Hotbed stopped working

I have a prusa mendel with a ramps 1.4 (?) board running marlin. I picked it up on craigslist from a kid who couldn't get it calibrated well enough to run it. I did the research and got it calibrated enough to print for a few months and then it sat for about a month not being used. I turned it on the other day and the hotbed will not heat up. I though maybe the bed was bad and I had a spare so I hooked up a new bed to it and that didn't fix it either. I get no voltage from the board when I set the bed to heat. Where should I go from here? I'm very new to all of this but I've learned very quickly up to this point. I don't know enough about the pcb to know what might be broken.

do you have a voltage tester? test the solder contacts. if anything else try to reflow solder to the contacts. if it still does not work, it is likely the board is damaged. they can be found on ebay. you could try to repair it, but it would likely fail again.


on another note are you sure the supply is working that it is used on, and has enough current. just thought to ask because the supply used needs at least 10-12amps to work.

I'm using a desktop tower power supply, it has plenty of amperage to power them. I pulled my board off and it looks like it's shot. I'm not sure if the damage was caused by something I did or the previous owner. I have a new Ramps 1.4 on the way.

Did you have heat sinks on the FETs (transostors) that control the heated bed. I noticed that the FET that controls the heated bed on my RAMPS 1.4 was getting so hot I couldnt touch it. Though it looks like your's got hot enough to melt solder.

I'd recommend that you make sure that the FET controlling (switching the power) to the heated bed has a heat sink on it, and I'd also recommend that you fan cool the whole of the RAMPS board (the stepper drivers and the FETS, as all of them are prone to over heating).
If your stepper driver's don't have heat sinks, I'd recommend you buy some and stick them on. Loads of places on eBay sell them dirt cheap, and they stick on with special double sided tape (usually supplied with the heat sinks)

yes, there was a heatsink on the hotbed FET and the board is out in the open, not enclosed. I'll add a fan to the board when the new one comes in and hopefully that helps.

I'd double check for shorts on the hot bed as well, as the FET getting that hot seems unusual even for non fan cooled

make sure the mosfet that handles the current draw for the heated bed has a heat sink attached. then as long as it is exposed (not covered or in a case) convection should be enough to cool it. a fan over the entire ramps board, does reduce the temp. most mosfets parts are rated for above 150C. there is some debate on cheaper fake mosfets that can not pass the current. make sure your part is from a reputable vendor.

New problem- I ordered a new Ramps 1.4 board and set it up yesterday. Since I also bought a new hotbed, I used the new one. Running the printer through pronterface, The hotbed will only heat up to about 70C before clicking off and cooling down again. It will not hold a temp above 70C.
I am getting quite annoyed with this setup so my thought is to run a 12V Dimmer switch to the hotbed so I can manually adjust the voltage going to the bed and still keep the temp sensor connected to the ramps board to monitor the temp though pronterface. Will this setup work? It would keep the load off the mosfet and hopefully preserve the new ramps board for longer.

Again, any thoughts would help.

Hello,

Are you using marlin firmware? If so:

open your firmware in the arduino environment,
on the configuration.h tab, line 163 is

#define BED_MAXTEMP 150

where 150 is your maximum bed temp in degrees.... maybe this is set to a lower number? or your thermistor is reading incorrectly?

just below that line is:

// If your bed has low resistance e.g. .6 ohm and throws the fuse you can duty cycle it to reduce the
// average current. The value should be an integer and the heat bed will be turned on for 1 interval of
// HEATER_BED_DUTY_CYCLE_DIVIDER intervals.
//#define HEATER_BED_DUTY_CYCLE_DIVIDER 4

You could possibly uncomment that part to enable PWM which should do the same job as a dimmer switch would.

To me it looks like you MOSFETs got fried.
Maybe you want to have a look at this article: [reprap.org]
However, I tried their proposed solution (putting a N4001 diode in antiparallel) and still got problems also in a RAMPS 1.4.

TOMMM's solution I didn't try yet, but seems quite in the line of the article above and makes sense.

There is another more popular solution (although less elegant in terms of electric engineering) which is to put an electric relay connected to you new RAMPS in the D8 terminal. This way, the hotbed is connected to the power supply via the relay, which is controlled by the RAMPS.

Hope this helps



www.poti-poti.org

Edited 2 time(s). Last edit at 02/17/2014 02:34PM by ndujar.