First, a small correction to the specifications listed on the campaign page: the heater outputs on the boards that ship will be rated for 12 A, rather than 15 A. This change comes from switching to a more robust heater driver that switches the high side of the heater instead of the low side, giving the board even better protection in real-world fault conditions. Now on to the update:

On the campaign page, we’ve talked about Prunt Board 3 being designed with a level of protection well beyond typical 3D printer control boards. In this update, we’ll go connector by connector and explain what that means in practice.

USB

The USB port and the surrounding components are isolated from the rest of the board using a TI ISO7721 digital isolator. That means a serious electrical fault on the printer side is far less likely to reach your computer, Raspberry Pi, or other host device. On our board, it is extremely unlikely for a fault to occur which can cause damage in the first place unless you manage to connect mains power directly to the board, but if it does, the host device has an extra layer of protection.

Thermistor inputs

Thermistors are one of the most likely failure locations in a 3D printer’s electrical system. Thermistor wires are routed close to heaters, and a damaged cable, loose connector, or melted insulation can easily short a thermistor to a heater wire.

On many boards, protection is incomplete. The high side of the thermistor input is usually an undersized pull-up resistor, which can easily overheat from a short to 24 V. The low side is often even worse: on many boards, it is connected directly to ground with no meaningful protection at all, so a short to 24 V sends a high current through a cable that’s only intended to handle a few milliamps. If you’re lucky, this will blow a fuse, but more often than not, something will melt or catch on fire first since the high-current fuse on a heater just won’t kick in fast enough.

Prunt Board 3 protects both sides. The pull-up resistors are sized and rated to survive the relevant fault currents, and the low side includes a PTC resettable fuse, which will limit current much sooner than a heater fuse will trip. Just as importantly, the input network lets the board measure voltages above the normal open-circuit thermistor voltage. That means the firmware can detect when a thermistor is shorted to a heater. Combined with all the other extra components on the board, we can even tell you exactly what a thermistor is shorted to in many cases.

Endstops

Endstop inputs often look simple, but they are another place where mistakes are easy. Optical sensors, inductive probes, and other proximity sensors all tend to get connected to these inputs, and many industrial options will take a 24 V input and output 24 V rather than the 5 V that most boards expect.

Prunt Board 3 uses protected endstop inputs that tolerate sensors that output higher voltages, including common 24 V proximity sensors, without exposing the MCU to unsafe conditions.

The 5 V endstop supply is also separate from the board’s main 5 V supply. This matters because a wiring fault on an endstop or sensor should not take down the whole controller. With a separate supply, the board can detect and report a fault instead of simply losing power or behaving unpredictably.

Stepper drivers

The TMC2240 stepper drivers used on Prunt Board 3 already include built-in protection features, but the surrounding board design still matters.

One important addition is the large amount of bulk capacitance on the board. Stepper motors can generate significant back-EMF, especially during sudden stops from an unexpected reset or emergency stop. If all motors stop abruptly at once, that energy has to go somewhere. The capacitance on Prunt Board 3 helps absorb those voltage spikes and keeps the supply rail at a safe level.

This is the kind of protection that usually goes unnoticed when everything is working normally, but it makes the board much more robust when something unexpected happens.

Fans

Fan outputs are often barely protected on 3D printer control boards. In many designs, the fan is connected directly to 24 V and switched with a low-side MOSFET. At best, there may be a 10 A to 20 A fuse shared between several fan outputs and other parts of the board. This is obviously not good enough for the 3 A JST XH connectors, which are used for fans.

Prunt Board 3 treats fans as real power outputs that deserve proper protection instead of just putting them on a shared fuse without much thought.

On the high side, each fan output is protected by a TPS27S100 smart switch with robust current limiting, which always keeps the output current below the connector’s current rating. A shorted fan cable will not be able to burn a trace or wire (assuming the wire is not undersized), destroy a MOSFET, or take down unrelated outputs. The use of a switch here also means that fan outputs are really off instead of floating at 24 V, which provides an extra bit of protection when the board is inactive.

On the low side, Prunt Board 3 uses a VNS1NV04DP-E smart switch, adding current limiting there too, where other boards typically use an unprotected MOSFET.

The tachometer inputs are protected as well. They are safe across the full 0 V to 24 V range, just the same as endstop inputs.

Heaters

Heater outputs are probably the most important outputs to protect properly. A typical board relies on a fuse; this is generally fine and will prevent a fire, but a fuse is usually not fast enough to protect the switching components themselves. A MOSFET can be destroyed much faster than a fuse can react. A 3D printer board designed to be installed in custom printers should not be damaged by miswiring outside of the board.

Prunt Board 3 uses a TPS27SA08 high-side heater driver with fast over-current detection. This reacts far faster than a fuse and shuts the output down before the fault can develop into board damage. If a fault is applied near instantly and right next to the connector, then you still might be able to damage this component, but that is generally not what a fault looks like if it develops while the printer is running. The switch is also completely safe when attempting to start up into a dead short, so miswiring is not an issue.

The heater outputs also provide current sensing, which lets the board detect abnormal heater behaviour. This allows for the detection of developing faults before they turn into major issues. For example, as the temperature of a heater goes up, the current through that heater will reduce just a little bit, so if we detect a lower current but we do not see the value of a thermistor changing, then we can shut down the heater and report that the thermistor has likely come loose instead of starting a fire.

Because the heaters are high-side switched, the board can also shut off the 24 V supply to the output during a detected failure. This is an important difference from typical low-side-switched designs, where a heater may have a short to ground, and the board has absolutely no way to switch it off.

The same design philosophy is used throughout Prunt Board 3: faults should be detected, isolated, and reported wherever possible. A wiring mistake or damaged peripheral should not destroy the control board, and a control board fault should not endanger the host device connected to it.

If you haven’t already, be sure to https://www.crowdsupply.com/prunt-3d/prunt-board-3 to help us bring Prunt Board 3 to life.