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Electrical & Wiring·Lesson 19 of 34

Troubleshooting Electrical Faults

Diagnose brownouts, tripped breakers, CAN errors, and the faults that cost matches.

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Think in terms of the power path

When something doesn't work, trace the power and signal path methodically from the battery outward. Most faults fall into a few categories.

Brownouts

A brownout happens when battery voltage sags too low under heavy load and the roboRIO protectively cuts power to outputs to avoid rebooting. At Stage 1, when the bus drops below about 6.8V, the 6V output on the PWM pins begins to drop (the 5V and 3.3V user rails are not disabled yet). The full output-disable brownout (Stage 2) is what shuts down the 6V/5V/3.3V user rails and the motor outputs, and its threshold depends on the version: the roboRIO 1.0 is fixed at 6.3V, while the roboRIO 2.0 defaults to 6.75V and is software-adjustable. Below ~4.5V the device blacks out entirely (Stage 3).

Causes and fixes:

  • Tired battery - check internal resistance; retire anything over ~0.020 ohms.
  • Too many motors stalling at once - add software current limiting on CAN controllers.
  • High-resistance connections - a loose lug or cold crimp drops voltage; re-crimp and tighten.
  • Use the PDH voltage display or the Driver Station log to watch voltage while driving.

Tripped or blown breakers

  • A breaker that keeps tripping signals a mechanism jam, a stalled motor, or an undersized breaker for the load - investigate the mechanism first.
  • A blown fuse (roboRIO/radio) means a short or overload on that 10A circuit - find the short before replacing.

CAN bus errors

  • A device disappears - check its connectors and crimps; CAN is a chain, so a break kills everything downstream of it.
  • Erratic behavior - look for duplicate CAN IDs.
  • Flaky comms - verify termination (~60 ohms) and that CANH/CANL aren't swapped.
  • Use Phoenix Tuner X (CTR) or the REV Hardware Client (REV) to see which devices are present.

No comms / no robot code

  • Radio LEDs - confirm the VH-109 is powered and linked.
  • roboRIO LEDs - green Power and a healthy Status LED.
  • Check the Ethernet link between roboRIO and radio.
  • Confirm the Driver Station shows communication and robot code green.

Diagnostic tools

  • Multimeter - check voltage and continuity, measure CAN termination resistance.
  • Battery tester (Battery Beak) - resting voltage and internal resistance.
  • Driver Station Log Viewer - voltage, brownout events, and current over a match.
  • PD telemetry / PDH LED display - spot the channel drawing too much.

A repeatable process

  1. Reproduce the fault.
  2. Check the simplest causes first (battery, main breaker, loose connectors).
  3. Use telemetry to localize the bad channel or device.
  4. Fix one thing, re-test, and confirm before moving on.

Sources

Key takeaways

  • Brownouts come from sagging voltage under load - the roboRIO sheds load near 6.8V; full brownout is 6.3V (RIO 1.0) or a 6.75V default (RIO 2.0, adjustable).
  • Fix brownouts with healthy batteries, tight low-resistance connections, and software current limiting.
  • A repeatedly tripping breaker usually means a jammed mechanism or stalled motor, not just an undersized breaker.
  • CAN faults trace to loose connectors, duplicate IDs, or missing termination - diagnose with Phoenix Tuner X / REV Hardware Client, a multimeter, and the Driver Station Log Viewer.

Go deeper

Lesson quiz

Required

Answer all 3 questions correctly to complete this lesson.

1.A robot's motors briefly cut out and the roboRIO Power LED turns amber/orange during high-current maneuvers. What is the most likely cause?

2.The roboRIO's Power LED is solid RED. According to WPILib, what should you check?

3.Devices keep dropping off the CAN bus intermittently. What is a fundamental requirement for a healthy FRC CAN network to check first?

Answer every question to submit.