RAMPS + Heated Bed

Hi -
So on the wiki page for RAMPS (http://reprap.org/wiki/RAMPS#Heated_Bed) it says that you just need to bridge the pads where the fuse normally gets soldered and then connect a thermocouple to the T1 pads...
I'm just a little confused (probably the n00b in me)... would there be a schematic somewhere I could see? I found one somewhere that showed a relay connecting pin D10 to the circuit I think...

I mean, so I wire the resistors in series (most likely) and then what?... would this be somewhere in the firmware? I understand that's firmware-specific... maybe I just haven't come along far enough to get into that... I'd just like to start with a heated bed some I don't mess up a lot of plastic to start with (or more than I know I already will mess up)..

Thanks!

The information there does not all apply. It needs cleaning up, and clarification. I would recommend the PCB-type heated bed. Ultimachine sells them, and I imagine others do too.

When wiring it up, there are pads on the PCB for wires, resistors, and LEDs. The resistors are there to keep the LEDs from blowing out by limiting current@12VDC. I have not seen what specific resistor is required, but then again, I left the LEDs and resistor off.

Wiring directly to the D10 terminal is not recommended, or at least that's what I have read. Instead, one wire from the heated bed should connect to Ground(-) on D10, and the other should connect directly to +12VDC on the RAMPS power connector. This allows the MOSFET to act as a switch without actually carrying the current for the bed. Feel free to correct me on this, as I am otherwise clueless.

Thanks for the info - I really don't want to spend $50 for the pcb... Might roll my own with aluminum resistors and a separate arduino uno + thermistor... Maybe throw in an LCD and some buttons also..

If I do, I'll be sure to put it on the wiki if it's any good..

Wish there was a well-documented way to get RAMPS to play nice with it :-/

I have the Prusa PCB heatbed from Ultimachine. It works but I can't get past 120C with my 18A 12V ATX power supply. I think this is probably a designed limit since the PCB material can't take that temp for too long.

I had a play with one of those recently and I only managed to get it to 120C with a 12V bench PSU. However I was losing about 1V in the wires to it, which where about 1m long. As power is V^2 that makes a big difference. You need thick wires and a very low RDSon MOSFET. You may also need a big load on the 5V rail to get the 12V to be actually 12V.

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I initially run the heated bed (4x 2R2 clad resistors) direct from Ramps but found that the fuse on the ramps board would kick in every now and then so I was abusing it. I switched to a simple coil relay and a cheap power supply (24volt) from ebay - works well easy to get to 110c (about 4 mins), have not tried higher yet. I switch the relay direct from the D10 pins, there does seem to be a issue with the firmware when it switches off though (tesla standard) sounds like it switches on and off in rapid pulses for a second or two?

@hexitex - I think you just had to connect one lead to the heater directly to the + terminal of the 12V power supply and then the other lead to the D10 - terminal. If you're using a different power supply on your heater, then you also have to connect the - terminal of the 12V power supply with the - terminal of the RAMPS (Arduino) power supply to complete the circuit. This way, the heater is not pulling its power from the RAMPS board. But the MOSFET still need to be able to handle the current load.

I haven't connected my RAMPS electronics yet. I still have the Mendel on TZ Gen3 remix. My question to others who are using RAMPS with a heated bed
is if the fan in the printed case would be enough to keep the MOSFET cool or if need to add a heatsink.

I use the printed PCB hotbed on my RAMPS, the GND is on connector D10, and +12 is wired direct to the power supply connector. I have a fan blowing sideways onto the Pololus, but not the Mosfet. It doesn't seem to get hot in this configuration.

If you are interested, I just uploaded the latest rev of my RAMPS enclosure to Thingiverse. It has clamps for threaded rod, and the fan is captive. The lid has built in cutouts for an LCD, and there is a built in RESET button on the side.

thanks brnrd, but i wanted to go to higher voltage as 12v would not get to the 120c needed for ABS first layer adhesion. I know ramps can run on 30v power supply but as volts increase so does the cost 24v seemed to be just below the expensive threshold (taking amps ito account as well)

The method I described should work for any power supply voltage like 24V. You just need to connect the negative output of the power supply to the same ground as the RAMPS shield. If you look at the circuit for the RAMPS shield, you'll see that the + terminal in D10 is just connected to +12 V on the board and the - terminal is what gets grounded when the power MOSFET is turned on.

It would probably be better to use a higher voltage since that would reduce the current required to reach a given temperature which would also reduce the temperature of the MOSFET.