Background: in a project I am using a PIC to alter the phase angle when to
fire a triac.  I detect the zero crossing then wait xxx uSec before firing.
 The
time delay into the phase is altered by using a rotary encoder, ie the
rotary
encoder controls the power output.

Problem: I need to make the power output linear with respect to the encoder
setting, ie when the encoder is set to 10% the output power needs to be
10%.
As you know, when controlling the phase angle of firing the power is not
linear
as it follows an 'S' type of response - a change in time before or after
the peak
of the sinewave results in a small change in power control, whereas a
change
in time firing near to the peak of the sinewave results in a vary large
change in
power output.

Has anyone got a formula to calculate the change in power compared to the
change in time in firing within the phase so as to allow me to linearise
the
response - or do you know of any articles which could point the way.

Many thanks

Stephen H Alsop    email: steve@s.ssystems.easynet.co.uk
S&S Systems Ltd   www: http://easyweb.easynet.co.uk/~s.ssystems