IET Labs VI-700, Operation Manual

Page: 13 / 26

VI-700 V&l Source
7
OPERATION
to the binding posts. If the output voltage is being
monitored, then the voltage at the posts and at the
load should be measured to determine the potential
drop between them. These precautions are espe-
cially important whenever the X0.l range is being used,
because even very small resistances can result in a
significant voltage relative to the least significant digit
resolution of that range, i.e. 100
µ
V.
If sensing at the load is desired to virtually eliminate
this lead drop, consult IET Labs for instructions on
implementing this technique.
In the case of the current output, no such problem
exists. The current indicated on the front panel is
exactly the current flowing through the test load.
3.6 Overload Indicators
In order to provide the user with certainty that the
output load capability is not being exceeded and that
the VI-700 is performing within specifications, two
OVLD indicator LED lamps are provided. As long
as the appropriate indicator is off, its associated out-
put is accurate as specified.
In the case of the current output, note that it is the
voltage or compliance across the terminals, which
cannot be exceeded, 20 volts in particular. An open
circuit is therefore an overload condition and will cause
that indicator to go on. This condition can be ignored,
or, if desired, a short circuit strap may be placed across
the current terminals to turn off this lamp.
In the case of the HP - High Power Option - where
up to 200 mA may be drawn, a shorting link will cause
this current to be constantly drawn from the current
terminals if the unit is on a high setting. This could
cause overheating, and it is advisable to leave the mA
terminals open. The current
OVLD
LED will re-
main on if the mA terminals are open, and this indica-
tor may be disregarded. The
OVLD
indicator will
behave normally under normal operation.
It is important to realize that the OVLD indicators
are conservatively set. The output in the overload
region may be used as long as care is exercised. The
voltage output remains precise for 50-100% current
overload with only a slight degradation in accuracy, of
the order of one LSD (least significant digit.) The VI-
700 can actually be used as a low power program-
mable power supply.
If both voltage and current connectors are used to
supply power simultaneously to outside loads, one or
both overload indicators may come on sooner than at
the limits indicated above since they are deter-mined
by the total output capability of the instrument.
There is a small amount of hysteresis built into the
overload indicator circuits in order to prevent them
from flickering as the limit threshold is approached.
This requires reducing the load substantially below the
limit whenever it is necessary to reset or turn off the
indicator.
3.7 HP - High Power Option - 200 mA
Load Current Capacity (Optional)
A unit with this option may be used in the same way
as a standard unit. Caution should be exercised since
a significant current is available and could overheat
the item under load. The current overload indicator
will be on; see the Overload Indicators section above.
3.8 HV - High Voltage Option - 200 V
Output (optional)
WARNING
OBSERVE ALL SAFETY RULES
WHEN WORKING WITH HIGH VOLTAGES OR LINE
VOLTAGES.
HIGH VOLTAGE MAY BE PRESENT AT
REAR PANEL WITH HV OPTION.
The rear output voltage is precisely 10 times the volt-
age set at the front panel. Up to 10 mA may be drawn.
Exceeding that current will not turn on the OVLD
indicator. Consult IET Labs if more current is re-
quired.
It is up to the user to consider placing a fuse or a
current limiting resistor in series with the
X10 200 V
OUTPUT terminals as a safety measure.