April 1974 Popular Electronics
Table of Contents
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are 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).
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
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 April 18, 2017