Tuesday, March 24, 2009

4-20 mA source circuit

0-1V to 4-20 mA Converter
Ensure +5/-5 dual supply for chip TL062 IC3. Gnd is common
ps ground, let grounds radiate from ground plane in one
side of PCB. R3-R8 is an attenuator that may need to be
designed or modified.


1-5 V to 4-20 mA Circuit
The input voltage to this circuit is assumed to be coming from
some type of physical transducer/amplifier arrangement,
calibrated to produce 1 volt at 0 percent of physical measurement,
and 5 volts at 100 percent of physical measurement. The
standard analog current signal range is 4 mA to 20 mA,
signifying 0% to 100% of measurement range, respectively. At 5
volts input, the 250 Ω (precision) resistor will have 5 volts applied
across it, resulting in 20 mA of current in the large loop circuit
(with Rload). It does not matter what resistance value Rload is, or
how much wire resistance is present in that large loop, so long as
the op-amp has a high enough power supply voltage to output the
voltage necessary to get 20 mA flowing through Rload. The 250 Ω
resistor establishes the relationship between input voltage and
output current, in this case creating the equivalence of 1-5 V in /
4-20 mA out. If we were converting the 1-5 volt input signal to a
10-50 mA output signal (an older, obsolete instrumentation
standard for industry), we'd use a 100 Ω precision resistor instead.


Howland Current Source for Grounded Load

Тhis genius idea is implemented in the famous Howland current
source. In this clever circuit, the excitation voltage V and the
resistor R form a basic current source. It produces a current
I = V/R - VL/R, which passes from the left hand side through the
load RL.


Constant-current circuits
Constant-current circuits are usually implemented with an op
amp and a discrete external transistor.

Circuit E is a current source, which requires close matching of
the R2-R3 and R4-R5 resistor pairs to ensure insensitivity to
changes in the supply voltage.
Circuit (E) Iout = Vin/Rload

Computer Controlled 100ma Current Source
Often in industrial control systems a constant current source
is needed, which is controlled by a computer and referenced to
circuit ground. The circuit below converts a zero to 5v
signal from a computer’s analog output into a current, with
a full scale of 100ma. The circuit shown requires a 9v DC
supply but any voltage from 9v to 12v will work.


Constant current source circuit
We can use a voltage reference turn into a Constant current
source circuit and use pnp-transistors for current-boosting


Voltage-to-Current Converter
Voltage-to-Current Converter or Current Source provided more
High-quality Signals Transmission in Long Lines instead of
Tension Transmission.

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