2
DESCRIPTION OF OPERATION
The Apollo Current-Loop Indicator
(APLCL)
consists of a digital voltmeter
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 “V
I
” 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 Apollo Current-Loop Indicators 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 Apollo Current-Loop Indicator can be set up over a very wide span range
by means of the coarse DIP switches S6-S10
(on the rear)
, and the fine
screwdriver adjustment pot, located behind the sealing screw on the front bezel
(left side)
. 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 front panel 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 Apollo Current-Loop
Indicator 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 Apollo Current-Loop Indicator, 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
(at the rear)
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 control
(front panel, right)
has a numerical readout
range of ±100 and brackets all the coarse
switched ranges.
SIMPLIFIED SCHEMATIC, APOLLO CURRENT-LOOP INDICATOR
