HP 8112A Operating, Programming And Servicing Manual page 192

Voltage and Current
Sensing Circuits
10.2-2 Servicing the Power Supply
Table 10.2-I. 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 10.2-2. Regulated voltage supplies
The smoothed f5 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.
A reference voltage of 5.1 V is obtained from the +15 V regulated
supply, using zener diode VRl (6.2 V),Rll and R12. UlC compares
the t5.1 V supply with this reference voltage and drives the
regulator transistor Ql, via driver transistor Q2, until there is zero
difference.
If the current drawn from the t5.1 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switches off Q2 and Ql, and hence the t5.1 V supply is withdrawn.
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 R13.
-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 Regulator Adjustor
V U3 R18
V U4
R24
v u5
R25
-15 v U6