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Why Use a Triac?
April 1974 Popular Electronics

April 1974 Popular Electronics

April 1974 Popular Electronics Cover - RF CafeTable 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.

Triacs 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?

Circuit Spotlight

By Leslie Solomon, Technical Editor

Why Use a Triac?, April 1974 Popular Electronics - RF CafeThe 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 April 18, 2017

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