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I have heard that when a VTX gets too hot, it can cause irreversible damage to the VTX. I have sometimes noticed that the VTX on my 5-inch drone gets very hot to the touch. How hot is too hot for a VTX and what are some signs that the VTX on my drone might be getting too hot?

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Transistor junction temperature (aka junction temperature or Tjunction) is the rated maximum operating temperature for a semiconductor before it will either permanently fail or produce possibly damaging outputs to other components in the circuit. Desktop computer CPUs often have a Tjunction of ~100 °C, and this site claims that most commercial-grade semiconductor parts have a max operating temperature of ~70 °C.

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(cit. ROC = Recommended Operating Conditions and AMR = Absolute Maximum Rating)

Abiding by the manufacturer's specs for thermal limits is usually a good idea assuming they're published, but staying below 70-80 °C is always good.

Redlining temperatures in between the ROC and AMR will result in decreased efficiency and abnormal behaviors of the transistors in the ICs and switching components in the VTX, like the power regulators (most of which use inefficient linear regulators which dump a lot of wasted energy as heat) and signal processors. Some of these abnormal behaviors may cause the kind of irrevocable damage to the VTX mentioned in the OP and will for sure decrease the VTX's power efficiency.

This can cause the VTX to fail mid-flight and leave you high and dry without a video feed. High VTX temperatures could also have adverse effects on other parts of your drone if in close proximity by heating them up as well.

Possible solutions

As Oscar Liang points out, you could try:

  • Replacing the (usually metal block) heat sink on the VTX with something better

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  • Take load off of the voltage regulator in the VTX by installing a more efficient switching regulator to step down Vbattery before it gets to the VTX.
  • Move the VTX to a location where it sees more airflow.
  • Try replacing the VTX; it could be dead/dying.
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