting-out.1299/

What it is - The heated bed MOSFET stp55nfl06l could be better and gets VERY HOT. And too, the 11A poly fuse is too small for the application and gets HOTTER!. Better choices are in the above referenced thread. The solution is a new better suited MOSFET that will run cooler and a larger poly fuse 12-15A. A get by before it fry is a fan but, that is a poor bandaid for a real wound. Once you fix it - it will work - fast - and cooler. It will not need a fan but, why not use one anyway for the steppers.