Dwyer DL8 - Manual (Page 11)

25 milliamp Channel
The 25 milliamp channel can be modified to span higher
ranges (greater than 25 mA DC) by also adding a single
resistor across the input terminals as shown in Figure 10.
The value of the resistor “R” must be calculated by the fol-
lowing formula based on the desired current input range (I
in
):
Make sure the resistor can withstand at least twice the
power you will be sending it. To calculate this, use the fol-
lowing equation:
P = 2 * I
in
2
* R
Where:
P = Minimum Power Rating of resistor (in watts)
I
in
= Maximum amperage expected through resistor
(in Amps).
R = Resistance of Resistor (in ohms).
For example, a 50 ohm resistor calculated to measure cur-
rents up to 75mA will require a power rating greater or equal
to 0.56 watts. Therefore, a one watt resistor will be fine.
For increased sensitivity at lower current ranges down to
2.5 milliamps, refer to the section for modifying the 200 mV
channel.
External Battery Operation
You can activate a special function on the DL8 designed to
optimize the use of power from external batteries while out
in the field. The battery-saving contacts can provide switch-
ing capability to draw power from the battery only when
needed, thus maximizing its life for extended field applica-
tions.
The internal battery switch allow you to power external
transducers with a separate battery pack. This capability
makes it easy for you to assemble your own self-contained
data logging kit for long-term in-field use where there is no
convenient power supply receptacle.
The internal switch can be activated during exit from setup
session by choosing “Yes” as answer to “Activate the
Logger’s external battery switch?” The battery switch sta-
tus can be checked in setup menu. If the battery switch is
activated Hi Cal for channel 1 (25mA) reads -5. If the switch
is not activated, this value is 0.
The switch works by closing eight seconds prior to a read-
ing, remaining closed until the reading is taken, then imme-
diately opening again before repeating the cycle.
It extends battery life by only drawing power, when neces-
sary, to meet the input requirements of the transducer or
transmitter circuit that you want to monitor.
To use the battery switch, the battery’s positive (+) terminal
must be wired to the “+ Battery In” terminal on the data log-
ger (refer to Figure 11). The power to your external trans-
ducers will then be available from the “+ Power Out” termi-
nal. The battery’s negative (-) terminal can then be wired to
either of the “-Common” terminals.
Maximum current that the battery switch can source is
100mA. Battery supply voltage can be from 9 to 25 volts.
Short circuit protection is continuous at 9 volts, but only one
second at higher voltages.
Things you should know about batteries
When using batteries for power your external sensors and
transducers, you should keep in mind a few key points.
1. The rated voltage of a battery only applies to initial use.
Once under load, the voltage will decrease gradually until
completely drained. By knowing how the battery voltage is
affected through use you can make sure that the voltage
requirements of your external transducers will be met. A
good rule of thumb to follow is to exceed the minimum
required input voltage for your transducers by at least two
times (where acceptable).
2. Nickel-Cadmium batteries (rechargeable) self-drain them-
selves at a rate approximately 1% of their remaining capac-
ity per day. This should be taken into account for extended
logging sessions.
R =
2,500
I
in
- 25
Figure 10: Modifying a 25 mA channel
Figure 11: Using the Battery Switch
Bulletin E-90-DL8 6/22/05 3:56 PM Page 11