What is a double base diode (Unijunction transistor) and how to detect it?
Double base diode, also known as Unijunction transistor (UJT), is a three terminal semiconductor device with only one PN junction.
At both ends of a high resistivity N type silicon wafer, two ohm contact electrodes (very small contact resistance contact electrodes) are made,which are called the first base B and the other side of the base Be: silicon wafer near the second base pole to make a PN junction, and the electrode led on the P type semiconductor is called the emitter E.
In order to facilitate the analysis of the operating characteristics of double base diode, the N type region between two base B and B2 is usually equivalent to a pure resistor RB, called the base resistor. It is an important parameter of the double base diode.
RB can also be seen as a series of two resistors in series, where RB1 is the resistance between the base Bi and the emitter E, and RB2 is the resistance between the base B2 and the emitter E. When working normally, the resistance of RB1 varies with the emitter current Ip, which can be equivalent to a variable resistor. The role of PN junction is equivalent to a diode VD.
The two most important parameters of the double base diode are the base resistance RBB and the partial pressure ratio.
RBB refers to the resistance between the two base poles in the open circuit state of the emitter, that is, RB1+RB2, usually RBB between 3~10kΩ.
The ETA is the ratio of the voltage between the emitter E to the base B, and the voltage between the base electrode B2 and B, which is usually between 0.3~0.85.
Detect a double base diode
1.Discriminant base
(1) Discriminant emitter E.
When the multimeter is placed in the RX1kΩ block and the positive and reverse resistance values between any two electrodes are equal (2~10kΩ) with two forms, the two electrodes are B1 and B2, and the remaining one is the emitter E.
(2) Judge the base B1 and the base B2.
The black pen is connected to E., and the two other electrodes are contacted with a red pen in order to measure two forward resistance values respectively. In manufacturing engineering, the second base B2 is near the PN junction, so the forward resistance between emitter E and B2 is small, and the difference between them is tens to tens of thousands ohm.
Therefore, when the resistance measured by the above method is relatively small, the electrode connected to the red pen is B2, and when the resistance is large, the electrode connected to the red pen is B1.
Note: the above judgment base B1 and B2 methods are not suitable for all double base diodes. The positive resistance between E and B1 with individouble pipes and the forward resistance between E and B2 are not significant. It is not very important to accurately determine which two bases of the double base diode are B1 and which is B2 in practice.
Even if B1 and B2 are reversed, they will not damage the tube, only affect the pulse amplitude of the output. When the output pulse is small, the original identified B1 and B2 two electrodes can be adjusted to try to determine the correct connection of B1 and B2 with actual use effect.
(2) Whether the pipe is good or bad, the multimeter is placed in RX 100Ω or RX1kΩ gear. When the black pen is connected to the emitter E, red pen to connect to B1 or B2, the positive resistance value of the double base diode PN junction should be a few to a few thousands ohm, a little larger than the positive resistance value of the ordinary diode. The red pen is connected to the emitter E. and connected to the black pen with B or B2. At this time, the reverse resistance value of the double base diode PN junction is measured, and it should be infinite when normal. The red and black pens are respectively connected to B and B2, and the resistance value between the double base diode B1 and B2 should be within the range of 2~ 10kΩ. Too large or too small, it can not be used.
3) measure the negative resistance characteristic between B1 and B2, and connect 10V DC power supply. The multimeter is placed in the RX100Ω or RX 1kΩ block, the red pen is connected to the B1 pole, the black watch is connected to the E pole, which is equivalent to the 1.5V forward voltage between E- B. When normal, the multimeter pointer should be stopped at the infinite position, indicating that the pipe is in the cut-off state, because the pipe is at the peak point below P, and it is far from negative resistance zone.
If the pointer deflects to the right, it shows that the tube has no negative resistance characteristic, which is equivalent to the volt ampere characteristic of an ordinary PN junction. Such pipes are not suitable for use.
(4) The partial pressure ratio derived from the internal structure of the double base diode is expressed as the measured partial pressure ratio ETA
When testing, use the RX 100Ω or RX 1kΩ gear of multimeter to measure the REB1,REB2 of the double base diode.And the RBB value can be used to calculate the partial pressure ratio.
This article is from Allicdata Electronics Limited.