1.r,rc,ujt firing

R, RC & UJT FIRING MODULE(Model No : PEC14M12)

User Manual

Technical Clarification /Suggestion :NNNN / FFFFTechnical Support Division,Vi Microsystems Pvt. Ltd.,Vi Microsystems Pvt. Ltd.,Vi Microsystems Pvt. Ltd.,Vi Microsystems Pvt. Ltd.,

Plot No :75, Electronics Estate,Perungudi, Chennai - 600 096, INDIA.Ph: 91- 44-2496 3142, 91-44-2496 1852Mail : [email protected],Web : www.vimicrosystem

Version 1.0

R, RC & UJT FIRING MODULE PEC14M12

Vi Microsystems Pvt. Ltd., [ 1 ]

Chapter-1

Introduction



1.1 INTRODUCTION

Power electronics deals with the application of solid-state electronics for the control andconversion of electric power. Conversion techniques require the switching ON/OFF of powersemi conductor devices.

Modern Power Electronics started with the invention of SCR commonly known as thyristor in1956. At present, a number of power switching semiconductor devices are available namelyBJT, MOSFET, IGBT and GTO etc. All these devices are turned ON/OFF through controlterminal, either base or gate as the case may be. This makes simple power circuit.

This manual is intended to give you description of R, RC & UJT Firing Circuits. An SCRcan be switched from OFF state to ON state by means of triggering. Gate triggering is themost common method of turning ON the SCRs, because it is an efficient and reliable method.

The gate control circuit is also called firing or triggering circuit. These gating circuits areusually low power electronic circuits. A firing circuit should fulfil the following two functions.

(i). If power circuit has more than one SCR, the firing circuit should produce gating pulsesfor each SCR at the desired instant for proper operation of power circuit.

(ii). The control signal generated by a firing circuit may not be able to turn-on an SCR. Itis therefore common to feed the voltage pulses to a driver circuit and then to gatecathode circuit. A driver circuit consists of a pulse amplifier and a pulse transformer.



Chapter-2

About Our Trainer



2.2 SPECIFICATION

1. Power Input : 230V AC, 50 Hz.

2. Transformer : 230V/24V AC

3. Load Rheostat : 50E/4A

4. Maximum Load Rheostat : 2 Amps

5. SCR (TYN612) : 600V, 12A

6. UJT : 2N2646

7. Zener Diode : 18V/1 W

8. Fuse : 0.5 Amps



2.3 FRONT PANEL VIEW



2.4 FRONT PANEL DESCRIPTION

The front panel of R, RC & UJT Firing module consists of

i. R Firing Circuit

ii. RC Firing Circuit

iii. UJT Firing Circuit

1. Power ON/OFF switch is provided to control the input AC mains supply to themodule.

2. AC ON/OFF switch is provided to control input AC voltage to the power circuit.

3. The input supply to the R, RC & UJT firing circuit are given through the T7- T9, T12-T13 and T1-T2 terminals respectively.

4. Test points are provided to view different signals in the power circuit.

5. P & N terminals are provided to apply 24V AC input to the firing circuits.

6. Pulse output from each firing circuits are terminated at respective G and K terminals.



TEST POINT DESCRIPTION

i. UJT FIRING

T3 & T4 - Test points to check the input DC current

T3 & T7 - Test points to check the input DC voltage

T4 & T7 - Test points to check the Zener diode voltage

T5 & T7 - Test points to check the capacitor voltage

T6 & T7 - Test points to check the output pulses.

ii. R FIRING

T10 & T11 - Test points to check the output trigger pulse.

iii. RC FIRING

T15 & T16 - Test points to check the output trigger pulse.

