The firing sequence of the six SCR bridge is diagramed below.
The shaded area is the allowable conduction angles for correct
phasing.
Catch diode, CR1, provides a current path when all six SCR’s
are off.
Voltage adjust pot R41 adjusts the power supply’s output voltage
from a minimum above 46 VDC (maximum CCW) to a maxi-
mum below 51 VDC (maximum CW). The output voltage regu-
lation is less than 0.5% from no-load to full-load.
To reduce output ripple, the delay angle balance adjust, R62 to
R66, are adjusted for even firing angles on all six SCRs in the
bridge. The output ripple is less than 150 mV p-p.
The output current is sensed by R100 and applied to U5 and R75.
Gain adjust, R75, scales the output current to 1V per 100 Amps
at J1-3 (positive) to J1-4 (return).
R71 is adjusted to give an overcurrent trip when output current
exceeds 315 amps. R71 controls the overcurrent threshold. If the
threshold is exceeded, U5 pin 14 will cause CR41 to fire which
turns on DS2, overcurrent LED. CR41 also allows CR27 and
CR56 to conduct and shut down the 24V regulator. With the 24V
regulator circuit off, all power to SCR bridge firing circuits is
removed. Thus, the output goes to zero, remaining there until the
24V regulator is reset by first removing, then reapplying, the
three phase AC input.
R84 is adjusted to give an overvoltage trip when the DC output
voltage exceeds 57 VDC. If the output voltage exceeds the
threshold set by R84, U1 pin 13 will cause CR40 to fire, which
turns on DS1, the overvoltage LED. CR40 also allows CR32 and
CR55 to conduct and shuts down the 24 volt regulator. As with
the overcurrent trip, shutting off the 24 volt regulator will shut
of the SCR bridge until the AC is removed and reapplied.
If the temperature of the heat sink assembly reaches 100°C,
thermal switch S1 will close. When S1 closes, CR42 fires, which
turns on DS3, the over temperature LED, and allows CR29 and
CR59 to conduct which again shuts off the 24 volt regulator. As
with the other faults, an over temperature trip causes the output
to go to zero and remain until the three phase input voltage is
removed and reapplied.
R74 is adjusted so that Q19 is ON (sinks current) from J1-7 to
J1-8, the OPTO GO/NO GO terminals, when the output voltage
is between 48 and 49 volts DC.
An output voltage signal provided between J1-6 and common
J1-4 is a scaled DC output voltage sample, equal to 5% of the
power supply DC output voltage.
1/2 PB1
1/2 PB3
1/2 PB2
1/2 PB1
1/2 PB3
1/2 PB2
Q1
Q6
Q2
Q4
Q3
Q5
Eab
Eac
Ebc
Eba
Eca
Ecb
Eab
360
300
240
180
120
60
t
o
o
o
o
o
o
SCR
2291-032
Figure 4-3. SCR Firing Sequence
Section IV - Theory of Operation
888-2365-001
4-9
WARNING: Disconnect primary power prior to servicing.
Summary of Contents for Platinum Series
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