2 Attachment(s)
Clarifying i3/i3v heat bed and heat bed relay wiring
This is an attempt to clarify the i3/i3v wiring for the heat bed and heat bed relay. This is written from the perspective of the smaller printers where the heat bed relay is optional. The difference for the 10 and 12-inch printers is that the larger heat bed requires the use of the heat bed relay; the relay is not optional on the larger printers.
RAMPS OVERVIEW
For a schematic of the RAMPS 1.4 board, look for the schematic image under the Summary heading at http://reprap.org/wiki/RAMPS_1.4. We'll be discussing functionality related to the Power and the Heaters & Fans blocks on the left side of the schematic. RAMPS has two 12V power inputs and three switched power control outputs.
The 12V 5A input (+12V on the schematic) provides power for the Arduino board and LCD smartpanel, the motors, and the cartridge heater on the extruder. This power input is protected by a 5-amp polyfuse (the yellowish postage-stamp component marked as F1). The 12V 11A input (+12V2 on the schematic) provides power for smaller heat beds or the heat bed relay control. This power input is protected by an 11-amp polyfuse (the larger yellowish postage-stamp component marked as F2). The polyfuses are essentially fuses that open up like a fuse during an overcurrent situation and automatically reset.
D8, D9, and D10 are the switched power control outputs. D10 is normally used for the cartridge heater on the primary extruder. D9 is normally used for a print cooling fan controlled through gcode commands, but it can be reconfigured for use as the heater control for a second extruder. D8 is normally used with the heat bed. For printers without a heat bed relay, D8 is equipped to handle the MK1 type heat bed power directly through the higher current 11-amp polyfuse and by having a heatsink on the MOSFET switch controlling the output.
The RAMPS board includes LEDs that can be used to monitor the on/off state of the three power control outputs. LED2 reflects the output state of D8. LED3 reflects D9, and LED4 reflects D10.
WIRING WITHOUT A HEAT BED RELAY
Separate 12V power connections are made to the RAMPS 11A and 5A input terminals. For at least the smaller printers, these will often come from the same power supply, but running separate wires is usually smart.
Attachment 5926
WHY USE A HEAT BED RELAY
For smaller printers where the heat bed relay is optional, the relay is more efficient at applying power to the heat bed, allowing it to heat up faster. When supplying power directly to the heat bed from RAMPS, there are voltage drops due to the current flow through the necessary fixed resistance of the 11-amp polyfuse and the on-resistance of the MOSFET switch. These voltage drops reduce the amount of power ultimately applied to the heat bed. A downside to using the heat bed relay is that the heat bed power is no longer protected by the 11-amp polyfuse on the RAMPS board.
10-inch printers use a larger area heat bed that draws more current than can be controlled directly by RAMPS. The 8x8 MK1 heat bed draws around 10 amps; the 10x10 heat bed requires up to 18 amps. This is why the 10-inch printers are shipped with a heat bed relay. The relays have to be used on the 10-inch printers.
HEAT BED RELAY WIRING
The information here is for the heat bed relay supplied by MakerFarm installed on an 8-inch printer. For those using their own relay, the connection concepts are similar. On the heat bed relay board, the relay is used as a switch between the IN and OUT terminals on connector J1. The two terminals on connector J2 are simply tied together on the board. The connections to J1 can be put in either the +12v line or the ground line, but general practice is to always switch voltages, not grounds.
So, the heavy duty wiring for the heat bed power from the power supply goes to the J1 IN terminal and either of the J2 terminals on the heat bed relay. The heat bed is connected to the J1 OUT terminal and the remaining terminal on J2. The heat bed relay ships with two lighter gauge wires soldered to the RELAY COIL terminals on the heat bed relay. These connect to the D8 output on the RAMPS board. Simply put, the Arduino and RAMPS act exactly the same with the heat bed relay involved - it's just that instead of supplying power directly to the heat bed, the D8 output now just energizes the heat bed relay.
The drawing shows a 12V 11-amp power source being connected to the heat bed relay. This is what would be required for the 8x8 MK1 heat bed. For the 10x10 heat bed, the heat bed power source and wiring must be able to provide 12V at up to 18 amps.
There's an important connection remaining to be discussed in the heat bed relay wiring. For the D8 output to be able to drive the relay, 12V must still be connected to the RAMPS 11A input. Those adding a heat bed relay to an existing printer can't just move the heavy gauge wiring from the 11A terminals to the heat bed relay; that would leave the RAMPS 11A input without any power. Some might run an additional pair of wires from the power supply for the 11A terminals. The heat bed relay, however, is a low current device. It's fine to just add short jumpers from the RAMPS 11A terminals to the 5A terminals. Just keep the polarity straight.
Attachment 5925
WHAT GAUGE WIRE SHOULD I USE?
There isn't a simple answer to this. While a lighter gauge will functionally work for heat bed wiring, heavier gauge wire will have less voltage drop, apply more power at the heat bed itself, and thereby allow the heat bed to warm up faster. Wiring between the power supply and the heat bed relay is likely fixed in place - go as heavy as you can here to minimize the power losses in the wiring. My preference would be to use #14 on the 8-inch heat bed and #12 on the 10-inch heat bed.
You want to maintain flexibility in the wires between the heat bed relay and the heat bed, so a lighter gauge wire might be appropriate there. One consideration, however, would be to use silicone insulated RC-hobby wire to the heat bed. The silicone insulation and very high strand count makes the wire very flexible, and a heavier gauge wire could be used.
WHAT DEFINES HOW D8, D9 and D10 ARE USED?
The D8, D9 and D10 labels refer to the pin on the Arduino Mega microcontroller being used to control that output. In Marlin, file pins.h defines the mapping between the microcontroller pins and firmware functions. For the motherboard type 33 configuration used in the MakerFarm i3v printers, pins.h defines HEATER_BED_PIN to the number 8, FAN_PIN to the number 9, and HEATER_0_PIN to the value 10. This sets the relationship between the bed heater, the print cooling fan, and the primary extruder to D8, D9 and D10 respectively.
CAN I REARRANGE HOW D8, D9 and D10 ARE USED?
If power to the heat bed is fed directly from RAMPS (no heat bed relay is used), heat bed control must remain on D8. That's the only output fed by the 12V 11A input terminals and protected by the 11 amp polyfuse, and the MOSFET switch for D8 is the only one equipped with a heatsink for the higher current involved.
If a heat bed relay is being used, technically you could alter pins.h to swap around the HEATER_BED_PIN, FAN_PIN, and HEATER_0_PIN definitions. It can be tricky to follow the motherboard ifdef flow of pins.h; be sure you're modifying the values for the right motherboard type and be sure that a pin is only allocated to one function.
HOW WOULD THIS CHANGE IN UPGRADING TO A DUAL EXTRUDER?
In Marlin, a dual extruder setup would typically operate under motherboard type 34. The defaults in pins.h for motherboard 34 define HEATER_1_PIN as pin 9, and FAN_PIN moves to pin 4. The second extruder heater would connect to the D9 terminals. There is no output driver circuit on RAMPS for the D4 pin; those with a dual extruder that also want to have a controllable print cooler need to tap into the D4 signal and add their own driver circuit for the fan control.
NOTE: The 12V 5A RAMPS input should only be able to handle the current for one extruder heater. Firmware logic evidently only turns on one extruder heater at a time, alternating the power between the two cartridge heaters.
