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BT21i Current sensor User’s Guide
Short description
The Current sensor BT21i is a general-‐purpose sensor to measure currents in AC and
DC circuits in the range between –5 and +5 A. It has two banana (4-‐mm) plugs for
easy connection.
The sensor contains a sensing element and a signal-‐conditioning amplifier. The
sensing element is a 0.04
Ω
resistor (2 W) connected between the red and black
terminals. As the current passes through the resistor, a small potential difference
can be measured across this resistor. This potential difference goes through a signal
amplifier and the output of the sensor is adjusted to the range of
±
7.5V, which can
be measured by an interface. The sensor is protected and currents up to 7 A will not
damage the sensor, but it should never be used for higher currents.
The sensor should be connected in series to a circuit element. Currents in either
direction can be measured. The current is indicated as positive when it flows from
the red terminal to the black terminal.
The Current sensor can be directly connected to analog BT inputs of the CMA
interfaces. The sensor cable BT -‐ IEEE1394 needed to connect the sensor to an
interface is not supplied with the sensor and has to be purchased separately (CMA
Article BTsc_1).
Sensor recognition
The Current sensor has a memory chip (EEPROM) with information about the sensor:
its name, measured quantity, unit and calibration. Through a simple protocol this
information is read by the CMA interfaces and the sensor is automatically recognized
when it is connected to these interfaces. If your Current sensor is not automatically
detected by an interface you have to manually set up your sensor by selecting it
from the Coach Sensor Library.
Calibration
The CMA Current sensor BT21i is supplied calibrated. The output of the sensor is
linear with respect to the input current. The supplied calibration function is:
I(A)= 0.78125 * V
out
(V) – 0.0047.
The Coach 6 program allows selecting the calibration supplied by the sensor memory
(EEPROM) or the calibration stored in the Coach 6 Sensor Library. For better
accuracy the pre-‐defined calibration can be shifted.
For even more accurate measurements a new user calibration (a standard, simple 2-‐
point calibration) can be performed in Coach using known currents.