T14 & T16 - Test points to check the capacitor voltage



2.5 HARDWARE DESCRIPTION

2.5.1 Block Diagram

The block diagram consists of

i. Step down transformerii. Diode bridge rectifieriii. UJT Firing circuitiv. R Firing circuitv. RC Firing circuit vi. Pulse Transformer

2.5.2 Block Diagram Description

i. Step Down Transformer

The 230V AC input supply is step down to 24 V AC using step down transformer. Thisvoltage is given as input to the firing circuit.

ii. Diode Bridge Rectifier

The diode bridge rectifier converts the input AC voltage into fixed dc voltage. The constantrectified DC voltage is obtained through the zener diode. This dc voltage is given to UJTFiring circuit.( This rectifier is only for the UJT Firing circuit).

iii. UJT Firing Circuit

The resistor, charging circuit (RC) Zener diode (Z) & UJT (2N2646) forms the UJT firingcircuit. The input is DC voltage. It is used to generate triggering signals to SCR.

Step DownTransformer

Diode Bridge UJT Firing Pulse Transfomer

RC Firingcircuit

Rectifier

circuitR Firing

230V AC50 Hz

230/24V

G

KcircuitTo G & Kof SCR



iv. R Firing Circuit

R Firing circuit is made up of variable resistance which controls the firing angle of SCR in therange of 0-90°. This circuit controls the gate current to the SCR.

v. RC Firing Circuit

RC Firing circuit is made up of resistance and capacitance. On varying the resistance, thecurrent through the gate varies and controls the firing angle in the range of 0-180°. Thismethod also controls the gate current to the SCR.

vi. Pulse transformer

The purpose of pulse transformer used in triggering circuit are

i. The isolation of low voltage gate circuits from high voltage anode circuit.

ii. The triggering of two or more devices from the same trigger source.

vii. SCR Device (TYN612)

SCR Device along with RC Snubber protection circuit is provided in the module. Using thedevice the study of SCR operation for different firing circuit can be done. Snubber circuit isprovided to limit the switching losses of the device.



2.7 Protection

1. Fuse is provided to protect the trainer from over current faults.

2. Earthing is provided to protect the trainer module.

3. RC Snubber circuit is provided to protect the SCR.

2.8 Precautions

1. Before doing connections make all switches in OFF position.

2. Load Rheostat should be kept in maximum position.

3. Do not view the AC input waveform and pulse waveform in CRO with dual mode.

4. 24V input AC to the SCR device should not be connected directly.



Chapter-3

Experiment Section



Experiment 1: Study of R-Firing Circuit

Aim:

To study the operation of resistance triggering circuit using R, RC & UJT Firing module.

Apparatus Required:

1. PEC14M12 Module2. Patch chords.

Accessories to be provided by the institution

1. CRO2. 50E/4A Rheostat3. Digital Multimeter - 01

Circuit Diagram

minR

50 Hz24V AC

K

50E/4A

RDSCR

A

v

bR

LoadRheostat

(POT)



Connection Diagram



Connection procedure:

1. Connect the input supply to the module.

2. Connect P & N terminals to T7 & T9.

3. Connect one end of load rheostat to anode (A) of SCR.

4. Connect the other end of the load rheostat to the P terminal of 24V AC supply.

5. Connect the cathode (K) of SCR to the N terminal of 24V AC supply.

6. Connect the G & K terminals of firing circuit to G & K terminals of SCR.

7. Connect the CRO groung to Anode of SCR. Connect channel 1 probe to T7 & channel2 probe to cathode of SCR.

Experiment Procedure

1. Verify the connection as per the connection diagram.

2. Switch ON the power supply.

3. Switch ON the power on/off switch.

4. Switch ON the 24V AC S/W.

5. Switch ON power supply to CRO & select suitable sensitivity and time settings. Useline trigger module.

6. Observe the waveform for input AC voltage & the load voltage.

7. Study the waveform for various firing angle by varying the POT in R firing circuit.

8. For a particular firing angle, plot the waveforms on a graph sheet to scale.

9. Measure the average DC voltage across the load & rms value of the ac input voltageusing a Digital multimeter.

10. Calculate the dc output voltage using the equation.

VV

dc 2

21

cos



and compare the measured value.

- Firing angle delayV - RMS value of the ac input voltage.

Model Graph

L,i

LV

V

e

t

t

t

s

ak



Experiment 2: Study of RC-Firing Circuit

Aim:

To study the operation of resistance capacitance triggering circuit using R, RC & UJT Firingmodule.

