Power Supply Card
12 - 1
Power Supply Card
The Power Supply Card supplies all low voltage power (26V and
below) to the injector system.
Refer to Figure 12.1 for a block diagram of the Power Supply Card.
+26 VDC and -26 VDC
Supply
One winding on the power transformer supplies fused 36 VAC to
bridge rectifier Z1. The center tap of the 36 VAC winding is ground.
Bipolar bridge output is filtered for +26 VDC, while outputs from Z1
are +26 VDC and -26 VDC.
+15 VDC and -15 VDC
Supply
The +26 VDC supply is regulated to +15 VDC by U2, voltage is filtered
by C12. The -26 VDC supply is regulated to -15 VDC by U3, voltage is
filtered by C15.
+5 VDC
Regulator Circuit
The +26 VDC supply is filtered then regulated to +5 VDC using Pulse
Width Modulator (PWM) U1 to switch a portion of the +26 VDC supply
through FET Q4. Pins 11 and 14 of U1, will pulse low then high, at a
frequency determined by R13, R14, and C8 (approximately 330 KHz).
The pulses are ORed through D3 and D4, and applied to Q3 and Q5,
which will pulse the gate of FET Q4. Zener diode D2 protects Q4 by
limiting the gate to source voltage to a maximum of 15 volts. The out-
put of Q4 is filtered by inductor L1, capacitors C1 - C3, and load resis-
tor R1.
Regulation of the +5 VDC supply is through the PWM IC and support
circuitry. Resistors R19 and R20 set up reference at U1 pin 2. The +5
VDC is coupled back to U1 pin 1, through a voltage divider network of
R16 and R18. The two voltages are compared to determine pulse
width at U1 pins 11 and 14. If the voltage is low, pulse width is length-
ened to increase voltage; if voltage is high, pulse width is shortened to
decrease voltage.
+5 VDC Overcurrent
Protection
The +5 VDC supply has overcurrent protection circuitry will shut down
the PWM IC (and +5 VDC circuit) if current exceeds 6-8 amperes. If
voltage across sampling resistor R2 exceeds a certain value, Q1 and
Q2 conduct, switching power to U1 pin 10 (Shut Down), which stops
pulses at pins 11 and 14. The PWM IC will restart and r5 VDC
if the overcurrent condition is eliminated.
12
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