On Tuesday 13 April 2004 08:45 pm, manifold wrote:
--- In Electronics_101@..., "Roy J. Tellason"
<...>
I did forget to add you will need a RC network across the triac to
protect it from possible high voltages.
That network is to prevent the device from triggering when the
voltage across it rises too rapidly, if I'm remembering right.
Yes, a fast dv dt across the anode and cathode terminals will cause a
SRC or TRIAC to trigger without any gate signal applied.
That's the expression I was reaching for and couldn't quite recall, dv/dt!
The voltage does not need to be particularly high so the RC network is not
there to protect against high voltages, it is there to slow things down a
bit.
This meshes with my understanding as well.
No, A DC gate signal will not 'turn on the TRIAC' it will only rectify
the current to the load if the power source is from the AC mains. A
TRIAC is the same as two reverse parallel SCRs. If a DC gate signal is
applied, only one of the 'SCR halves' will ever turn on.
Oh?
A DC signal should work fine for a photo-coupled TRIAC.I have driven
small motors and lights directly from an optocoupled triac using a DC
signal; it worked fine.
Yes, TRIACs almost always fail closed. Most light dimmers that go bad
stop dimming but the light will still go full on as long as the switch
works. Don't forget that relays can also weld their contacts together
and fail short. Look for a more positive way to shut off the circuit
if there is a failure like remove all power, short the motor and blow
the fuse, etc.
Yes, I was thinking that fuses were good to have in there. As well as way
over-spec'ing the triacs, as I sure haven't seen many fail.
