[TEST, PIC HEAVY] Cheap eBay 5A TTL driver

The PRV was just a helpful way to find that rectifier as I didn't have a link to it. A 10 amp 1000 PRV rectifier. The PRV is unnecessary except to find the diode. And those pulse trains seem to be from a switching source. Like Benm said, you can add capacitance to filter better if it is a problem.

Sceeker said:

And some times (rarely) I even measured a very constant voltage at 5.2V

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It doesn't matter what heat sinking you use. In order for heat to flow to the heat sink, there MUST be a temperature difference between the diode and the heat sink. Therefore, there MUST be a temperature rise in the diode. Therefore, there MUST be a decrease in forward voltage. That's just physics.

If you are measuring something different, there is a flaw in your equipment or your methodology.