3-6
MST-MHT SERIES 071320
•
Status bit 4 appears in the Questionable register of the MST 488-27HT controller indicat-
ing a catastrophic failure. Upon receiving status bit 4, the Controller removes that Con-
trol Bus address from the look-up table, preventing further commands from being
directed to the faulty power module.
b. Fault Shutdown with Output disabled - If an open sense lead condition is detected, the fault
shutdown procedure is as follows:
•
All output power and signal relays are immediately opened, isolating the Power Module
from the load;
•
Output voltage and current are programmed to zero;
•
If open sense caused the failure, status bit 10 appears in the Questionable register of the
MST 488-27HT controller.
3.7.1
FAULT RECOVERY
Recovery from any fault condition (described above) requires that the user cycle source power
off for a minimum of 5 seconds. After reapplication of source power, a IEEE 488 DCL (device
clear) command must be sent to the controller to restore the module’s Control Bus address to
the look-up table. This procedure eliminates any possibility of casual reapplication of a Power
Module to the system load after symptoms of a catastrophic module failure have been detected.
3.7.2
FIXED OVERVOLTAGE DETECTOR
The output stabilizer control circuitry of the MST-MHT Power Module includes a fixed overvolt-
age amplifier. This devices monitors the actual output voltage supplied by the Power Module
and compares it to a fixed reference voltage. In the event that a parameter exceeds the fixed
voltage limit by an amount equal to 10% of the Power Module’s rated voltage, the circuit initiates
a fault shutdown procedure as described in PAR. 3.7a. The tolerance of the fixed overvoltage
hardware trigger is the minimum value listed in Table 1-1, +6%.
3.7.3
OVERCURRENT TRACKING DETECTOR
The output stabilizer control circuitry of the MST-MHT Power Module includes a separate over-
current tracking amplifier. This device monitors the actual output current supplied by the Power
Module and compares it to the programmed current value. In the event that actual current
exceeds the programmed current by an amount equal to 10% of the Power Module’s rated cur-
rent, the circuit initiates a fault shutdown procedure as described in PAR. 3.7a The tolerance of
the current fault threshold is ±3%.
3.7.4
OVERTEMPERATURE DETECTORS
Any one of a number of internal or external conditions (i.e., elevated ambient temperature,
clogged vent holes, cooling fan failure, etc.) can cause elevation of internal heat sink tempera-
tures to unsafe levels. If unattended, the resultant temperature rise of the power devices will
lead to eventual destructive failure. MST-MHT Power Modules incorporate two internal thermo-
static switches to detect unsafe internal operating temperatures. One is located on the heat sink
related to the input preregulator and dc-dc converter, while the second monitors the heat sink
related to the output stabilizer. If the temperature of either heat sink rises above a predeter-
mined safe temperature, the related thermostat will change state, initiating a fault shutdown as
described in PAR. 3.7a. The thermostat will reset upon return of the heat sink to acceptable
operating temperature, however the module recovery procedure (see PAR. 3.7.1) must still be
performed.
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