|
April 1974 Popular Electronics
 Table of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
Popular Electronics,
published October 1954 - April 1985. All copyrights are hereby acknowledged.
|
The Triac (triode for
alternating current) is not a component
often used in RF and microwave circuit design, but being conversant in its operation
could make you popular at nerd parties. A triac is basically the equivalent of two
SCRs (silicon-controller rectifier, aka thyristor) connected back-to-back, allowing
it to conduct on both the positive and negative half-cycles of an AC connection.
Both devices are most commonly used in switching applications. The unique feature
of an SCR and triac is that once the gate voltage is sufficiently high to begin
conduction between the anode and cathode, it can be removed and conduction will
continue until the anode-cathode voltage is removed (i.e., holding current removed).
This 1974 Popular Electronics magazine article introduces and explains
the operation and applications for triacs.
Circuit Spotlight: Why Use a Triac?
By Leslie Solomon, Technical Editor
The use of the triac in various power control
systems may not be particularly innovative; but, in looking at the circuit, one
tends to wonder just why a triac was used instead of some other component - or components.
Many hobbyists are not really that familiar with the triac.
Since a triac can be considered to be a second-generation silicon controlled
rectifier, it is necessary to understand how the latter works before getting into
details on the former. An SCR is a four-layer pnpn semiconductor device having three
electrodes - cathode, anode, and gate. With a forward bias (positive voltage on
the anode, cathode connected to common), an SCR should behave like a conventional
diode. In that case, current would flow through the junction and through any load
in series.
However, the construction of an SCR is such that current cannot flow through
the junction unless both the anode and gate are simultaneously positive with respect
to the cathode. As soon as this happens, the SCR conducts fully, after which the
signal on the gate no longer has any effect. Thus, if pure DC (rectified and filtered)
is used as the power source, the SCR will not turn off as long as the anode voltage
is applied.
But in most SCR circuits, either raw AC or rectified but not filtered de is applied
to the SCR. This means that only the positive half cycle has any effect on the SCR
since the negative half cycle reverse biases the SCR and can't be used (see sketch
A). The amount of power controlled by the SCR depends on how long the positive voltage
is allowed to remain on the anode, thus supplying current to the load. The SCR turns
off automatically when its anode voltage drops to zero.
If the SCR is turned on late in the positive half cycle (sketch B), only a small
amount of current is available for the load; but when the gate signal is used to
turn the SCR on earlier in the positive half cycle, the current through the load
is increased. Keep in mind that the SCR turns off at each zero crossing and must
be retriggered in each positive half cycle. Varying the triggering is usually the
job of a phase-shift network which drives the gate (a circuit found quite commonly
in home light dimmers, power tool controllers, etc.).
Obviously, no matter how early in each positive half cycle the SCR is triggered,
the best it can do is pass half of the available power in each cycle - hardly a
profitable arrangement. To remedy the situation, bridge rectifiers are sometimes
used for full-wave rectification. (The negative half cycle gets "folded up" to become
a positive half cycle.) This approach permits using more of the available power;
but the rectifiers cost money too.
Now back to the triac, which is essentially a pair of SCR's connected as shown
in sketch C, with just one common gate for the two junctions. But the two junctions
are, so to speak, back-to-back so that the other two terminals can't be marked anode
and cathode. Instead they are called simply Main Terminal 1 and Main Terminal 2
(MT1 and MT2).
Unlike the SCR, the triac can conduct on both halves of the cycle - with MT1
positive on one half cycle and MT2 positive on the other half cycle. Thus the triac
can deliver more power than a single SCR, without a special power supply circuit.
Posted February 22, 2024
(updated from original
post on 4/18/2017)
|
