A
1
CALEX
FaxFACTS:
201
1997
Model 930 Programmable Current Source
http://www.calex.com
2401 Stanwell Drive
Concord, CA 94520-4841
(510) 687-4411 • Fax (510) 687-3333
Features
nProgram Output by Potentiometer Setting
or Voltage Input
nWide Supply Range from +12V to +32V
nFast Response from 10 µs to Step Change
nWell Regulated 10 megohms Output Impedance
Description
The Model 930 is a programmable current source that
provides an output current that stays constant with changes
in load impedance. The output voltage will vary as necessary
to maintain the desired value of constant current through the
load. Output current of the Model 930 can be set by means
of an external potentiometer or by a DC voltage. It has an
internal regulator and reference circuit to make the output
current independent of power supply variations. Supply voltage
can be unregulated and can range from +12V to +32V. The
compliance voltage (maximum output voltage for highest
load impedance) is dependent on the supply voltage level and
is rated at the supply voltage less 5 Volts.
Operation
The Model 930 output current can be programmed by a 1000
ohm potentiometer or by a voltage input. To set the current
with a potentiometer, connect the potentiometer end terminals
across pins 1 and 7 and the wiper arm to pin 3. Connect pins
9, 10 and 11 to each other. This will provide a linear adjustment
of output current from 0.5 mA to 50 mA. This is extended to
a range of 5 mA to 500 mA on the 296HP Power Booster
Mounting Kit.
For voltage programming, the potentiometer must be removed
(pins 1, 3, 7 left open) and a jumper must be connected across
pins 3 and 4. As with potentiometer programming, pins 9, 10
and 11 must be connected together. When connected for
voltage programming, an input voltage of zero to +10V
provides a linear output current of +0.5mA to +50mA.
Need more output current? It’s easy, simply parallel the
outputs of several programmable current sources. For
example, the outputs of two Model 930/MK 296HP units can
be connected to the same load to provide a combined output
of up to one ampere.
Output or Compliance Voltage
The key parameter in applying constant current sources is the
range of load impedance. This determines the range of output
voltage needed to maintain a constant current through the
varying load impedance. For example, if the load were to vary
from 1 ohm to 200 ohms and the desired constant current
were 50 mA, then the output voltage range needed would be
50 mV to 10V. The power supply must be 5 Volts greater than
the compliance voltage or 15 Volts, in this case.
FIGURE 1.
Model 930 Connected for Potentiometer Programming of I0.
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2
CALEX
FaxFACTS:
201
1997
Model 930 Programmable Current Source
http://www.calex.com
2401 Stanwell Drive
Concord, CA 94520-4841
(510) 687-4411 • Fax (510) 687-3333
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Mechanical SpecificationsSpecifications
Pin Assignments
0 1020304050607080
AMBIENT TEMPERATURE (Deg. C)
0
1
2
3
4
5
6
7
8
DISSIPATION (WATTS)
MODEL 930/MK296HP
MAXIMUM DISSIPATION vs. AMBIENT TEMPERATURE
3.1 W
0 5 10 15 20 25 30 35 40 45 50
LOAD CURRENT (mA)
10
11
12
13
14
15
16
MINIMUM SUPPLY VOLTAGE (VOLTS)
MODEL 930
MINIMUM VS vs. LOAD CURRENT for VOLTAGE PROGRAM
0 1020304050607080
AMBIENT TEMPERATURE (Deg. C)
0.0
0.7
1.3
2.0
POWER DISSIPATION (WATTS)
MODEL 930
MAXIMUM PACKAGE POWER vs AMBIENT TEMPERATURE
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A
3
CALEX
FaxFACTS:
201
1997
Model 930 Programmable Current Source
http://www.calex.com
2401 Stanwell Drive
Concord, CA 94520-4841
(510) 687-4411 • Fax (510) 687-3333
Mounting Kits
The Model MK 296 and MK 296HP Mounting Kits are for
resistive or voltage programming of the Model 930. The MK
296HP is a high current version with a power transistor to
increase the range to 0.5A. The Model MK 298 allows
adjustment of the “live zero” for 4 to 20 mA systems. All three
mounting kits consist of the PC board, PC connector with
built-in guides, and the necessary potentiometers. When
ordered with the Model 930 the unit will be delivered mounted.
They can be plugged into the 22-100MK Power Source Kit
(page F2) or used with any of the CALEX supplies or DC/DC
Converters.
