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RE: How hot is too hot?

Posted by Paul Joppa on June 2, 2017 at 19:36:32:
The usual (industrial?) safety rule is surface temperature no more than 140F (60C) - basically, you have enough time to perceive the heat and take your hand away before getting a burn.

Inside it is probably hotter. Each component will have a max temperature rating in the spec sheet. For reliable operation, you must have a margin. In my experience that margin is larger than you think. For example, I try to keep silicon solid-state junction temperatures at 100C or less, though the spec is 150C. Some resistors at max spec temperature are hot enough to melt the solder that holds them in place. It's a judgement call for each component - a lot of work!