[chbot] TRIACs cont'd ...

[Mark Atherton]

Robin,

I just took a look at 
http://www.nxp.com/documents/data_sheet/BT137-600E.pdf - see page 
7/13, table 7, top entry.

There appears to be almost a 4:1 trigger current difference between 
MT2+ and MT2- firing with +ve gate current (2.5 vs 11mA).

If you could use -ve gate current, the ratio drops to nearly unity (4 vs 5mA).

Assuming that previous observations were correct, this may be as 
simple as moving your digital GND reference on the TRIACs.

-mark