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Diagnostic Checklist

This checklist is needed when the device does not work or behaves strangely.

The idea is simple: first remove the danger, then check power and wiring, and only after that move to firmware, sensors, and loads.

1. Stop and Turn Off Power

First, turn off the device.

Especially if there is:

  • smell;
  • wire heating;
  • terminal heating;
  • smoke;
  • strange sound;
  • sparking;
  • heater will not turn off;
  • breaker or fuse trips.

Do not change wires under power.

2. Disconnect Power Load

Before diagnostics it is better to temporarily disconnect:

  • heater;
  • powerful LED strip;
  • servo;
  • motor;
  • external power load.

First verify controller and logic are alive without dangerous load.

If device has 110-230V AC, do not hold open mains part under power on the bench.

3. Check Power Supply

With multimeter check:

  • is there voltage at power supply output;
  • is voltage correct;
  • is polarity correct;
  • does voltage sag under load.

For example:

  • 24V line should be about 24V;
  • 12V line should be about 12V;
  • 5V line should be about 5V;
  • 3.3V line should be about 3.3V.

Acceptable deviation depends on power supply and device, but an obvious error like 24V instead of 12V should be found before connecting load.

4. Check Polarity

Check where plus and minus are.

Do not trust only wire color.

Check:

  • board markings;
  • power supply markings;
  • module pinout;
  • multimeter;
  • connection diagram.

Reversed polarity often kills module instantly.

5. Check Common GND

If there is external module or separate power, check common GND.

This is important for:

  • MOSFET module;
  • servo;
  • PWM fan;
  • HX711;
  • RFID;
  • external sensors;
  • separate DC-DC.

If signals go between devices, they must have a common reference level, unless the circuit uses special isolation.

6. Check Terminals and Connectors

Inspect:

  • wire inserted all the way;
  • conductor clamped, not insulation;
  • no exposed strands;
  • no darkening;
  • no smell;
  • terminal does not shake;
  • connector does not hold "on faith";
  • case cover does not pinch wire.

Move wires with power off. If contact clearly drops mechanically, fix wiring first.

7. Check Controller Without Load

Connect controller without power load.

Check:

  • is it detected via USB;
  • does firmware start;
  • is there communication with host;
  • does microcontroller not heat;
  • does power not flicker;
  • are logs visible.

If controller is not detected even without load, do not reconnect heater and power modules until you find the cause.

8. Check Firmware and Configuration

Check:

  • firmware built for correct board;
  • correct serial/CAN/USB path selected;
  • pins in config match pinout;
  • sensor_type correct;
  • min_temp and max_temp reasonable;
  • outputs not accidentally inverted;
  • no pin conflicts.

If heater behaves opposite after configuration change, for example "off", but temperature rises, turn off power and check config and power circuit.

9. Check Sensors

Before heating check sensors.

For thermistor:

  • temperature like room temperature;
  • no MINTEMP;
  • no MAXTEMP;
  • readings do not jump;
  • resistance like expected;
  • when heated by hand readings change correctly.

For other sensors:

  • power correct;
  • interface selected correctly;
  • address or pins match config;
  • wires short and not hanging.

10. Check Output Without Load

If need to check MOSFET, relay, or board output, do it first without dangerous load.

Options:

  • small test fan;
  • LED with resistor;
  • multimeter;
  • weak safe load.

Do not start diagnostics with powerful heater.

For heater, before first turn-on, numerical check is useful: measure resistance and compare to expected R = U^2 / P. If there is metal case or PE, check no short to case. For 12V/24V heater first run is better through lab power supply with current limit or temporary fuse.

11. Check Power Switch Separately

For MOSFET/SSR/relay check:

  • correct load type: AC or DC;
  • correct voltage;
  • correct terminals;
  • control input;
  • common GND if needed;
  • heatsink if needed;
  • input and output not mixed up.

AC SSR and DC SSR cannot be swapped.

For SSR separately check "off" state: with control signal removed load must not heat. First better use safe small load, lamp, or multimeter, not main heater.

12. Connect Load with Brief Test

When basic checks passed, connect load briefly and under supervision.

Check:

  • load turns on;
  • load turns off;
  • terminals do not heat;
  • wires do not heat;
  • MOSFET/SSR does not overheat;
  • sensor shows realistic temperature;
  • controller does not reboot;
  • fuse does not trip.

First heater test should not be long and unattended.

13. Check After Few Minutes of Operation

After few minutes check:

  • terminals;
  • wires;
  • power supply;
  • MOSFET;
  • SSR;
  • case near heater;
  • sensor readings;
  • controller stability.

If something unexpectedly became hot, turn off power and return to checking power, wiring, and power switch.

What Not to Do

You cannot:

  • repeatedly turn on device if something heats or smells;
  • hold 110-230V AC open on the bench;
  • change wires under power;
  • increase fuse "so it does not blow";
  • disable thermal protection for testing;
  • short emergency thermostat;
  • test powerful heater without temperature sensor;
  • leave first test unattended.

Short Order

If you need very short order:

  1. Turn off power.
  2. Disconnect power load.
  3. Check power supply voltage.
  4. Check polarity.
  5. Check common GND.
  6. Check terminals and wires.
  7. Check controller without load.
  8. Check firmware and pinout.
  9. Check sensors.
  10. Check output without dangerous load.
  11. Check MOSFET/SSR/relay.
  12. Connect load with brief test.
  13. Check heating of wires, terminals, and power parts.

Key Points

  • Diagnostics start with turning off power and reducing risk.
  • Power load better disconnected for basic checks.
  • Power, polarity, GND, and terminals checked before firmware.
  • Heater cannot be used as first test indicator.
  • If protection triggered, find the cause, not disable protection.

References