The Diac can be thought off as two thyristors connected in
antiparallel(A triac without a gate electrode):
O
.......|.......
: .--*--. :
: | | :
: _|_ | / :
: \|/ -+- :
: -+- /|\ :
: / | "|" :
: | | :
: `--*--' :
:......|......:
O
The diac only have two
terminales. It will be triggered when
the
voltage over it rises high enough, and stay in the
conducting state until the current flow through it falls bellow
the holding current.
The Diac is mostly used in triac triggering circuits. They
may easily be mistaken for diodes, since the '
package' is
similar, but a diac will have two rings, one in each end:
________
___| || |___ DIODE
|_||_____|
________
___| || || |___ DIAC
|_||__||_|
symbol:
|
__|__
\/ /\
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|
|