HP 8112A Operating, Programming And Servicing Manual page 192

Voltage and Current
Sensing Circuits
1 0.2-2 Servicing the Power Supply
Table 1 0.2-1 . Power supply rectifiers
The raw voltage outputs are all smoothed by capacitors, as shown
in Figure 10.2-3. The following supplies are then fed to voltage
regulators, with potentiometers to adjust the final voltage level:
Table 1 0.2-2. Regulated voltage supplies
The smoothed ±5 V DC outputs of CRl , which drive the fan, also
provide the basis for the voltage and current sensing circuits which
control the +5.1 V, +5 V and -5.4 V supplies.
+5. 1 V supply
A reference voltage of 5 . 1 V is obtained from the +15 V regulated
supply, using zener diode VRl (6.2 V) ,Rl l and R12. UlC compares
the +5.1 V supply with this reference voltage and drives the
regulator transistor Q l , via driver transistor Q2, until there is zero
difference.
If the current drawn from the +5.1 V supply is excessive, a distinct
voltage drop develops across R2. UlD detects this and its output
switches toward the negative supply. This forward-biases diode CR8,
switches off Q2 and Q l , and hence the +5.1 V supply is withdrawn.
+5 V supply
The same principles of operation apply to the +5 V voltage
regulator, U2C, and current sensor, U2B . The 5 V reference is
obtained from the 5 . 1 V reference via Rl3.
-5.4 V supply
The -5.4 V reference is obtained from the 5 V reference using U2D as
an inverter with a gain of 1 .08. The voltage regulator UlB and the
current sensor UlA operate as above except that the comparator
output is normally negative and switches positive to withdraw the
supply.
Rectifier Output
CRl ±5 V DC
CR2 ±23 V DC
±15
DC
CR3 V
Supply Regulator Adjust or
v U3 R18
+23
v
-23 U4 R19
v
+15 U5 R24
v
-15 U6 R25