Service manual

108 Replaces page 54
Overcurrent Circuit Troubleshooting (Figure 3-9)
This circuit limits the maximum current the load can sink for different input voltage and/or power conditions. The primary
components in this circuit are amplifier U14 and transistors Q11 and Q12.
At power on the secondary power clear (
SPCLR ) signal provides a High level via D35 to drive U14-7 Low turning Q11 on.
With Q11 turned on, IPROG goes High (less negative) and turns off the input power FETs (load will not sink current).
When the input voltage is approximately 6.3V or lower, diode D53 is forward biased causing voltage divider R214, R213,
R59, and R58 to hold U14-6 at approximately - 7 volts. This will clamp the maximum input current capability between 60
and 70 amps
As the input voltage increases from 6.3 to 65 volts, diode D53 is reverse biased and the input voltage will appear across the
voltage divider. This causes the voltage at U14-6 to decrease from - 7 volts to - 0.8 volts. At an input of 65 volts, diode
D54 turns on and holds U14-6 at - 0.8 volts and limits the maximum input current capability to less than 9 amperes.
When the input voltage reaches 75 volts the OV circuit goes to -13 volts and pulls IPROG Low (more negative) via diode
D17. The input power stages will now attempt to sink more current and decrease the input voltage. If the combination of
input voltage and current (power) is greater than the power stages can sink when OV condition occurs, the overpower circuit
(see next paragraph) will override the OV circuit and limit the maximum current capability of the load.
The -15VX bias voltage is a delayed bias derived from the normal -15V supply. When the load is first turned on, -15V is
not present and U14-6 is at common potential. This causes Q11 to conduct pulling IPROG High. Q12 is also on
connecting Q11 to the + 15V bias. When -15VX comes on, Q12 turns off causing U14-6 to go more negative than U14-5.
This turns off Q11 allowing IPROG to go negative. VR27 supplies Q11 collector current once -15VX is available.
To troubleshoot the current limit circuit, check test points
, , , and using the measurement conditions and
readings specified in Table 3-3.