Dave Plowman writes:

You mean it will be an inductive load, or switch on and off at double
mains frequency?

What tends to happen is that the triac turns on briefly when triggered
'on', but then turns off again within tens or hundreds of microseconds
because not enough current is flowing, i.e. less than the triac's
holding current - it takes time for current to build up (or decay) in an
inductance.

Depending on the triac and the resistance/inductance of the load, it is
sometimes possible to correct this problem with a 'snubber', a small
resistor in series with a capacitor, which you connect across the load.

To experiment, start with those old standby values of 100 ohms (1/2
watt should be plenty) and 0.1 microfarads (rated for 240V. A.C., not
D.C.). Increase the capacitance and decrease the resistance until it
works.
Of course it's much better to have a scope and look at what is
happening.

And there are no guarantees!



--
Roy Millar, Use m o u l i n e t @