Grounding tip makes temperature display erratic

RoosterCogburn

Hello,

I just got mt TS100 and want to report an interesting problem I discovered with it.

I'm using a switching power supply with floating 24V 6A DC output. Like many power supplies, mine is not earthed and leaks AC voltage. I can measure ~80VAC between the TS100 tip and mains earth. Nothing terribly unusual, most switching power supplies leak around half mains voltage AC. For ESD safety, the TS100 has a ground screw terminal so it can be guaranteed that the iron tip is at ground level. I can verify that the tip has a low-impedence connection to the ground screw, and connecting the ground screw to mains earth does remove the voltage potential from the tip, as expected. The TS100 is now ESD safe.

However, doing so makes the temperature display on the TS100 jump around erratically. It fluctuates by as much as ~10-70C with each update. It seems grounding the tip / screw causes the temperature sensor to give false readings.

I tried to investigate further. I tried grounding my power supply instead, tying negative DC out to mains earth. This causes no temperature problems, and since the tip is connected to negative DC in by a 100nF capacitor, there is little AC potential between the tip and mains earth. Now I can also ground the tip directly through the ground screw, removing the last bit of AC without causing any further issues. To summarize:

- no grounding on iron & no grounding power supply: ~80VAC between tip and mains earth, temperature display stable
- no grounding on iron & grounding power supply: ~1.5VAC between tip and mains earth, temperature display stable
- grounding on iron & no grounding power supply: ~0.015VAC between tip and mains earth, temperature display erratic
- grounding on iron & grounding power supply: ~0.015VAC between tip and mains earth, temperature display stable

This seems like a defect / design error to me.

Is this how it's supposed to work? Is my TS100 broken?

Thanks!

ny!

This is how it works, and the structure will improve later.