24-01-2013, 10:53 AM
STUDY OF GATE FIRING CIRCUITS
STUDY OF GATE FIRING.docx (Size: 14.68 KB / Downloads: 69)
AIM:
To observe the output waveforms of resistance, Resistance- Capacitance and UJT gate
firing Circuits of SCR.
APPARATUS:
S.No EQUIPMENT Qty
1. R-firing circuit Kit 1
2. RC firing circuit kit 1
3. UJT firing circuit kit 1
4. CRO with (1:10) Probe 1
5. Patch cards 1 set
THEORY:
R-firing Circuit:
Uni-Junction Transistor: UJT exhibits negative resistance characteristics; it can be used as
relaxation oscillator. The external characteristics RB1 and RB2 are resistances which are small in
comparison with internal resistances R1 and R2 of the UJT base. The emitter potential V is varied
depending on the charging rate of capacitance C. The charging resistance Rc should be such that
the load line intersects the device only in the negative resistance region. η is called as the
intrinsic standoff ratio. It is defined as
( ) /( ) B1 B1 B2 R R R
UJT is a highly efficient switch .It’s switching time is in a range of nano seconds. Since UJT
exhibits negative resistance characteristics it can be used as a relaxation oscillator.
PROCEDURE FOR R-FIRING CIRCUIT:
1.Turn the potentiometer fully anti clockwise, connect load as shown by jumpers,
2. Connect SCR in the ckt by using shorting links as shown by the dashed lines.
3. Connect the Oscilloscope across the load.
4. Vary the firing angle and observe the waveforms on the CRO
5. Draw the corresponding waveforms.
PROCEDURE FOR RC – FIRING CKT:
1. Connect the load and SCR in the CKT by jumpers as shown in the ckt diagram.
2. Tune the potentiometer fully anticlockwise.
3. Connect oscilloscope in the load divider and switch on the power supply.
4. Vary the firing angle and draw the corresponding waveforms.
PROCEDURE:
UJT firing circuit:
1. Connect the circuit as shown in figure.
2. Connect a capacitor C1 in series with variable resistance.
3. Place the knob of variable resistance at either of the extreme positions and place one capacitor in
series and take the reading of firing angle at that time period. i.e. total time is equal to the sum of
turn off and turn on times.
4. Vary the resistance to the other extreme position and note down the readings.
5. Replace the capacitor with another one and calculate the RC from noted reading.
6. Calculate the RLmax and RLmin from the above readings.