HP 5340A Service Manual

Model 5340A
Theory of Operation
4 - 276. In the event that the +15 volt reference fails or falls below 11 volts, A32CR3 conducts to
turn off A32Q2 thereby disabling the supply. This prevents A32U1 from locking in the off
position. The series pass transistor in
A32U1 drives A32Q5 to handle the larger current require -
ments of the regulator. A32R17 sets the current limit point and A32R16 and R18 set the current
foldback characteristics.
4 - 277.
+5 VOLT DIGITAL REGULATOR (PART OF A32). The +5 volt digital regulator (Figure
8 - 34) is similar to the +5 volt RF regulator except for no preregulator, additional series pass
transistors, and two current limit circuits. In order to handle the 3 amp load, an external series
pass transistor (Q2) and driver (A32Q3) are provided. A32R9 serves as the collector load for
A32Q3. The operation of U2 is similar to that described in Paragraph 4 - 270. A32R10 provides
current limiting for the
IC. For example, if Q2 opens, the series pass transistor in A32U2 and
A32Q3 would be seriously overloaded. In this case, the drop across A32R10 drives A32U2 into
current limiting.
A32Q4 provides current limiting for loads above 3 amperes. When the drop
across A32R13 and R15 forward biases A32Q4, A32U2 is driven into the current limit mode. The
collector of
A32Q4 connects to the compensation input (see Figure 4 - 52) which is in parallel
with the current limit amplifier in the
IC. The current foldback circuit is made up of A32Rll and
R12. A32R14, A32CR6, and A32C10 provide regulation for the current foldback point to prevent
line voltage variations from changing the foldback characteristics.
A32CR5 and A32C9 provide
ripple filtering for the current foldback circuit.
4 - 278. A29 +15 Volt Regulator Assembly, 05340 - 60025
4 - 279. The +15 volt regulator (Figure 8 - 33) is similar to the +5 volt RF regulator described in
Paragraph
4 - 275. The supply provides approximately 170 milliamperes. The preregulator is
composed of
A29Q1, A29Q2, and A29CR1-CR3. A29Q3 is the series pass transistor.
Current
limiting
is set by A29R7 and A29R10. A29R6 and A29R9 set the current foldback characteristics.
A29R5 adjusts the output voltage.
4 - 280. A28 - 15 Volt and t175 Regulator Assembly, 05340 - 60022
4 - 281. The - 15 volt regulator (Figure 8 - 33) supplies about 180 milliamperes and consists of a
preregulator, regulator, series pass transistor and driver, and current limit circuits. A28Q1 and
A28Q3 serve as a constant current source for A28CR2. Zener diode A28CR2 establishes a con -
stant 19.6 volts for the regulator reference input. R5 sets the proper current input for the
regulator reference.
A28U1 is a negative supply voltage regulator similar to the positive voltage
regulator previously discussed. The reference current input at
A28U1 flows internally through the
IC to pin 1 and A28R6 and A28R7.
Adjusting A28R7 sets the output supply voltage. The boost
output at
A28U1(7) drives Q7 and QlO to regulate the output voltage. A28C4 provides com -
pensation to prevent high - frequency oscillation. Current limiting for the IC is determined by
A28Rll.
A28R16 is the current limit resistor and works in conjunction with A28Q8. When the
current limit point is reached, the drop across
A28R16 turns on A28Q7 which diverts drive
current from
A28Q10. A28R13 and A28R14 determine the current foldback limiting
characteristics.
I
4 - 282. The +175 volt regulator provides +175 volts at about 28 milliamperes to drive the display
tubes in
A25. Since no filter capacitors are used for rectifier A30CR1, a pulsating dc is developed
for driving the display tubes.
A28Q2, A28R2, and A28CR1 serves as a current source for A28CR3
and A28CR4.
A28CR3 and A28CR4 provide the +175 volt reference for A28Q4. A28Q4 drives
A28Q6 which serves as the series pass transistor.
Current limiting
is set by A28R15 and the
current foldback limiting, chgracteristics are determined by
A28R8 and A28R10. When the drop
across
A28R15 is sufficient 'to turn on A28Q9, A28Q9 diverts current from A28Q4 thereby
reducing the output voltagb to maintain a constant current. With overloads, the voltage across
A28R8 and A28RlO bias3siA28Q9 to conduct further to reduce the output current below the
current limit point set ,by
A28R15. This is foldback action and prevents excessive power dis -
sipation in the regulator. A28R9 suppresses as oscillations. A28C6 filters out the pulsating dc from
A28Q9 to prevent A28Q9 from conducting at the peak levels of the pulsating dc.
-I
4 - 53

HP 5340A Specifications

General

Brand	HP
Model	5340A
Category	Cash Counter
Language	English