Red Lion PAXLCL - Manual (Page 6)

6
DESCRIPTION OF OPERATION
The PAX Lite Current Loop Meter consists of a digital volt meter combined
with an analog scaling circuit (shown above). The unit was designed primarily for
use with 4-20 mA and 10-50 mA current loop signal circuits. However, it can also
be adapted to other current ranges, such as 0-50 mA, 0-20 mA, 0-10 mA, and in
a great many applications it can be used even with 0-5 mA and 1-5 mA current
loops. In addition, input current can be reversed in polarity resulting in negative
numerical readout with a minus (-) sign displayed. Input terminals 3 and 4 are
connected in series with 10-50 mA current loops, and Terminal 3 and 5 are series
connected with 4-20 mA loops. In either case, the voltage drop generated across
the shunt resistor(s) ranges from approximately 0.12 V min. (@ 4 or 10 mA) to
0.59 V max. (@ 20 or 50 mA). The buffer amplifier (K1) conditions and filters
the input signal voltage and applies it to the input of the scaling circuit. The
procedure for scaling PAX Lite Current Loop Meters is simplified by dividing the
scaling process into two separate components, span adjustments and offset
adjustments which are defined in the following discussion.
SPAN ADJUSTMENTS
Span is defined as the numerical range that the display traverses, disregarding
decimal points, when the input signal current is varied from minimum (4 or 10
mA) to maximum (20 or 50 mA). For example, if a unit is to display 25.0 @ 4
mA and 100.0 @ 20 mA, the span is 750 (the difference between 250 and 1000).
Had the minimum display been -25.0 @ 4 mA and +100.0 @ 20 mA, the span
would be 1250 (1000 - (-250) = 1250). (Note: the terms “GAIN”, “SCALE”, and
“SENSITIVITY” are also frequently used interchangeably with the term
“SPAN.”) The PAX Lite Current Loop Meter
can be set up over a very wide span range by
means of the coarse DIP switches S6-S10,
and the fine screwdriver adjustment pot,
located at the back cover. The coarse span
switches add parallel input resistors to the
summing amplifier (K2), thereby increasing
its gain, or sensitivity, as more summing
resistors are added. Effectively, adding more
parallel input resistors, increases the slope of
the transfer curve (at right) and increases the
numerical readout for a given input signal
current change. The input summing resistor values are weighted in a binary
progression, so they can be switched in combinations to give 32 discrete steps of
span. The fine adjust control brackets these coarse steps and can be adjusted to the
exact span needed.
The approximate span contributed by each switch is shown on the rear label.
These values are based on the standard current-loop spans of 4 to 20 mA (16 mA
current variation) and 10-50 mA (40 mA current variation). In other words, if S7
only is turned “ON”, the numerical readout will display a change approximately
1050 for a current swing of 16 mA (4-20 mA input) or 40 mA (10-50 mA input).
If S8 were also turned “ON”, the numerical readout would swing approximately
1575 (1050 for S7 + 525 for S8) for the same signal current variation. The fine
control has a continuous span range of approximately 0-150.
OFFSET ADJUSTMENTS
In the foregoing discussion of span, the transfer curves were shown as
“ZERO-BASED”, i.e., the numerical readout displays “0” when the signal
current goes to zero. With current loop ranges such as 0-5 or 0-10, or 0-20 mA,
and with Bi-Polar (+/-) signals, this is often the desired condition. However,
with 4-20 and 10-50 mA current loops, the minimum current level of 4 or 10
mA usually represents the zero level of the parameter being displayed. There are
also many applications where the
minimum (or zero level) represents some
value that does not fall on a zero based
transfer curve. To accommodate non-zero
based applications, the PAX Lite Current
Loop Meter has provisions for offsetting
the transfer curve over a wide range.
Essentially, offset moves the transfer
curve up or down to change its intercept
with the numerical readout axis, but it
does not change the slope (SPAN) of the
transfer curve. In the PAX Lite Current
Loop Meter, offset is accomplished by
adding (or subtracting) a constant at the input of the summing amplifier (K2).
This offset constant is summed in with a switched binary resistor network and
a fine adjust offset control in a similar manner to that used for span adjustment.
Switches S2-S5 can be turned on in combinations to give 16 different coarse
offset levels. Each switch is labeled to show the approximate amount of offset
contributed when it is turned “ON”. Switch 1 selects the polarity of the
switched-in offset value and allows offsetting the transfer curve “UP” (adding
the offset constant) or “DOWN” (subtracting). The fine offset control has a
numerical readout range of ±100 and brackets all the coarse switched ranges.
-500
NUMERICAL READOUT
0
4 8 12 16 20
+500
+1000
+1500
+1999
INPUT SIGNAL CURRENT
Span Adjustment
SPAN 525 (S8 ON)
SPAN 2100 (S6 ON)
SPAN 1050 (S7 ON)
-500
+500
0
+1000
+1500
+1999
INPUT SIGNAL CURRENT
+500 OFFSET
NUMERICAL READOUT
4 8 12 16 20
Offset Adjustment
"ZERO BASE" NO OFFSET
-300 OFFSET
-300 OFFSET
+500 OFFSET
INPUT CURRENT SIGNAL
17.4
Ω
RANGE
10-50MA
11.8
Ω
3
+LOOP
4
(-)
-LOOP
RANGE
4-20MA
(-)
5
-LOOP
K2
5.5K
S6 R
S7 2R
S8 4R
S9 8R
S10 16R
12R
(+)
S5
32R
S1
S4 S3 S2
50R 16R 8R 4R 25R
OFFSET
DIRECTION
SWITCH
(+)
ADD (-)
SUBTRACT
FINE
OFFSET
ADJUST
25K
R +V -VR
CURRENT
SHUNT
RESISTORS
K2
V
I I
V
FINE SPAN ADJ.
5K
SPAN=150
SPAN=0
INPUT SIGNAL SCALING CIRCUIT
BUFFER
AMP
SPAN ADJUST
OFFSET ADJUST
A/D
VOLTMETER
CIRCUIT
S3 S2 S1
DIGITAL DISPLAY
DECIMAL
POINT
SWITCHES
DIGITAL VOLTMETER
O
V
1400 700 350 175
2100
1050
525
260
130
mA
S4
SWITCH
ANNUNCIATOR
BACKLIGHT
PAXLCL SCHEMATIC
4.0 S
CALING THE
M
ETER