HP 6038A Service Manual Download

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Down Programmer

This circuit allows the output voltage to be lowered rapidly when required. In order to lower the output voltage it is
necessary to discharge the output filter capacitors (typically, through the load). In situations that require the output voltage
to drop more rapidly than can be accomplished through the load, the Down Programmer discharges the capacitors and pulls
the output line low. Five conditions can conditions can trigger down programming: programming of a lower output voltage,
overvoltage, overtemperature, remote disable, or primary power failure. The Down Programmer turns on when either
MASTER ENABLE is low or the CV ERROR VOLTAGE is more negative than about -6Vdc.

The + 8.9Vdc bias supply for the Down Programmer stores enough energy in its input capacitor to operate the Down
Programmer after loss of primary power. This ensures that the Down Programmer will be able to discharge the output
circuit completely when primary power is turned off.

Bleeder Circuit (6038A only)

This circuit enables the output capacitor to discharge faster by providing ample bleed current at various output levels,
(thereby improving Down Programming times). The path for the bleed current is provided by one of two transistors, A1Q1
or A1Q2. At output voltages below 13 to 15.5Vdc, transistor A1Q2 is turned on to supply milliamperes of bleed current.
When the output voltage is above 13 to 15.5Vdc, transistor A1Q1 is turned on, turning off A1Q2.

Fuse A1F3 provides protection to internal components should A1Q2 short and draw excessive current. Down programming
response time at no load will be considerably longer if components malfunction in the bleeder circuit or if fuse A1F3 is
blown.

Constant-Voltage (CV) Circuit

The constant-voltage circuit compares the output voltage to the user-set CV PROGRAM VOLTAGE to produce CV
CONTROL VOLTAGE. Two comparison amplifier loops accomplish the comparison. In the outerloop, CV error amplifier
A2U8 compares V-MON, a buffered fraction of the sensed output voltage OVS, to the programming voltage from the HP-
IB board, to create the CV ERROR VOLTAGE. Then in the innerloop, amplifier A2U10A compares this error voltage to
IVS, a buffered fraction of the innerloop output voltage, to produce the CV CONTROL VOLTAGE. The CV ERROR
VOLTAGE is also diode-OR connected through diode A2CR21 as an input to the down programmer.

V-MON also connects through protective circuitry to rear-panel terminal VM for remote monitoring of the output voltage.
As output varies from zero to full scale, V-MON varies from 0 to + 5 volts.

Settings of the CV programming switches--the B6, B5 and B4 MODE switch settings--allow the CV PROGRAM
VOLTAGE to come from the HP-IB board, from an external voltage applied between rear-panel terminals VP and P, or
from an external resistor between VP and P. When using an external resistor, current from the CV constant-current source
flows through the applicable resistance to develop the CV PROGRAM VOLTAGE.

In CV mode the CV CONTROL VOLTAGE varies between about - 0.5Vdc and about + 1.0Vdc. It is most negative when
the load is drawing no power. As the load draws more power, the voltage becomes more positive. The CV CONTROL
VOLTAGE is at the cathode of diode A2CR24, part of the diode-OR input to the control-voltage comparator. Diode
A2CR20 prevents voltage overshoots during transient load changes and program changes.