Fried Teensy Troubleshooting

It sounds like you have checked for any shorts to ground between the 5v, and signal pins so I'm not 100% sure. You didnt mention if you checked the 3.3v to gnd on the robot side - I assume you did. Make sure to also inspect each limit switch and make sure there are no shorts to the robot chassis - this is common. Here is some additional troubleshooting tips from chat:

Unfortunately, the symptoms you’re describing are consistent with catastrophic damage to the Teensy’s 3.3 V rail or internal regulator. A ~4 Ω resistance between 3.3 V and GND, combined with the chip getting hot and no USB enumeration, strongly suggests the MCU is no longer recoverable. At that point replacement is the correct next step, so you’re already doing the right thing there.

That said, the important part now is finding the cause before powering a new board, since Teensys don’t fail like this spontaneously.

Recommended next steps before installing a new Teensy

-

Disconnect the Teensy completely from the AR4

-

With the Teensy removed, re-measure resistance on the AR4 harness:

• 3.3 V → GND

• 5 V → GND

Both should read high resistance (kΩ+). This confirms there isn’t a short on the carrier wiring itself.

-

Verify external voltage sources

-

Ensure no external device is back-feeding voltage into any Teensy pin.

-

Common culprits:

• 5 V accidentally present on a signal pin

• External encoder or IO device powered from 5 V while driving a 3.3 V signal

• Ground reference mismatch from an external power supply

-

Inspect encoder and IO wiring carefully

-

Pay particular attention to:

• Encoder V+ pins

• Signal lines adjacent to power pins

• Any connectors that could be rotated or misaligned

-

A brief over-voltage event on a 3.3 V pin can permanently short the internal regulator.

-

First power-up with a bench supply (recommended)

-

When your new Teensy arrives:

• Power it standalone on USB first (not connected to the AR4)

• Confirm it enumerates and runs a basic blink sketch

-

Then install it into the AR4 and power the system using a current-limited bench supply if possible.

• Set a conservative current limit so a fault can’t destroy the board instantly.

-

Check for inductive kick or ground transients

• While less common, a missing ground or momentary disconnect during operation can create voltage spikes.

• Ensure all grounds are solid and that motor power grounds and logic grounds are tied exactly as shown in the documentation.

A note on failure timing

The fact that the system ran for ~15 minutes before failure usually indicates:

• A wiring or voltage issue that only manifests once everything is energized and active

• Heat-related breakdown of a marginal condition

• Or an intermittent mis-connection that became permanent

This is not a known AR4 or Teensy design issue, when this happens, it’s almost always external voltage or wiring related.

Chris

I appreciate the quick reply! I did find an intermittent short between a limit switch and the chassis, which I repaired. I'll do other checks before installing the new Teensy.

As follow up, after fixing the short between a limit switch and the chassis and replacing the Teensy, everything seems to be working okay. Thanks again for all of the help!