Instructions

UM-0085-B09 DT80 Range User Manual Page 306
RG
Current
Current (rate of electrical charge movement) is measured by inserting a known shunt resistance into the circuit, and
measuring the voltage across it. Using Ohm's Law, Current = Voltage / Resistance.
The I channel type returns the current value in milliamps (mA).
The DT80 incorporates an internal 100Ω shunt resistor between the # terminal and analog ground. Alternatively, an
external shunt may be used. The value of the external shunt must be known and must be specified when the channel is
defined.
For example, the channel definition
3#I will measure the current which enters at channel 3's # terminal, flows through
the internal shunt, and returns via the AGND/EXT# terminal.
On the other hand,
2+I(51.2) will return the current flowing through a 51.2Ω shunt wired between the + and #
terminals of channel 2.
Channel Options
The following channel options are commonly used when measuring currents:
•
A (attenuator) specifies that the DT80's input attenuator should be enabled, which allows voltages of up to 30V
(up to 50V in Series 4) to be measured across the shunt resistors. Note that this option is not available if the
internal shunt is used.
• the excitation options specify how the current source is powered:
N (assume external power supply; default),
V (enable internal 4.5V voltage source on * terminal) or E (assume external supply connected to EXT*
terminal).
• E (external excitation) specifies that the power supply for the current sources is connected to the DT80's EXT*
terminal. This will then be automatically connected to each current source for the duration of a measurement,
and then disconnected.
•
MDn (measurement delay) specifies that the DT80 should wait n ms (default 10ms) after selecting a channel
before starting the actual measurement. This can be useful in conjunction with the E option, as it allows the
sensor some time to stabilise after power is applied to it.
• the channel factor specifies the shunt resistance in ohms (default 100.0 Ω)
•
GLx, ESn, as for voltage measurements
The following sections describe some common wiring configurations for measuring current.
C1 – Shared Terminal Current Inputs using External Shunts
In this configuration up to three separate current sources can be measured. This is done by measuring the voltages
across the shunts in the shared terminal configuration; see V1 – Shared Terminal Voltage Inputs(P305).
To avoid cross-channel coupling, connect the bottom of the shunts with the minimum of shared resistance to the sense
point.
The resistance of each shunt should be specified as the channel factor.
Figure 128: Wiring for shared-terminal current input using external shunt
To measure
Use the command
I1
1*I(R1)
I2
1+I(R2)
I3
1-I(R3)