Power Supply; Oven - HP 3300A Technical Manual

Model 3300A
4-27. POWER SUPPLY (Figure 6-5).
4-28. The power supply consists of two full wave
rectifiers CR1 thru CR4 and four series regulated
supplies. AllCR1 provides a stable reference for the two
negative regulated supplies which in turn are the
references for the two positive regulated supplies. The
two 20 volt supplies are double regulated. The operation
of the four supplies is similar: A differential amplifier
senses and amplifies any change.
applied through a driver stage to the series regulator
which then changes its conduction to oppose the
change.
4-29. Operation of the positive and negative supplies
is similar. Diodes CR2 thru CR5 and CR7 thru CR9
determine the maximum current permitted to flow in the
series regulating transistors.
+26.5 volt supply, it can be seen that an increase in
current through R5 and R6 increases the overall forward
bias on the diode network CR2 thru CR5.
magnitude of this forward bias is determined by the sum
of the forward biased base-emitter diode voltage of Q1
and Q2 in addition to the voltage drop across the R5-R6
The change is
Referring to Figure 6-5,
The
combination. When this forward bias increases to a level
sufficient to allow the diodes to conduct, any increase in
the collector current of Q4 will pass through the diodes
and not enter the base of Q2. This, in thru, limits the
maximum current in the series regulating transistors.
4-30.

OVEN.

4-31. (See Figure 6-5.) The desired oven temperature
is automatically maintained by a thermal control loop.
The loop consists of a thermistor, a signal amplifier, a
power amplifier, and the heater resistors. The operation
of the loop is as follows:
(thermistor) decreases with an increase in temperature
which causes the base current of A11Q9 to increase.
The corresponding decrease of A11Q9 collector voltage
is coupled into the base circuit of the power amplifier Q7.
The collector current of Q7 then decreases which
decreases the current through the heater resistors
generating less heat and the temperature decreases.
The response of the loop is improved by the physical
location of A11R27 in close proximity to the thermistor.
4-3
Section IV
The resistance of RT1