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Connect a SevenSwitch to a RAMPS

One of the oddities of the RepRap community is the heated bed connector on the very popular RAMPS controller. It's undersized for modern beds and this is known for years, but nobody fixes it (which would trivial).

Anyways. One of the solutions to work around this situation is to use a SevenSwitch to do the hard work. Here we show how to connect this switch to a RAMPS and how to tweak the popular firmwares to work with the new switch instead of the old one.

Finding a Signal Pin

The red oval marks the Servos connector.

The red oval marks the Servos connector.

First step is to find an appropriate signal pin on the RAMPS. “Signal” means, a pin which can be switched between 0 V and 5 V by the firmware, preferably using PWM. A RAMPS has a whole lot of extension connectors, we choose the Servos one. This connector features 4 PWM-able signal pins as well as convenient GND pins.

Some vendors solder in pins there already, others don't. If there are none, you have to do this yourself. Standard 0.1“ spaced pins are fine. Another option is to solder the signal wire directly into the board.

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Pinout of the RAMPS Servos connector(s).

Pinout of the RAMPS Servos connector(s).

There are 4 signal pins available for interfacing in the servo section: D4, D5, D6 and D11. Each of them works equally well. Connecting more than one SevenSwitch is possible, too.

Wiring

sevenswitch_1.2_ramps_wiring.jpeg

The above picture shows how things get together:

  • Thin lines are signal wires which draw negligible current, so one can use thin wires. Typically used wire is 0.14 mm² or AWG 35.
  • Thick lines mean wires carrying substantial current, so it needs thick wires. Required thickness depends on the current going to the heated bed; using a 1.5 mm² (AWG 15) wire is almost always sufficient. For a little extra bed power, many use 2.5 mm (AWG 11) size.
  • Dismantling an old power cable meant to connect a 110/230 volts device to mains power is a convenient way to harvest such thick wires. Works on 12 volts just fine.
  • The green line is the signal wire. This one turns the switch on and off.
  • Power supply doesn't have to be the same as the one supplying RAMPS and stepper motors. A separate one works entirely fine.
  • Polarisation of these two wires going to the heated bed doesn't matter.
  • Do not connect the heated bed, yet. Without the bed connected there are no currents flowing, so nothing can overheat. The green LED will show wether the switch is working.

Why not ...

Some people tend to try connecting a SevenSwitch a different way. Let's elaborate on this.

... using D8, the original heated bed connector?

Using D8 is a possible, but poor choice for several reasons:

  • D8 is switched by a high current MOSFET, which is like using a truck for transporting a single cookie. There's no need for this MOSFET.
  • Due to being behind a MOSFET, signal on D8 is inverted. Without firmware modification, commanding the heater to be off turns it actually on and vice versa.
  • There are better uses for this output, e.g. for driving a fan, lighting, heaters, whatever draws less than 10 amps and has no coil inside.
  • If the RAMPS is operated with 24 volts, it'll destroy the SevenSwitch's signal pin, which can withstand 20 V, only.
connect_sevenswitch_to_ramps.1463753221.txt.gz · Last modified: 2018/05/27 16:10 (external edit)