Bulletin 257-2EX H-R-SCR-1 A Student Manual
EXPERIMENT NO. 6
SINGLE-PHASE, FULL-CONTROLLED, PUSH-PULL FULL-WAVE
SCR RECTIFIER WITH UJT TRIGGERING
The full-wave rectifier using two diodes and a center-tapped transformer is well known. If
SCRs replace the conventional diodes, control of load current is obtained. If an SCR replaces
one of the conventional diodes, a circuit is called half-controlled. If both conventional diodes
are replaced by SCRs as in Figure 6-1, the circuit is called full-controlled.
SCR5
I
m
T
T
GC
A'
I
P2B
1
w
-'l
I &
#l 2 ai
1
I
TRIG.
18 A C
DC
LOAD
I
I
0-0-
I
-
I
1
I
l@ A C
A
-
-
2
TRIG.
#2 1
G c
hI
w
;
1I
a
P2C .
SCR6
FIGURE 6-1: SINGLE PHASE, FULL CONTROLLED FULL WAVE, PUSH-PULL BRIDGE
WITH UJT TRIGGERING AND BYPASS DIODE.
Due to slight differences in the SCRs themselves, the UJT’s that trigger them, or in the gang
potentiometer, the load waveform may be a little asymmetrical.
If an inductive load is used, the bypass diode Dl must be added to conduct the flow of
current caused by the collapsing field of the inductor away from each SCR. Each SCR could
conceivably be held ON continuously through its respective half-cycle by inductive circulating
current if Dl were not present. Thus, each gate would lose control and a continuous full-wave
output would be produced across the load. The bypass diode insures each SCR turn-off during
each respective half cycle.
Bulletin 257-2EX H-R-SCR1 A Student Manual
PROCEDURE:
Step 1.
Connect the circuit shown in Figure 6-2.
Step 2.
Observe and record in TABLE 6-1 the waveforms of the voltage across SCR5 (A-C),
across SCR6 (A-C), between the Gate and Cathode of both SCR5 and SCR6, and
across the load for minimum (Fully Counterclockwise), about 3/4 clockwise, and maximum settings of gang rheostat P2.