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Operation Overview 31
Protection Features
The Electronic Load includes the following protection features:
•
Overvoltage
•
Overcurrent (hardware and software)
•
Overpower (hardware and software)
•
Overtemperature
•
Reverse Voltage
The appropriate bits in the status registers are set when any of the above protection features are active. Also, the Prot
annunciator comes on and the front-panel alphanumeric display indicates which conditions have been detected. For
example, if an overtemperature (OT) condition has been detected causing the input to be turned off (protection shutdown,
PS), the display will indicate "PS OT".
Resetting Latched Protection
All of the protection features latch (remain set) when they are tripped, except for the hardware overcurrent and reverse
voltage. The latched protection features can be reset via the GPIB (*RST or INP:PROT:CLE commands) or at the front
panel (
key). Of course, the condition that caused the protection feature to trip must be removed or it will trip
again as soon as it is reset.
To protect the Electronic Load from possible damage, the input voltage must not exceed the specified
maximum input voltage rating . Never apply the ac line voltage to a load’s input binding posts.
Overvoltage
The overvoltage protection circuit is set at a predetermined voltage, which cannot be changed. if the overvoltage circuit has
tripped, the load will attempt to limit the voltage by drawing current from the DC source. The load limits the value of
current drawn such that the resulting power is within the power rating. The overvoltage (OV) and voltage fault (VF) status
register bits are set when the OV condition occurs, and will remain set until they are reset as previously described.
An overvoltage condition does not cause the input to be turned off. However, a Fault signal output at the rear-panel control
connector will indicate when either an overvoltage condition or a reverse voltage condition has occurred. The Fault signal
is latched true (high TTL level) when the VF bit in the status register goes true. The Fault output signal (see Chapter 3) can
be used to trip an external circuit breaker or control a relay (e.g., Agilent 59510A Relay Accessory) in order to disconnect
the Electronic Load input from the source it is testing when an overvoltage or a reverse voltage condition occurs.
Overcurrent
The Electronic Load includes both hardware and software overcurrent protection features.
Hardware. When operating in the CR or CV mode, it is possible for a load to attempt to sink more current than it is rated
for. Under this condition, the load current will be limited by a current limit circuit, which is set at a value slightly above the
current rating . It protects both the Electronic Load and the device under test from operating too far beyond specified limits.
The hardware current limit circuit does not turn the load’s input off. The overcurrent (OC) bit in the status register is set
when an OC condition occurs, and is reset when the OC condition is removed.
Software. In addition to the hardware overcurrent protection circuit, the Electronic Load allows the user to define a current
protection limit in software which will shut down the input if the limit is exceeded. The protection limit can only be
programmed via the GPIB. It is turned on/off using the CURR:PROT:STATE ON|OFF command. The software current
limit level (in amps) is set using the CURR:PROT command. A programmable delay (in seconds) before trip is also
provided.