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<article class="guide-article">
<header>
<h1>Voron Klipper Runaway Protection Guide</h1>
<p class="lead">Ensure your Voron printer has proper safety safeguards against thermal runaway, driver failure, and endstop faults.</p>
</header>
<section>
<h2>Why Runaway Protection Matters on Voron Printers</h2>
<p>
Voron printers are enclosed, high-temperature machines. A runaway heater, disconnected thermistor, or stuck-on MOSFET can cause a fire in minutes. Klipper has multiple layers of safety protection, but they must be correctly configured. This guide covers all the critical safety systems you must verify on your Voron build.
</p>
</section>
<section>
<h2>1. Thermal Runaway Protection</h2>
<h3>Heater Temperature Monitoring</h3>
<p>
Klipper's built-in thermal runaway detection monitors temperature changes against expected behavior. If the heater is on but temperature doesn't change (thermistor fault) or keeps rising past the target (stuck MOSFET), Klipper triggers an emergency shutdown.
</p>
<p>Critical configuration in printer.cfg:</p>
<pre><code>[extruder]
max_temp: 285
min_temp: 0
max_extrude_only_distance: 50
max_extrude_only_velocity: 25
max_extrude_only_accel: 500
</code></pre>
<pre><code>[heater_bed]
max_temp: 130
min_temp: 0
</code></pre>
<h3>Verify Thermistor Connection</h3>
<p>
Loose thermistor connectors are a common cause of thermal runaway. On Voron builds:
</p>
<ul>
<li>Check the thermistor JST connectors on the toolhead PCB (SB2209, SB2240, or umbilical board)</li>
<li>Ensure the glass bead thermistor is fully inserted into the heater block and retained with a screw or crimp</li>
<li>Verify the wire insulation isn't melted or chafed at the strain relief</li>
</ul>
<h3>Thermal Runaway Test Procedure</h3>
<ol>
<li>Heat the hot end to 200°C and note the temperature graph in Mainsail/Fluidd</li>
<li>Disconnect the thermistor (simulate failure) — the printer should trigger a shutdown within seconds</li>
<li>Reconnect, heat to 200°C, then manually blow cool air at the heater block — the PID should compensate. If temperature drops below min_temp, the printer should shut down</li>
</ol>
</section>
<section>
<h2>2. MCU Watchdog Timer</h2>
<p>
The MCU watchdog reboots the microcontroller if the host computer (Raspberry Pi) stops communicating. This prevents the printer from running with unresponsive firmware.
</p>
<p>Enable the watchdog in your MCU configuration (build-time option in Klipper firmware):</p>
<pre><code>make menuconfig
# Enable extra low-level options
# Enable watchdog (if available)
</code></pre>
<p>
In printer.cfg, set the communication timeout:
</p>
<pre><code>[mcu]
serial: /dev/serial/by-id/usb-Klipper_stm32...
baud: 250000
restart_method: command
</code></pre>
<p>
The `restart_method: command` allows Klipper to issue a soft restart. If the mcu becomes unresponsive for more than the `communication_timeout` (default 25 seconds), the printer enters shutdown state.
</p>
</section>
<section>
<h2>3. TMC Driver Over-Temperature Protection (OTP)</h2>
<p>
TMC2209 and TMC5160 drivers have internal temperature sensors. When the driver exceeds 150°C, OTP kicks in and reduces current or disables the driver. Klipper reports this in the console.
</p>
<pre><code>[tmc2209 stepper_x]
run_current: 0.8
hold_current: 0.5
driver_SGTHRS: 100
driver_OTP: 1
</code></pre>
<p>
The `driver_OTP: 1` flag enables over-temperature pre-warning. Monitor driver temperature with the Klipper console:
</p>
<pre><code>DUMP_TMC stepper_x
# Look for "otpw" or "ot" flags
</code></pre>
<p>
Common causes of driver OTP:
</p>
<ul>
<li>Inadequate heatsinking (drivers need heatsinks and airflow in the electronics compartment)</li>
<li>Run current set too high (max 1.2A for TMC2209 on most Voron boards)</li>
<li>StealthChop mode at high current generates more heat than SpreadCycle</li>
</ul>
</section>
<section>
<h2>4. Endstop Failure Safeguards</h2>
<p>
A failed endstop switch (stuck closed) can cause the gantry to crash into the frame. Klipper provides several safeguards:
</p>
<h3>Homing Override Limits</h3>
<pre><code>[stepper_x]
homing_retract_dist: 5
homing_speed: 50
second_homing_speed: 10
homing_positive_dir: true
position_endstop: 0
position_max: 250
position_min: -5
</code></pre>
<p>
Setting `position_min` to -5 (negative) creates a software limit that prevents the axis from moving beyond the physical endstop even if the switch fails.
</p>
<h3>Endstop Status Monitoring</h3>
<p>
Use the Klipper console to verify endstop state:
</p>
<pre><code>QUERY_ENDSTOPS
</code></pre>
<p>
Each endstop should show "open" when not triggered and "TRIGGERED" when activated. If an endstop shows "TRIGGERED" at all times, the switch or wiring is faulty.
</p>
</section>
<section>
<h2>5. Klipper Shutdown Actions</h2>
<p>
Configure what happens when Klipper detects a fault. By default, all heaters and motors are disabled. You can add notification macros:
</p>
<pre><code>[gcode_macro _ON_ERROR]
gcode:
{ action_respond_info("Printer fault detected: {params.REASON}") }
UPDATE_DELAYED_GCODE ID=shutdown_check DURATION=0
</code></pre>
<pre><code>[delayed_gcode shutdown_check]
initial_duration: 0
gcode:
{% if printer.pause_resume.is_paused %}
TURN_OFF_HEATERS
{% endif %}
</code></pre>
</section>
<section>
<h2>Safety Checklist for Voron Builds</h2>
<ul>
<li>✓ Heater thermistors securely fastened, no exposed wire near heater block</li>
<li>✓ Max temps set below component ratings (extruder: 285°C, bed: 130°C)</li>
<li>✓ MCU watchdog enabled in firmware build</li>
<li>✓ TMC drivers have heatsinks and open-top covers for airflow</li>
<li>✓ Endstop switches wired normally-closed (NC) for fail-safe</li>
<li>✓ Fuses or PTC resettable fuses installed on heater lines</li>
<li>✓ Klipper `max_power` set to no more than 0.8 for silicone AC bed heaters</li>
<li>✓ Smoke detector near the printer enclosure</li>
</ul>
</section>
<section class="summary">
<h2>Summary</h2>
<p>
Runaway protection on Voron printers requires multiple overlapping safety layers: Klipper's thermal runaway detection with properly set min/max temps, MCU watchdog timer, TMC driver OTP monitoring, endstop fail-safes with software limits, and a well-configured shutdown sequence. Test each protection system during your build commissioning — never assume they work until verified. A Voron with all safety systems enabled and tested is a safe and reliable printer.
</p>
</section>
</article>