Jamie t wrote in message
...
N_Cook wrote:
Jamie t wrote in
message
...

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.



Thanks for that, I'll try exploring further. No snubber in this use.
Would you know of a term for the observed broad spike, of amplitude
twice or
more the normal pk-pk amplitude, generated and passing through to the
secondaries , increasing the rectified voltage to exceed normal ratings
?
Pseudo bucky inverter fault ?


A bad transformer can cause that. Do you have a megga meter you can use
on the transformer? Also, if I can remember that circuit, a cap failing
only at higher voltages can generate intermitting shorts and if in the
proper circuit, they'll just recover and go on. This kind of activity
can cause all kinds of strange spikes to appear.

Bridge rectifiers have been known to intermitly short, and then cause
a big spike to appear at the secondary. The filter caps (large one's)
can also do this..
So, you have a lot to look at here. If the transformer seems to be
running hot, I would disconnect the secondary of the transformer and
allow it to run unloaded, then perform some test via a scope and do
temperature checks.

P.S.
I've found transformers to become shorted that shows no sign of
electrical damage from heat but yet, exhibit very low Lx at the primary
side with secondary fully floating.. When doing a Lx check on the
primary the Lx value should not exceed a value that would generate over
the max expected load.. Some designs actually saturate or near saturate
the power transformer with no load which will cause it to heat up. This
is normal in design for a couple of reasons but should not measure
values of Sqr(Lx^2+R^2) that would exceed Max operating point. Normally,
this type of design is to reduce side effects from coupling circuits and
reduce cost in materials.





This was a consistent spike (3mS or so broad ) , once per mains cycle. It
was possibly variable in degree as 0.1 amp or so of "saturation" current one
time and 1 amp or so another time. Repeated firing up with a TIC226 and no
return , will try with a TOP66 higher rating triac , perhaps today and see
if still no return to spikes, before a replacement TIC236 placed in there.
The transformer powered straight from the mains was perfect, no saturation
current / core or wiring rattle or whatever gave that nasty noise in a
toroidal tx