HP 6236A Operating And Service Manual page 14

4-34 Reference and Bias Supply
4-35 The reference and bias supply powers the operation-
al amplifiers and provides the bias and reference voltages used
throughout the supply. A shunt zener regulates its +7.5V
output. A series transistor regu lates its -12.4 V output,
using 6.2-volt zener VR1 as its voltage reference. The -12.4V
output provides a constant current to VR 1, which is the pri-
mary voltage reference for the entire supply.
4-36 Two equal resistors are connected in series across
the -12.4V output. To regulate this output, voltage com-
parison amplifier U4 compares the voltage across one of
these resistors to the -6.2V reference and controls the con-
duction of series regulator 011 through driver 012. The
voltage drop across 011 is controlled by feedback so that
the voltages at the two inputs of U4 remain equal. Driver
012 controls 011 by shunting part of the base bias supplied
by R68.
4-37 During turn-on, the -6.2V reference supply is
temporarily shorted by 014 in the turn-on control circuit.
By trying to match this low reference, 011 is initially turned
off. While 011 is turned off, R69 bypasses current to the
-12.4V output until the output reaches -9 volts and the
turn-on control circuit removes the short from the reference
and enables the -12.4-volt regulator to operate normally.
4-38 Turn-On/Turn-Off Control Circuit
4-39 Immediately after the supply is energized and until
the output of the -12.4-volt regulator reaches about -9
volts, the turn-on control circuit withholds turn-on bias
from series regulator transistors 01, 03, and 07 and holds
4-4
the -6.2V reference at a low value. This prevents an out-
put voltage transient by ensuring that the operational am-
plifiers are energized and other essential bias voltages are
present before the series regulator transistors are turned on.
The circuit also prevents an output transient when the sup-
ply is turned off by removing the turn-on bias from the se-
ries regulators and shorting the -6.2V reference supply as
the voltage of the -12.4 V supply falls below -9 volts.
4-40 013 switches the bias to the -20-volt regulator on
and off, 014 switches the short across the -6.2-volt refer...
ence supply, and 015 switches the bias to the +20-volt and
+6-volt or +18-volt regulators. 015 remains turned off until
VR2 conducts at 9 volts to switch it on. While 015 is off,
it holds 013 biased off and 014 on; when 015 conducts,
it turns 013 and 014 off.
4-41 Meter Circuits
4-42 Voltmeter. Two of the resistors in resistor network
Z1 are range resistors for the voltmeter. The accurate ratio
of these resistors permits a single calibration potentiometer,
R58 to adjust both ranges simultaneously.
4-43 Ammeter. The range switch connects the ammeter
across the current monitoring resistor of a supply: R48 in
the +6-volt or +18-volt supply, R8 in the +20-volt supply,
or R28 in the -20-volt supply. Each of these resistors con-
ducts a constant bias current for its series regulator transistor
in addition to the supply's output current. If no compen-
sation were used, this additional current would raise the
indicated output by up to 8% of full scale. The resistor net-
works connected to each range of the ammeter selector
switch apply a bias to the meter to offset this error. R59
calibrates all ammeter ranges.