Service manual

Replaces page 55 109
Figure 3-9. Overcurrent Circuit Troubleshooting
Overpower Circuit Troubleshooting (Figure 3-10)
This circuit limits the power sinking capability of the load to one of two different power ranges depending upon the
temperature of the input power FET heat sink assembly.
The circuit monitors the input voltage and the input current in order to limit the current when an overpower condition exists.
The circuit consists of amplifier U12, power select components (U9, R92-R95), and four comparators (U10) as shown in
Figure 3-10. Signal levels representing the input voltage and input current are summed with a reference voltage
( + 12V REF) via resistors to determine if an overpower condition exists. The signal levels are scaled to allow different
combinations of voltage and current (e. g., high voltage and low current, high current and low voltage, etc.) to be compared.
If external power is available, the circuit increases the current and voltage limits to allow the load to operate in the extended
power range. Thus, the load has two power limits, normal (300 to 396 watts) and extended (400 to 530 watts).
When extended power is unavailable, the
EPC EN_
(extended power capability enable) signal is false (High) and the EPU
(extended power unavailable) bit 9 in the status register is true. When extended power is available, the
EPC EN_
signal is
true (Low) and the EPU bit is false. To check the EPU bit, send the string "STAT:CHAN:COND?".
When the overpower circuit is active (limiting input power capability), the comparator circuit becomes a relaxation
oscillator and output voltage will go between -14 volts and 0 volts (see waveform at test point
on Figure 3-10).
Troubleshooting the power limit circuit consists of checking test points
through using the measurement conditions
and readings specified in Table 3-3.