Block Diagram - Keithley 148 Instruction Manual

MODEL 148 NANOVOLTMETER
CIRCUIT DESCRIPTION
FIGURE 13. Block Diagram of Model 148 Power Supplies.
b.
The unregulated supply consists of a full-wave rectifier, diodes
D212 and D213, and a dropping resistor, R232. The AC CONNECTED Lamp,
DS201, is in series with resistor R226, which is connected directly
across this supply. For battery operation the primary of transformer
T201 is disconnected.
c.
Voltage from the unregulated supply or the battery is applied to the
dc-to-dc inverter circuit. Transistors 428 and Q29 form a switching network
to supply an interrupted voltage to transformer T202. The switching fre-
quency is about 2 kc, well away from the carrier frequency.
Transformer T202
has a saturable ferrite core.
The inverter circuit supplies voltages to
the three regulated supplies, which are basically the same series regulator
design.
d.
For the +12 volt supply, diodes D206 and D207 full-wave rectify the
signal from transformer T202. The signal is applied to the pass transistor,
Q30.
The output of Q30 is divided by resistors R208 and R209 and compared
to the zener diode reference, D205. Transistors Q34 and Q33 amplify any
potential difference and apply a signal t,o the base of transistor 932.
Diodes D204 and D205 fix the emitter voltage of 432; therefore, the
collector voltage of Q33 directly determines the current through Q32,
which comes from the -12 volt supply. The current drop through resistor
R216 determines the voltage at the base of transistor Q31, and therefore
.the output voltage through the base-emitter junction.
Diode D203 limits
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