If I have an electric countertop burner that I put on a timer for yom tov, can I adjust the heat setting while the burner is on? Does it matter if I’m turning the heat up when the burner is already on or down when it’s already off (there’s a light that says if it’s currently heating or not)?
Do you have general guidelines on interacting with electronic devices on Shabbat/Yom Tov?
There are two issues here—havarah/mekhabeh (igniting/extinguishing), and use of electronics.
1. Regarding havarah/mekhabeh (igniting/extinguishing)
One cannot make a new fire on Yom Tov or extinguish a fire (Shulchan Arukh OC 502:1, OC 514:1). This means, that if the heating element (which is not visible) is not red hot, one cannot adjust the knob up if that will lead to it getting to be red hot (fire requires that it give off both heat and light according to halakha—Maggid Mishnah on Mishnah Torah, Sabbath 12:1:4, s.v. vehamechamem, Kuntres Acharon on Shulchan Arukh HaRav, Orach Chayim 495, etc.). Similarly, one cannot adjust the knob down so that it goes from red hot to black (even if it remains on). It is immaterial whether it is in the process of heating up or not (although if it was not red hot but already in the process of getting to be red hot, and you turned the knob even higher, that would not be a problem). So the question that you have to ask is: at what temperature (knob position) is the hidden heating element red hot? Once you know that, you know the line that you have to stay above (or stay under, as the case may be).
[N.B. – There are quite rare cases of stovetop burners—where there are 2 heating elements combined under one burner. In these cases, going up in temperature might take you from being one burn red hot to two burners red hot. I cannot imagine that that is the case here. (See here: https://www.partselect.com/PS244048-GE-WB30X354-Tilt-Lock-Hinge-Mounted-Surface-Burner-8.htm)]
2. Regarding the use of electronics
Logically, if you can do bishul, havara, lisha (cooking, igniting, kneading, etc.), on Yom Tov for cooking purposes (Shulchan Arukh OC 495), there is no good reason that you cannot use electricity other than uvda d’chol (weekday activity). In your case, there is basically no uvda d’chol because you are merely turning a mechanical knob, so there is no experience of electricity/electronics, and it feels exactly like what you do on Yom Tov to cook food, i.e., no different (experientially) than turning a mechanical knob to raise the gas on a gas stove. That being the case, I cannot see why it should be a problem.
In other words, I think it is permissible to adjust the temperature on the burner, as long as the red LED light is covered well so you don’t see it going on and off as a result of your turning the knob (if it won’t do this as a result of your adjusting the knob, even if it does so on its own from time to time, then there is no need to cover it).