2 Thyristor Triggering Devices – UJT and DIAC

1. Unijunction Transistor

A Unijunction Transistor is a 3 terminal device with a single PN junction and is basically used to trigger a SCR or a TRIAC. It is a unidirectional device.

UJT Construction

A unijunction Transistor is constructed using a lightly doped N type silicon bar, onto which a heavily doped P-type bar is alloyed. Metallic contacts are embedded on three sides, from which three terminals are taken out, which are named as Emitter, Base1 and Base2.

UJT Operation

A UJT can be seen as equivalent of a diode connected to the junction of two resistors. The resistors are the internal resistances of the two bases. The supply voltage is normally applied across the two base terminals and the input voltage is applied to the emitter terminal and the base1.  The device will conduct when the applied voltage exceeds the diode forward voltage as well as the junction voltage of the two resistors. In other words, when the applied voltage exceeds the peak voltage, the device will conduct.

Initially as the applied voltage is lesser than the peak or threshold voltage, a negligible amount of current will flow and the device is in cut off region. Once the applied voltage reaches the threshold level, the device starts conducting and current flows through the device. As voltage decreases, the current increases and thus the device is in negative resistance region. This decrease in voltage occurs till the applied voltage reaches a valley point voltage and a saturation point is reached.

Application of UJT to trigger a TRIAC

UJT can be used in a relaxation oscillator used to produce pulses to trigger a TRIAC.

In the above circuit the applied AC voltage is rectified using a bridge rectifier and regulated using a zener diode. This regulated DC voltage is applied to the capacitor, which starts charging through the variable resistor. Once the capacitor voltage reaches the peak or threshold voltage, the UJT starts conducting and the capacitor starts discharging through the UJT and the primary of the transformer and a pulse voltage is produced across the secondary of the transformer, which is given to the gate of the SCR to trigger it. Once the SCR is triggered, it will start conducting, irrespective of any gate voltage.

2. DIAC

A DIAC is a combination of two Shockley diodes (conducting current in one direction) connected back to back such that the device conducts in both directions. It is a bidirectional device which conducts on being triggered with a voltage. It is actually a thyristor and conducts only when a voltage exceeding a particular level is applied. This is the breakdown voltage or V_BO which can be a momentarily rising voltage. DIACs are widely used as a switch to trigger devices used in circuits like a Lamp dimmer, motor speed control etc. Its main application is to phase shift a Triac. The only difference from a UJT is that a DIAC is a bidirectional device.

DIAC Operation

When operated with a DC voltage, a DIAC is exactly like a diode. But with AC voltage, the DIAC conducts for each of the half cycles, only when the voltage reaches a certain level. The DIAC is a small diode similar to the rectifier diode. But unlike the rectifier diode, it is bidirectional and conducts in both directions. But it conducts only when the voltage through it rises above its breakdown voltage, typically 30 volts. When this occurs, the DIAC enters into the region of negative dynamic resistance leading to a sharp decrease in voltage drop across it. Negative resistance implies that the current starts increasing and the voltage across it starts decreasing. This leads to a sharp increase in current through the DIAC. It remains in the conducting mode until the current through it drops to a particular value specific for the device. This current is called holding current IH. Below the value of the holding current, the Diac again enters into a high state of resistance and non conducting mode. This characteristic make the DIAC an ideal switch in power control systems. This behavior of the DIAC is bidirectional and occurs in both the directions of the current.

DIAC Operation in AC

In AC circuits, where it is required to give AC supply to loads, the DIAC can be used as a switch to trigger a load switch. Normally a TRIAC or a SCR is used to control the supply of AC current to loads like Incandescent lamp or a fluorescent lamp and acts as a load. However it is not safe to directly connect the TRIAC directly to the AC supply and for this reason a different device is needed to control the AC supply to the TRIAC. This is where the role of a DIAC comes.

During the positive half cycle, MT1 is positive with respect to MT2, which is negative. Thus the 1^st junction is reverse biased and the second is forward biased.  As we know for a reverse biased junction, current will not flow until the applied voltage reaches a breakdown level. Similarly in the DIAC, only a negligible amount of current will flow through the device. Once the applied voltage exceeds the reverse breakdown voltage of that junction, the current starts flowing and the device conducts.

During the negative half cycle, MT2 is negative with respect to MT2 and MT2 is positive. The device will start conducting only when the applied voltage exceeds the break down voltage.

Transistor Equivalent of a DIAC

A DIAC can be seen as an equivalent of a transistor with no base connection and both the junctions having identical characteristics. When one of the junction is forward biased, the other is reversed biased and the break over voltage is the reverse breakdown voltage like in a zener diode or a forward break down voltage. If the polarity of the voltage applied across the DIAC is reversed, still it will conduct and this is the reason why a DIAC is seen as a bidirectional device.

DIAC Construction

DIAC is a three layered structure and has no gate electrode or a control terminal. They are considered as Symmetrical trigger diodes due to the symmetry of their characteristic curve. Their terminals are not labeled as anode or cathode and can be connected either way round. The terminals may sometime label as A1 and A2 or MT1 and MT2. It has two junctions – one a forward biased one and another a reverse biased junction. It is constructed in the same way as a transistor with the only difference being in the fact that in a DIAC both the junctions are doped with equal concentration. It is packaged like a pn junction diode.

An Application of DIAC to Trigger TRIAC

DIAC controls the phase angle of the firing of TRIAC, so the current through the lamp can be controlled. Variable Resistor and the capacitor act as the phase shift network. When the voltage across the capacitor reaches the break over voltage of the DIAC, it starts discharging through the DIAC. The DIAC starts conducting and this gives a triggering pulse to the Triac gate and the Triac starts conducting.

Know more about DIAC and TRIAC.

Photo Credit

• Unijunction Transistor by wikimedia