Apparatus Required:




Circuit Diagram

G

D2

A+ -

SCR

C

D1R

50E/4A

K

24V AC50 Hz

v

LoadRheostat

(POT)



Connection Diagram





2. Connect P & N terminals to T12 & T13.



5. Connect the cathode (K) of SCR to the N terminal of 24V AC supply.

6. Connect the G & K terminals of firing circuit to G & K terminals of SCR.

7. Connect the CRO ground to Anode of SCR connect channel -1 probe to T12 &channel 2 probe to the cathode.





4. Switch ON the 24V AC S/W.

5. Switch ON power supply to CRO & select suitable sensitivity and time settings. Useline trigger module.

6. Observe the waveform for input AC voltage & the load voltage.

7. Study the waveform for various firing angle by varying the POT in R firing circuit.

8. For a particular firing angle, plot the waveforms on a graph sheet to scale.

9. Measure the average DC voltage across the load & rms value of the ac input voltageusing a Digital multimeter.

10. Calculate the dc output voltage using the equation.

VV

dc 2

21

cos



and compare the measured value.

- Firing angle delayV - RMS value of the ac input voltage.

Model Graph

L,i

LV

V

e

t

t

t

s

ak



Experiment 3: Study of UJT Firing Circuit

Aim:

To study the operation of UJT triggering circuit using R, RC & UJT Firing module.

Apparatus Required:




Circuit Diagram

D1

C

50V/4A

SCR

G

E

R1

B2

B1

D3

D4 D2

Z

R

24V AC

ALoad

Rheostat

(POT)

K



Connection Diagram







4. Connect the G1 & K1 terminals of the UJT firing circuit to gate(G) and cathode (K)terminals of SCR.





4. Connect the CRO ground to ground terminal of UJT Firing circuit.

5. Switch ON the power supply to CRO & select suitable sensitivity and time settings.Use line trigger module.

6. Observe the waveform for input ac voltage & pulsating DC output.

7. Connect channel 2 probe to terminal T4 and to observe the zener diode voltage.

8. Connect channel 2 probe to terminal T5 and to observe the capacitor voltage.

9. Study the waveform for various firing angle.

10. For a particular firing angle plot a waveform on a graph sheet to scale.

11. Observe the capacitor voltage is zero at the end of every half cycle.



Model Graph

V

0

0

0

dc

zV

Vc

gV

acV

t

t

t

t

t



Chapter-4

Theoretical Concept



The following section is discussed about the various types of firing circuits.

4.1. R Firing Circuit:

This is the simple method for varying the triggering angle upto 90º. Instead of using gatepulse, here gate current is used to trigger the SCR. The circuit diagram is shown in fig 4.a.

Fig 4.a. R Firing circuit

The circuit operates as follows,

1. When a gate current Ig is equal to Ig(min), SCR turns ON while es goes positive and eLis approximately equal to es.

2. As es goes negative, SCR turns OFF and the load voltage (eL) is zero during thisperiod.

3. The purpose of the diode(D) in the gate circuit is to prevent the gate cathode reversebias during the negative half cycle.

4. The same sequence is repeated, when eS again goes positive.

5. By varying Rv, the load voltage can be controlled.

6. If Rv increases, the firing angle ’ will increase & vice versa.

7. The limiting resistor Rmin is used to limit the gate current to a safe value while varyingRv.

8. The stabilizing resistor Rb should have such a value that maximum voltage drop acrossit does not exceed maximum possible gate voltage Vgm. This can happen only when Rvis zero.

b

v

oV

-+Load

minR

50 Hz24V AC

K

RD

R

SCR

A



When the supply voltage has reached its peak, Emax

From the voltage distribution

The thyristor will trigger when the instantaneous anode voltage, es, is

where Ig(min) = Minimum gate current to trigger the thyristorVd = Voltage drop across the diodeVg(min) = gate-voltage to trigger, corresponding to Ig(min).

The waveform for R firing circuit is shown in Fig 4.b.

