N_Cook wrote:

I replaced the BC214 with a BC212 and as no TIC236 to hand , with a TIC226
for proving purposes, temp. There is now no sign of the broad spike trying
with the 25 ohm dropper, even to 100 percent mains. The 214 tests as 420 hFE
and I'll assume the triac is faulty. My triac test is little more than
go/no-go, as only substituting in a lamp dimmer set up, so may well work
with light duty and non inductive load. The 5.1V device tested as a 5V zener
, I wonder why 2x 1N4005 and not 2x 1N4001 in that area.


A Triac can short half of it's element. We have situations like this
with universal motors (series motor) for vacuum loaders. They use a
simple thyristor phase control with a small uC to operate them..
Once in a while, one will short in a why that allows only a DC to
appear to the motor. (Half wave /cycle), since these motors can operate
on DC/AC, hence universal motors, they normally operate at half speed
when the triac fails with no gate control signal. In other words, they
start up at half speed and just keep running, and when demand comes on,
they go up to full speed.
Diode test between B1 B2 terminals don't always reveal a shorted unit
because the meter is not placing the triac under the same conditions. A
short is obvious when performing a diode test between B1 and B2.

Also, Triac's are known to fire off the other side of the thyristor
at the transition, when you don't want it to. In other words, you may
want lets say the + side and then decide at the base of the + side, not
want the (-) side how ever, at times the (-) side will fire and vise versa.

Sensitive Gate triacs are tricky business.

They also have snubberless types.