Zealot H743 FC Internal Voltage Drop Due To IMU Heater

All, Latest Copter, MP, H743 firmware, dead-cat 450 quad.

I observed what I initially thought was an issue with the H743 voltage/current sensor: It would not consistently report the calibrated voltage on the HUD or via telemetry. The display was often 0.5V low, and would then slowly (over 5-8 seconds) return to within 0.03V of the calibrated voltage. This ‘drop-then-return correctly’ cycle repeats every 15-20 seconds.

The cycling and voltage drop are a result of the IMU heater requesting heat.

To prove this, I set BRD_HEAT_TARG to -1, which kills the heating element. After rebooting, the HUD reported voltage and actual voltage correctly remain within 0.03V of each other. I determined that the voltage drop is ocurring within the FC itself, not caused by a voltage drop in the external wiring due to increased current demand.

In my case, the false low voltage resulted in an unanticipated premature RTL Failsafe event, and a minor crash.

Test setup consists of a variable voltage lab power supply and several DVMs of known accuracy.

Representative test result:

  • Calibrated voltage at 11.90V HUD says 11.93V, system wide current is 340 mA
  • When IMU heater fires, HUD says 11.41V, current is 410 mA
  • After ~20 seconds, displayed voltage slowly increases to 11.92V, current 340 mA
  • Above cycle repeats every 20-30 seconds

So an internal-to-FC ~70mA current bump is causing the voltage sag - I’m not sure of a work around yet.

No .bin file, this was all bench work.

I’m thinking that the power bus (or internal traces/wires) in the H743 might need a revisit…or power the heating element from a different source?

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Thanks for the report. A dataflash log would be really handy - you can set LOG_DISARMED to 1 to start a log generating, and then back to 0 to close that log off.

I can reproduce something similar to what you’re seeing here, but with nothing like the range:

We have a customer who reported the same issue. He solved it after reconnecting the power connector.

Is this specific to power via VBAT? Or can it be mitigated by using a redundant power supply via the Power Module connections?

Thanks for the responses, Jason and Yuri_Rage. I have a bit more information. The original voltage bounce was seen when running a 3S configuration. Since then, I upgraded the drone to 4S, and at 15-16V the drop due to the IMU heater is < 0.1 V or so.
I am presently powering the '743 exclusively from the 6 conductor harness from the power module. At some point, Jason, I’ll try the alternate power connection. But I can live with drop when running 4S. FYI: @timtuxworth … I am getting another Qiotec for an old IRIS I picked up and am totally rebuilding as 4S with 2216 motors…

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