Ioannis wrote...
The ones I've seen are regular transformers with multiple taps on the
secondary, for experimental reasons. You can get a 220V X 3Amp input for
example and it may have taps for 440V, 110V, 50V, 25V, 10V etc, on its
secondary, such that the Amperage on the secondary times the Voltage is
equal to 220 X 3.
The ones I've seen were used to fire up Carbon arcs in Physics labs.
No. A Triac is a semiconductor device - stands for TRIode AC switch - and
is basically a bi-directional thyristor.
It is a device that is normally off but will break down and conduct if a
large enough voltage is applied in either direction and the breakdown
current will "latch-on" and continue to flow until it falls below a
threshold value, when it will turn off. This basic behaviour describes a
Diac (the two terminal DIode version), which has a fixed breakdown voltage -
a Triac has a third terminal (the gate) that allows the breakdown voltage to
be varied by variation of the gate current. It can be operated such that it
turns on simply when the supply AC voltage exceeds the breakdown voltage or,
more usually, forced on by an applied trigger current.
The device is commonly used as a lighting dimmer when the device is turned
on at varying points of each half cycle, under the control of the gate
current and then stays on until the supply voltage falls almost to zero.
Jayesh asked HOW does it work. The device is based on the Shockley Diode,
which is a four terminal PNPN device.
Effectively this behaves like two overlapped bipolar transistors - one PNP,
the other NPN - with the collector of each connected to the base of the
other:
P
N - N
P - P
N
The P end of the device is the anode and the N end the cathode.
Connected in this way, neither transistor could ever turn on. However if a
sufficiently large voltage is applied to the anode then the lower transistor
will start to conduct due to avalanche breakdown. This in turn will supply
a base current to the upper transistor, which will also turn on. Now each
transistor will hold the other one on and current will continue to flow even
when the voltage is reduced below the breakdown voltage. The current
continues to flow until it falls below a critical holding current whereupon
both transistors will turn off.
The Shockley diode can only conduct in one direction. The Diac is a more
complicated two-terminal five-layer NPNPN device where the top P-N and the
bottom N-P junctions are each shorted out by the contacts. This behaves
just like two back-to-back Shockley diodes, connected in parallel, and so it
can be turned on in both directions and so switch both positive and negative
half-cycles of an AC signal - basically latching on at some fixed point when
the voltage rises above the threshold voltage and staying on until the end
of the half-cycle.
Both the Shockley diode and the Diac have a fixed switching voltage. This
can be varied by the addition of another contact - the Gate. In the
Shockley diode this is achieved simply by applying current to the P-type
layer in the middle of the device (the collector of the PNP and base of the
NPN effective transistors). This contact can be used in one of two ways.
Either the gate current can be used to vary the breakdown voltage of the
device and thus contol where it turns on in the first quarter cycle of the
AC supply or - more commonly - to supply a trigger pulse that forces the
device to latch on at any desired part of the half cycle. Once latched on
then the device stays on until the output current falls to zero, just as in
the Shockley diode. This device is a Silicon-Controlled Rectifier (SCR),
which can only switch current in one direction.
The Triac is effectively a parallel-coupled pair of back-to-back SCRs, which
allows the current to be switched in both directions. In the case of the
Triac an extra layer of N-type semiconductor is required to be added to the
Diac stucture to create the gate contact. This leads to a more complicated
six-layer, three-contact structure that can be triggered in either direction
and control both half cycles of the AC supply, which is what is required for
lighting control.
I hope that this description is not too confusing and reasonably accurate.
Cheers
David
