report ujt

7/29/2019 Report Ujt

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MARIO CHAPARRO- HOOSMAN MONROY POWER ELECTRONICS 1

Abstract-In this lab, we perform a typical installation of a junctiontransistor UJ T joined, resistors, capacitor and power supply were

calculated previously, although the development of practice, we hadto make some changes in resistance and the capacitor, for moreaccurate results.

INTRODUCTION

The uni-junction transistor, consists of two regions with threeexternal terminals, two base and an emitter, the emitter region

(p) is heavily doped with impurities, the region is weakly n-doped, for this reason resistance between bases (RBB) is high.

When not driving, the voltage drop can be expressed asfollows:

V1= R1/R1+R2 (VB2B1)

A. Operation of a UJ TThe operation of a UJT is very similar to an SCR. The

electrical characteristics of this device through the connectionof the emitter voltage (VE) with the emitter current (IE). We

define two critical points: point of peak or peak-point (VP, IP)and point-point valley or valley (VV, IV), both verify the

condition of PVS / ck = 0. These points, in turn, defines threeregions of operation: cutting region, and negative resistance

region saturation region, as detailed below:Regin de corte.

In this region, the emitter voltage is low so that the intrinsic

tension keeps emitting diode reverse biased. The emittercurrent is very low and it is verified that VE


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E. PROCEEDINGSRelaxation oscillator.

This circuit serves to generate signals for control of powerdevices as thyristors or triacs. The capacitor is charged up to

the trigger voltage of the transistor UJT, this happens whenthis is discharged through the junction E-B1.

The capacitor discharges until the voltage reaches a valley

called (Vv) of approximately 2.5 volts.

With this voltage UJT transistor turns off (stops drivingbetween E and B1) and the capacitor begins loading again.

If it is desired to vary the oscillation frequency can be changed

so the capacitor C as the resistor R1. R2 and R3 are also

important to find the oscillation frequency.

For our practice we used a 2N4870 or 2N2646 UJT

- Power supply of 20 V.

- Frequency of about 500 Hz

f = 1 / RC ln (1/1 - ).

- Resistance R2R2= 0,7 RBB / VR2= 470

- The capacitor can be raised by any tension.

- The capacitor discharges until the voltage reaches a valley

called (Vv) of approximately 2.5 V.

- It is very important to know that R1 must have values that

must be within acceptable limits for the circuit can oscillate.These values are obtained with the following formulas:

Maximum R1 = (Vs - Vp) / IpMinimum R1 = (Vs - Vv) / Iv- Vs = 10V

- Vpk = manufacturer's specification- Ip = manufacturer's specification

- Vv = manufacturer's specification

- Capacitors: 0.1 Uf

SIMULATION

Saturation resistance RB1

Charging and discharging the capacitor

RESULTS.

When mounting with the estimates obtained earlier, oscillator

showed no rectification at the appropriate output, and sometests were made in order to obtain a good result.


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The above graph was obtained with a 0.1 uF capacitor,

although we obtained a good sign in and out, we wanted to geta higher frequency. We obtained a frequency of 230.4Hz, v

3.68 Vrms, Vpp 5.4 V.

In this figure shows a sawtooth signal at a frequency of1025Khz, Vrms 255mv, Vpp 640mv.

The above graph was obtained by varying the base resistors,RB1 = RB2 = 330 and 47, R = 20, the remaining

components are left the same as had been working. Weobtained a 17.86 Hz input frequency, Vrms 356mv, v 10.4

Vpp output frequency of 5.97Hz.

The figure above shows the result of recalculating the baseresistors, the input voltage and other components. We

obtained a frequency of 350 Hz, with Vcc = 15 V, RBB = 7 K,

R = 10 K, RB2 = 470K, RB2 = 470, C = 220 nF.

CALCULATIONS IDEALS.

Ve= 0.6 (Vbb (Rb1+Rb1)/Rbb+Rb1+Rb2

Rb1=100k

Fr= 1/Tc500Hz= 1/Tc

12=20(1-e-0.002/RC

)0.6-1=(1-e

-0.002/R(0.1uF))

-0.4=(-e-0.002/0.1nF

)0.4=(-e

-0.002/0.1nF)

0.91=(0.002/1nF)R=(0.002/0.91(1nf)

R=2k

Rbb=10k

Rbb= RB1+RB2= 10k0.6=RB1/10k

RB1=6kRB2=4k

F. CONCLUSIONS An additional fact that the manufacturer gives the

current required is to be found between E and B1 forthe UJT triggering = Ip.


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Through several attempts we could verify that theoscillator is not working properly, this is due to the

value of the base resistance, as they must be almostprecise in order to generate the pulses, this has to

recalculate the values to reach the desired output.

Another of the problems that had to face was thegeneration of short pulses at low frequency, because

the charge on the capacitor is too slow, this capacitybeing great, making it impossible to be loadedquickly. To do this we need to verify the calculations

of the capacitor and put one with proper TAO, it isoften best done empirically and changes between

different capacitors.

Something to keep in mind, is the polarization of thetransistor, as we can be wrong about assigning theissuer bases.

If no signals are obtained transistor bases, maybe oneis disconnected or the transistor is burned, sometimes

it may be better to try another UJT.

G. REFERENCES Analog Electronics I. Espluga Nelson. The transistor

UJT uni-or union. 2004.

Design and Integration of Mechatronic Automation.Alexis Andres Rodriguez Diaz. 2007

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