I have asked a question and received some comments and an answer. Having solved the original problem, I now have another question related to the same circuit. Should I ask a new question or edit the original question? If I should ask a new question, should I link to the new question from the original question? My thinking is that it should be an ongoing discussion regarding the same circuit but I'm not sure of proper etiquette.
If you were to edit your original, answered question, then either:
the original answers would no longer make sense, which is undesirable because it makes the page less useful and takes away from the answerers' efforts, or
you would be asking two questions in one post, which is undesirable because then the answers are likely to answer one and miss the other, or do a good job on one and bad on the other.
For these reasons, editing a question like this is a bad idea, and you should instead ask a new question (and, as already said, link to the old one if you like, but make sure it stands on its own as well).
However, if your new question is really just being more precise about old question, so you're more tweaking “what makes a good answer” than “what is the question”, then it might make sense to edit the question. Be careful.
A different case: If you've asked a question that hasn't been answered (or has been answered extremely poorly), then editing it into a better question about the same topic does no harm.
In addition to what W5VO said (ask new question, link to old), make sure the new question contains enough information to stand on its own. The old question should only be used for background information, not essential information needed to answer the new question.
It's really annoying when it would require following links to get basic necessary information to answer a question. I usually stop reading at the link, downvote due to poorly written, then vote to close as unclear since I'm not going to follow the link.