For the MK 298 the output current can be programmed for
input voltages other than 0 to 10 Volts by changing the value
of RS.
The formula for RS is: RS = Vin (max)
10 Ι0 (max)
Where: Vin (max) = max programming (input) voltage
Ι0 = max output current
Note: +VS must be a regulated (0.1% or better) source, and
be 14 Volts to 24 Volts.
To Calibrate: repeat as necessary
1. With the max Vin adjust the span pot. for max Ιout
2. With min Vin adjust the zero pot. for min Ιout
Model 930/MK296HP
Model 930/MK296
Model 930/MK298 Mounting Kit Model 930/MK298
Mounting Kit Model 930/MK296HP
For Voltage Programming
the pot must be removed
and jumper J1 must be
added
A
4
CALEX
FaxFACTS:
201
1997
Model 930 Programmable Current Source
http://www.calex.com
2401 Stanwell Drive
Concord, CA 94520-4841
(510) 687-4411 • Fax (510) 687-3333
Transducer Bridge Drive
A bridge circuit located a significant distance from the exciting
and measuring system can be effected by unknown and or
changing drive line resistance. This can be eliminated by
driving the bridge with a constant current. The following
equation is used to determine the transducer resistance. The
voltage across the bridge is determined by the transducer
resistance.
The transducer resistance, RG, when in the upper arm, is
determined by the following equation:
RG = r (Ι x r - 3 Vo) / (Ι x r + Vo)
where r is the resistance of the 3 fixed arms of the bridge. Ι
is the constant current drive, and Vo is the bridge output
voltage. If RG is in the lower arm of the bridge, the minus and
plus signs in the equation are reversed.
Resistance Temperature
Measurement (RTD)
RTD’s are used to make accurate and stable temperature
measurements as one arm of a Wheatstone Bridge. Although
the RTD is more linear than the thermocouple, it still requires
curve fitting for best accuracy. A bridge circuit produces a
non-linear output for a linear change and thus requires an
additional curve fit. A single RTD driven by a constant current
source eliminates the errors associated with the bridge
method, including variable line resistance, although 4 wires
are required.
The Model 930 is an excellent constant current source for this
application. The 930 scale factor is set by an external RS
value of 10,000 ohms, providing a full scale output of 100
microamp. This resistor should be a 0.1%, 25 ppm, metal film
resistor. The other resistor values shown in Figure 4 provide
a small adjustment range around a 100 microamp output
current. This low level of current minimizes RTD self heating
error and produces a voltage change of 38.5 microvolts per
degree across a standard 100 ohm RTD with an alpha of
0.00385.
The components shown in Figure 4 will provide a current
source with a stability of 0.02% per degree ambient, from 0
to +70°C.
If a copper RTD is being used, the current source is easily
changed to 500 microamps by replacing the 10 kRS with a
2000 ohm, 0.1%, 25 ppm stable resistor.
Slip-ring Application
Slip-rings are often required to interface across a rotating
boundary. As an example, consider a temperature or strain
measurement that uses a 100 ohm transducer. Used in a
bridge circuit, every 1 ohm of varying line resistance could
contribute up to 1% error for each line that crosses the slip-
ring interface.
By using the sensor as a single resistance element and
constant current excitation as in Figure 5, the errors due to the
slip-ring changing resistance can be eliminated. If the 100
ohm transducer required 10 mA of drive current, the Model
930 could be operated from a 15 Volt supply allowing the slip-
ring and line resistance to vary from zero ohms to 900 ohms
without effecting the drive to the transducer. This also
eliminates the curve fitting step required by non-linear bridge
circuit outputs.
The measurement path would, in most cases, contribute a
negligible error. A typical volt meter has 10 megohms input
resistance, so a 1000 ohm series resistance change due to
the slip-rings would contribute only 0.01% of reading error.
Voltage-to-Current Converter
A Model 930 can be teamed up with a Model 178
instrumentation amplifier to convert low-level transducer
bridge voltages to output currents compatible with process
control equipment. The Model 930 is connected for a voltage
input. The output current range can be selected by varying
the impedance between pins 10 and 11.
FIGURE 6.
FIGURE 5.
FIGURE 4.
Other Applications
Other applications include charging batteries at a constant
current rate, Hall effect sensors, microplating using the
electrolytic process, and testing gyro torquers.
