iET PRS-300 Series, Руководство по эксплуатации, Страница: 21 / 50

PRS-300 Series
OPERATION 11
3.4.8 STO, RCL Keys
The STO and RCL key are used to store
user de fi ned values into memory. Press STO
key followed by 0 to 9 in the numeric keypad
stores the currently shown resistance value into
that memory location.
Press RCL key followed by 0 to 9 in the
numeric keypad recalls the resistance value
saved into memory to the currently displayed
value.
3.4.9 VOLT Key
The VOLT key should normally be set
to 0.5 V which allows us of the full resistance
range of the PRS-300. Pressing the
VOLT key
followed by a numeric value representing the
maximum voltage being used in the applica-
ton will limit the lowest resistance that can be
entered to prevent damage to the PRS-300.
The limited is based upon a maximum of
1 W being applied to the resistors. Note that
0.5 W is the maximum recommended and op-
eration at 1 W can cause the internal resistors
to drift beyond speci
fi cation.
3.4.10 2 and 4 Wire Kelvin Lead Con-
nections
2-wire or 4-wire operation can be selected
from by pressing the MENU button, selecting
CONFIGURE , pressing the “2” will change
between 2-wire and 4-wire modes.
The use of 4W mode with Kelvin leads
minimize the effects of contact resistance and
gives best performance. In 4-wire remove the
shorting links from the high and low terminal
pairs.
Figure 3-6 4-Terminal shorting links not con-
nected
2W mode should be used when connect-
ing to a 2 wire device or if the PRS-300 will be
used as a standard 2-terminal resistor such as
RTD simulation. In 2-wire mode shorting links
must be installed across high and low terminal
pairs.
Figure 3-7 Two terminal with shorting links
3.4.11 Thermal emf Considerations
The PRS Series uses high-quality, low-
emf components. Thermal emf is primarily
attributable to the temperature difference
between the leads of the relay and the contacts
when temperature is applied to the coil. This
emf is of the order of 5
μ V per relay, but is not
usually additive. The typical worst case is <15
μ
V.
If the effect of tens of microvolts is
signi fi cant to your application, connect to the
instrument with low-thermal-emf materials
only. Copper wire and copper alloys are rec-
ommended; brass and steel should be avoided.
Tinned copper and solder are acceptable.