Print bed will never go over 85-90C

Hey Guys, Im having problems with my heated print bed. I am using the Prusa PCB with a ramps 1.4 and a 12v switching Power supply. The problem is no matter how long I let it go I can never get over 90c on my bed and Im having a hell of a time getting abs to stick reliably. The bed usually gets to about 90c at the center and towards the edges as low as 75. Ive tried insulating the bottom of the PCB with cardboard and that helped even the temp some but Im still never over 90. Any ideas?

have you checked the resistance of the heat bed?

No I havent, Honestly I dont know what it should read. If you know I can check it when I get home.

I think it is between 1.0 and 1.2 ohm. im sure someone will correct me if im wrong. if it is too high you cant get enough current through it to heat up enough.

I feel like a broken record (remember those) saying this, but check your solder connections, and then check them again. A weak solder joint caused this same issue for me, and cleaning out joint and re-soldering fixed it.

Also, what gauge wire are you using between the RAMPS and the PCB?

Ill re check them again. I also have continuity between them through the board. I check the resistence and its 1.2 at the solder joints and 1.3 at the ramps board. I am using 12 ga speaker wire for the connection between both the hotend and the heated bed. I was originally using 18ga but I got rid of it when I realized that my wire was heating alot.

What is the power output of your power supply?

Measure the following points for voltages:

Out of the PSU
Across the input to the 11A RAMPS connector
Across the RAMPS output to the heated bed (D8 connector) *
Across the heated bed itself (after the wires from RAMPS) *

* Make sure everything is room temp and you've just told the bed to be turned on. This will ensure the FET is switched on properly.

If you're getting a significant drop of anything more than 0.6V between any of those places (eg: Volts across RAMPS output - Volts across Heated bed > 0.6V), you may need to change things.

The usual one is between the heated bed and the RAMPS board. YOU can lose a LOT of volts here using cable that simply isn't up to the job. A heated bed can pull between 10 and 13A depending on the setup, so the wires MUST handle that current, else they will heat up, and you'll lose lots of voltage across them. The cable from the power supply also can suffer this issue too.

Also, if you've not got a heatsink on the FET that drives the heated bed, I'd recommend at least bolting a tiny piece of aluminium to it and making sure that the fan you use to cool your pololu's at least slightly points in the direction of the FET. Especially with the somewhat "common" FETs that most people use on RAMPS boards (like the STP55NF06L, which is now a poor choice), keeping the FET cool will reduce the voltage drop it adds to the circuit. A much better alternative is to replace the FET with a better one like an IRLB8743, which should work fine without a heatsink.

That said, the local hackerspace machine still has issues after we upped the PSU and heated bed cables, put in a new FET with a very low RDSon value (the IRLB8743 noted above), removed the PTC fuse (which was also causing an issue and losing us 0.5V on average, and sometimes cutting out completely), and replaced various connectors that were "iffy". In the end, we find that running the RAMPS board on a slightly higher voltage (13.8V vs 12.0V), solved the issue for us.

Hope this helps.

voltage 12v maybe to low ,change 24v pcb will be hot soon.my reprappro huxley pcb is powered by 19v,it take me about 20 mintues from 20c to 110c

My power supply is adjustable and I have it cranked up to about 12.5v That said, I havent checked the voltage drops. I will check them out when I get home today. I Currently dont have any fans or cooling over my pololu's but have considered the need and May look into adding them this weekend as well.

What voltage is the ramps board safe too? I have a separate 19v power supply I could add to the heated bed area of the ramps board but I couldn't find any clear documentation to say whats safe.

There's a number of factors that determine how much you can run the RAMPS board at. Please read all of the below before you go ahead and try a higher voltage.

The Pololu A498x stepper drivers are rated to 35V, and I tend to leave a 2V margin, so that sets the first (absolute) maximum for your setup at 33V. The higher a voltage you run, the more heat the stepper drivers will need to dissipate, so make sure you have a good fan pointed at the board and heatsinks on all your stepper drivers. If you use DRV8825's, they have a max voltage of 45V, which with a 2V margin is 43V.

A number of the capacitors on the RAMPS board should be rated to slightly higher than whatever voltage you can push it to. eg: If the caps are rated to 16V, I would not push the board past 14V, leaving a nice 2V margin of error. The caps you need to check are C2, C3, C4, C6, C7, C9 and C10. There are various markings on the caps that indicate the voltage rating, but if you don't know them you may need to Google to figure out what they are.

The PTC fuses (F1 and F2) have their own voltage ratings. F2 (which is used for the heated bed) is specifically rated to 16V, so if you're going over 16V, you should replace this fuse with a wire link, and then put a proper fuse between your PSU and the RAMPS board. F1 is rated at 30V, so with a margin of error that comes down to 28V.

Then there is your Arduino Mega. Some of the clones have switch mode regulators on them and will happily take higher voltages than 12V. Note however that really, you don't want to go too far over 12V if you can help it. I've seen genuine Arduinos handle 13.8V at their input. If you're unsure, remove diode D1 from the board, and find another way to run the Arduino (eg: via USB cable). You can happily run voltages over 12V to the stepper motors, hot end and the heated bed, provided they can take the extra voltage.

IMO, with diode D1 in place, you should be able to use anything up to about 13.8V without changing a thing.

If the caps and your Mega support higher voltages, then you can probably take it to 19V or 24V easily. If the caps support it, but the Mega doesn't, you will need to remove diode D1. In either case, I would recommend removing F2 and using a real fuse instead.

I personally wouldn't run over 27V into a RAMPS board ever, but that's me being slightly conservative with the maximums.

Note: If you're JUST after pushing the heated bed higher, you can run dual-voltage inputs so that the heated bed gets a higher voltage than the rest of the system. If you run almost everything at 12V (hot end, fan output, motors), and want to run the heated bed at 19V, you only need to remove F2, replace it with a wire link, add a real fuse between your RAMPS board and PSU and then connect up 19V to the heated bed power input (the two pins nearest D8, with the pin nearest D8 being the positive). This way, only your heated bed will get 19V.

FWIW: I run my setup entirely on 24V, and have removed D1 and replaced F2 with a link. My PSU has a fuse between it and the RAMPS board. I have replaced the standard 6.8 ohm hot end resistor with a 22 ohm resistor. This gives me slightly faster hot end heat times than 12V @ 6.8 Ohms, while not using too much extra power. My motors run fine, though they are slightly warmer than when I was originally running 12V. YMMV.