Fig. 4.b Waveform for R Firing circuit

L,i

LV

V

e

t

t

t

s

ak



4.2. RC Firing Circuit:

The RC firing circuit which is normally used for simple phase control gives variation of firingangle beyond 90. The circuit diagram is shown in fig 4.c.

Fig. 4.c RC Firing circuitThe circuit operates as follows,

1. When es goes positive and the capacitor voltage Vc equals to gate trigger voltageVgt(=Vg min + VD1), the SCR will turn ON.

2. After this, the capacitor holds to a small positive voltage as shown in the waveform.

3. During negative half cycle, capacitor c charges through D2 with lower plate positive tothe supply voltage Emax.

4. The diode D1 prevents the break down of the gate to cathode junction during negativehalf cycle.

5. In the range of power frequencies, the RvC for zero output voltage is given by

R Cv 137

2.

Where T= 1/f = period of AC line frequency in seconds

6. The maximum value of Rv is given by,

oV

Load

G

D2

A+ -

SCR

C

D1R

K

24V AC50 Hz

v



Where VD1 = Voltage drop across the diode D1 eS = Supply voltage at which SCR turn ON

The waveform for RC firing circuit is shown in Fig 4.d.

Fig. 4.d Waveform for RC Firing circuit

Emaxes

t

L

g(min)

t

V

e

-Emax

t

VAK

cV



4.3. UJT FIRING CIRCUIT:

4.3.1. UNI JUNCTION TRANSISTOR:

UJT is commonly used to generate triggering signals for SCR. UJT has three terminals calledthe Emitter E, Base one, B1 and Base two, B2. The basic structure and circuit symbol of UJTis shown in Fig 4.e and 4.f.

Fig. 4.e. Basic Structure of UJT Fig. 4.f. Circuit Symbol for UJT

Between B1 and B2 the unijunction has the characteristics of an ordinary resistance. This isthe interbase resistance RBB and has value in the range 4.7 to 9.1K. The VI characteristicsof UJT is shown in fig 4.g.

Fig. 4.g. V-I Characteristics of UJT

RB1

B2R

N-type

P-type

E

B1

B2

B1

B2

E

PI

VP

I VEI

EV

Peak Point

Cutoffregion

Valley Point

regionSaturation

RegionResistanceNegative



4.3.2 Synchronized UJT Triggering [or Ramp Triggering

A synchronized UJT triggering circuit using an UJT is shown in fig 4.h. Diodes D1-D4rectifies the input AC to DC. Zener diode Z is used to clip the rectified voltage to a standardlevel Vz, which remains constant except near the Vdc zero.

Fig. 4.h Synchronized UJT Triggering circuit

The zener voltage Vz is applied to charging circuit RC. So capacitor C charges by current i1.When the capacitor voltage reaches threshold voltage Vz, the E-B1 junction of UJT breaksdown and C charges through primary of pulse transformer sending a current i2.

As the current i2 is in the form of pulse, SCR with positive anode voltage would turn ON.Varying R can control rate of rise of capacitor voltage. This method of control is called rampcontrol. The firing angle can be controlled up to 150. The waveform for generation of outputpulses for the given circuit is shown in fig 4.i.

Fig.4.i Waveform for synchronized UJT triggering circuit

24V AC

R

Z

D2D4

D3

B1

B2

R1

E

G1

C

D1

K1G2

K2

ZV

R2

Vc

+

-

PulseTransformer

i1

Vdc

+

-

+

-

Vc

dcV

cV Vc

dcV

Vz

Pulse Voltage

zV t

t



(5.1)

The period of oscillation, T, is independent of the dc supply voltage Vs, and is given by

The line synchronized UJT trigger circuit can be used for,

1. Single phase AC controller

2. Single phase half wave controlled converter

3. Single phase controlled rectifier with center-tapped transformer (bi-phase half waveconverter)

4. Single phase half controlled bridge rectifier.



2.6 DATA SHEETS

1. SCR (TYN612)



2. UJT (2N2646)

1.r,rc,ujt firing

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