DIAC (DIODE FOR ALTERNATING CURRENT)

INTRODUCTION

The DIAC (diode for alternating current) is a diode that conducts electrical current only after its breakover voltage, V_BO, has been reached momentarily.

When breakdown occurs, the diode enters a region of negative dynamic resistance, leading to a decrease in the voltage drop across the diode and, usually, a sharp increase in current through the diode. The diode remains in conduction until the current through it drops below a value characteristic for the device, called the holding current, I_H. Below this value, the diode switches back to its high-resistance, non-conducting state. This behavior is bidirectional, meaning typically the same for both directions of current.

DIAC operation

DIAC circuits use the fact that a DIAC only conducts current only after a certain breakdown voltage has been exceeded. The actual breakdown voltage will depend upon the specification for the particular component type.

When the DIAC breakdown voltage occurs, the resistance of the component decreases abruptly and this leads to a sharp decrease in the voltage drop across the DIAC, and a corresponding increase in current. The DIAC will remain in its conducing state until the current flow through it drops below a particular value known as the holding current. When the current falls below the holding current, the DIAC switches back to its high resistance, or non-conducting state.

DIACs are widely used in AC applications and it is found that the device is "reset" to its non-conducting state, each time the voltage on the cycle falls so that the current falls below the holding current. As the behaviour of the device is approximately equal in both directions, it can provide a method of providing equal switching for both halves of an AC cycle, e.g. for TRIACs.

Most DIACs have a breakdown voltage of around 30 volts, although the exact specifications will depend upon the particular type of device. Interestingly their behaviour is somewhat similar to that of a neon lamp, although they offer a far more precise switch on voltage and thereby provide a far better degree of switching equalisation.

Diac structure

The DIAC can be fabricated as either a two layer or a five layer structure. In the three layer structure the switching occurs when the junction that is reverse biased experiences reverse breakdown. The three layer version of the device is the more common and can have a break-over voltage of around 30 V. Operation is almost symmetrical owing to the symmetry of the device.

A five layer DIAC structure is also available. This does not act in quite the same manner, although it produces an I-V curve that is very similar to the three layer version. It can be considered as two break-over diodes connected back to back.

For most applications a three layer version of the DIAC is used. It provides sufficient improvement in switching characteristics. For some applications the five layer device may be used.

SIDAC 

A silicon diode for alternating current (SIDAC) is a less commonly used device, electrically similar to the DIAC, but having, in general, a higher breakover voltage and greater current handling capacity.

The SIDAC is another member of the Thyristor family. Also referred to as a SYDAC (silicon thyristor for alternating current), bi-directional thyristor breakover diode, or more simply a bi-directional thyristor diode, it is technically specified as a bilateral voltage triggered switch. Its operation is similar to that of the DIAC, but SIDAC is always a five-layer device with low-voltage drop in latched conducting state, more like a voltage triggered TRIAC without a gate. In general, SIDACs have higher breakover voltages and current handling capacities than DIACs, so they can be directly used for switching and not just for triggering of another switching device.

The operation of the SIDAC is functionally similar to that of a spark gap, but is unable to reach its higher temperature ratings. The SIDAC remains nonconducting until the applied voltage meets or exceeds its rated breakover voltage. Once entering this conductive state going through the negative dynamic resistance region, the SIDAC continues to conduct, regardless of voltage, until the applied current falls below its rated holding current. At this point, the SIDAC returns to its initial nonconductive state to begin the cycle once again.

Somewhat uncommon in most electronics, the SIDAC is relegated to the status of a special purpose device. However, where part-counts are to be kept low, simple relaxation oscillators are needed, and when the voltages are too low for practical operation of a spark gap, the SIDAC is an indispensable component.

DIAC applications

One of the major uses of DIACs within TRIAC circuits. TRIACs do not fire symmetrically as a result of slight differences between the two halves of the device.

The non-symmetrical firing and resulting waveforms give rise to the generation of unwanted harmonics – the less symmetrical the waveform the greater the level of harmonic generation.

To resolve the issues resulting from the non-symmetrical operation, a DIAC is often placed in series with the gate. This device helps make the switching more even for both halves of the cycle. This results from the fact that the DIAC switching characteristic is far more even than that of the TRIAC.

Since the DIAC prevents any gate current flowing until the trigger voltage has reached a certain voltage in either direction, this makes the firing point of the TRIAC more even in both directions. In view of their usefulness, DIACs may often be built into the gate terminal of a TRIAC.

DIACs are a widely used electronic component. The chief application of DIACs is for use in conjunction with TRIACs to equalise their switching characteristics. By equalising the switching characteristics of these TRIACs, the level of harmonics generated when switching AC signals can be reduced. Despite this, for large applications, two thyristors are generally used. Nevertheless the DIAC / TRIAC combination is very useful for lower power applications including light dimmers, etc.