15
Short Term Operation up to 600 Watts
The instrument has provision for applications which require the dissipation of higher than normal
powers for a limited period of time. It imposes a limit on the combination of power and time by
first displaying a warning message and then disabling the input. Full details of the constraints that
apply to this mode are given later, on page 24.
Voltage and Current Limit Conditions
The unit has provision for the user to specify limits on the permitted measured value of voltage or
current. If either of these limits is exceeded then the input will be disabled.
Power Limit
The unit continuously monitors the internal power dissipation and varies the speed of the fan
accordingly. If the dissipation rises above about 430 (or 610) Watts, a hardware power limit circuit
will come into operation and attempt to constrain the load current to control the dissipation. The
unit is then operating in a non-linear mode, which will change the stability conditions. If the power
limit circuit fails to prevent the power rising above a slightly higher fault threshold (perhaps
because of instability) then the fault detector will be tripped and the load will cease to conduct.
Input Condition Lamps
Two lamps above the Input Enable switch indicate the operating state of the unit. They are both
off when the input is disabled. The green lamp lights when the input is enabled, and if the load is
operating normally then the yellow lamp will not be lit.
The yellow lamps lights if the load cannot conduct the required current, with a message on the
status line at the top right of the display distinguishing between the three possible reasons:
• High Power: the power limit circuit is operating as described above.
• Dropout: the voltage applied from the source is below the setting of the Dropout voltage.
• Low Voltage: the power stage is in the minimum resistance condition, because the
voltage available from the source is insufficient to maintain the current level required.
The minimum resistance condition will occur either if the source is switched off and is not
providing any voltage at all, or if the voltage drop across the connection leads is causing the
actual input voltage at the load to be below its minimum operating level. Note that if the source
voltage is suddenly applied while the load circuit is in this state, then a current transient will
probably occur.
If only the yellow lamp is lit, with the green lamp off, then a persistent fault condition exists.
Fault Conditions
The unit detects (in hardware) the following fault conditions:
• Current above about 92 Amps.
• Power in excess of about 450 (or 630) Watts (that the power limit circuit has not
succeeded in controlling to the lower threshold as described above).
• Voltage above about 106 Volts.
• Excessive difference between external and internal voltage sense values.
• Excessive heatsink temperature.
• Fan failure.
The fault detectors for excess current, power and voltage have filter networks with a
time-constant of a few milliseconds to allow brief transients to be handled.
When any of these fault conditions occurs, the input is disabled, so the unit will cease to conduct
current and a fault message will be displayed. An excessive current or power condition will
disappear as soon as the input is disabled, but any of the other conditions will cause the yellow
lamp only to remain lit, and the message Fault to show on the status line, until it is cleared.
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