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3 - Installation
26
at the load. When using remote sensing, you must program the OVP trip voltage high enough to
compensate for the voltage drop between the output terminals and the load. Also, if the sum of the
programmed voltage and the load-lead drop exceeds the dc source's maximum voltage rating, this
may also trip the OV protection circuit.
Stability
When the unit is configured for remote sensing, it is possible for the impedance of the load wires
and the capacitance of the load to form a filter, which becomes part of the unit's feedback loop.
This can degrade the unit's stability and result in poor transient response performance. In extreme
cases it may also cause oscillations. The wiring guidelines previously discussed under "Wire
Considerations" will eliminate most stability problems associated with load lead inductance. If
additional measures are required:
♦ keep the load capacitance as small as possible
♦ use larger diameter load wires to reduce resistance
OVP Considerations
The dc source's OVP circuit contains a crowbar SCR, which effectively shorts the output of the dc
source whenever the OVP trips. If an external voltage source such as a battery is connected across
the output and the OVP is inadvertently triggered, the SCR will continuously sink a large current
from the battery, possibly damaging the dc source.
To avoid this, program the OVP setting to its maximum value to prevent it from inadvertently
tripping. Additionally, an internal fuse is connected in series with the SCR. This fuse will open to
prevent large currents from damaging the SCR. If this internal fuse has opened, The FS status
annunciator will be set. Refer to the Service Manual for instructions about replacing this fuse.
In addition, the OVP circuit's SCR crowbar has been designed to discharge capacitances up to a
specific limit. These limits are:
HP 6631B
127,000 µF.
HP 6632B and 66322A
50,000 µF.
HP 6633B
20,000 µF.
HP 6634B
10,000 µF.
If your load capacitance approaches this limit, it is recommended that you do not intentionally trip
the OVP and discharge the capacitance through the SCR as part of your normal testing procedure,
as this may lead to long-term failure of some components.