On Wed, 30 Sep 2020 01:22:30 -0700 (PDT), David
wrote:


Interestingly, Paul's post
notes "There is a large resistor that does the dropping to bring the
240V down to some lower level."


I have replaced (removed) a couple of these boards over the years. They had failed, and the big resistor was quite charred in both cases!


In the later of the two videos quoted, the big resistor looks like
11K, which works out at 2.6W at 240V half-wave rectified. This is fine
in a 3W resistor in free air, but the trouble is that it isn't.

The triac in the 2nd video needs 5mA max to trigger, so probably the
big resistor could be increased to 22K. In fact the man in the first
video with unknown triac says it's 22K and that his meter agrees. The
colours look more brown than red but maybe it's because they're burnt.

If only 5mA is needed, a 0.1uF X-rated capacitor could be used
instead, but this would cost more like 50p.
--
Dave W