What Is a Unijunction Transistor?

Single junction transistor (UJT for short) is also called base diode. It is a semiconductor device with only one PN junction and two resistance contact electrodes. The ohmic contact leads to two bases b1 and b2. A P region is made as an emitter e by an alloy method on a side slightly b2 in the middle of the silicon wafer.

only one

The resistance between the two bases b1 and b2 is called

Single junction transistor

Judging Single Junction Transistors

Single-junction transistors have large

(1)

The width of the pulse voltage output by the single-junction transistor trigger circuit is mainly determined by the time constant of the capacitor's amplification: t = R1c. R1 or C is too small, the discharge is fast, the width of the trigger pulse is small, and the thyristor cannot be triggered. Because the thyristor takes a certain time from the blocked state to full conduction, generally below 10uf, the width of the trigger pulse must be above 10uf. If you choose C = 0.1, 1uF, R1 = 250, 100, you can get a pulse width of tens of microseconds. However, if the value of C is too large, since the minimum value of the charging time constant (RP + R) C is determined by the minimum control angle, then (RP + R) must be small. As mentioned above, this will cause the shoot-through of the single junction transistor. phenomenon. If R1 is too large, when the single-junction transistor has not been turned on, its leakage current may generate a large voltage on R1. This voltage is added to the control pole of the thyristor and causes false triggering. Generally, the non-triggered voltage of the thyristor is 0.15, 0.3V, so the above voltage should not be greater than this value. The amplitude of the pulse voltage is determined by the DC power supply voltage and the voltage division ratio of the single junction transistor. If the power supply voltage is 20V and the voltage division ratio of the transistor is 0.5, the voltage on the capacitor is about 10V when the single-junction transistor is turned on. Except for the tube voltage drop, an output pulse voltage with an amplitude of 7 ~ 8V can be obtained. According to the above data, the width and amplitude of the output pulse can meet the requirements of triggering thyristors ^[3] .