Characteristic curve of diac

What is the characteristic of a diac? Is diode characteristic symmetrical? Diac Characteristics Curve. Figure shows the characteristic curve of a diac.

It is a symmetrical curve , meaning that it behaves the same way for current in either direction.

The breakover voltage of diacs is around V (typically 20–V). When the applied voltage surpasses V BR, a diac sharply conducts and the current is abruptly increased. Physically it looks like a regular semiconductor diode. As the voltage is increased from zero in either direction, a small amount of leakage current occurs, as shown in characteristic curve. When VBR is reached in either direction, the diac fires (ie, starts to conduct).

When breakdown occurs, the diode enters a region of negative dynamic resistance, leading to a decrease in the voltage drop across the diode an 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, IH. The curve looks like the English word Z.

The slow curve before going to the straight line is due to the leakage current. V-I characteristics of a diac is shown below. Volt-ampere characteristic of a diac is shown in figure.

Its looks like a letter Z due to symmetrical switching characteristics for each polarity of the applied voltage. The diac performs like an open-circuit until its switching is exceeded. At that position the diac performs until its current decreases toward zero. DIACs come in a variety of formats.

DIAC Characteristics From the figure above, we can see that a DIAC has two p-type material and three n-type materials. Also, it does not have any gate terminal in it. A current–voltage characteristic or I– V curve is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or potential difference across it.

The triac has on and off state characteristics similar to SCR but now the char acteristic is applicable to both positive and negative voltages. The DIAC can be turned on for both the polarity of voltages. This is expected because triac consists of two SCRs connected in parallel but opposite in direc tions.

The First quadrant represents the positive half cycle where the current will be flowing from MTto MTand the second quadrant represents the negative half cycle where the current will be flowing from MTto MT1. The break-over voltage of the DIAC remains unchanged because of the absence of the GATE terminal. For the positive half cycle, the characteristic obtains in the first quadrant.

I-V characteristic.

For the negative half cycle, the characteristic obtains in the third quadrant. The IV characteristic is as shown in the below figure. The characteristics curve is shown in the below Diagram.

In above figure a simple DIAC TRIAC Power circuit is shown, the working of this circuit is can be. DIAC , Diode AC bi-directional switch is widely used with TRIACs to improve the fact that its switching characteristic is far more even than that of the TRIAC. Here is a macro model of the DIAC.

The model has two parameters. VK specifies the breakover voltage at which the DIAC begins to conduct. RS specifies the limiting series resistance of the device.

The diac , because of its peculiar construction, does not switch sharply into a low voltage condition at a low current level like the SCR or triac. Instea once it goes into conduction, the diac maintains an almost continuous negative resistance characteristic , that is, voltage decreases with the increase in current. This means that, unlike the SCR and the triac, the diac cannot be expected to maintain a low (on) voltage drop until its current falls below a holding current level.

From the figure above, we can see that a DIAC has two p-type material and three n-type materials. As discrete components they may be contained in small leaded packages, they can be obtained in surface mount packages, in large packages that bolt to a chassis, or a variety of other packages.