Current-mode control loops (particularly the popular 4- to
20-mA controls) are used in many industrial applications
because of their immunity to induced EMI from motors,
contactors, relays, and other sources. Off-the-shelf process
controllers often have 4- to 20-mA (sometimes 0- to 20-mA)
output options for adjusting speed, pressure, temperature, or
some other parameter in a closed-loop control system.
The receiving circuit needed for the 4- to 20-mA control signal
isn’t extremely complex, and several components are available
that are designed specifically for this purpose. However,
the cost of these parts was a bit higher than I expected (more
than $10) in small quantities.
While looking for a low-cost alternative, I discovered the
INA134 from Texas Instruments. It’s a very versatile, unitygain
audio differential amplifier with a wide supply range.
Using the dual version (INA2134) and just a few precision
resistors, I came up with a 4- to 20-mA receiver circuit that
costs less than $2.60.
The circuit in Figure 1 was simulated with MultiSim8 (Electronics
Workbench) using the INA134. (Translating the pinouts
for the INA2134 is easily accomplished.) Circuit stimulus
is provided by an ac current source centered at 12 mA with an
8-mA peak signal (resulting in a 4- to 20-mA swing) at 10 Hz.
Any reasonable frequency can be used, but 4- to 20-mA controls
are typically slow-varying signals.
One section of the INA2134 provides an offset for the
output. The 1% resistors shown in conjunction with the lasertrimmed
precision resistors within the INA2134 provide a
fairly precise +2-V dc offset. The circuit runs off a 24-V dc single
voltage supply, so this offset is needed to ensure the output
doesn’t get too close to the ground rail. (The specification for
the part limits the output to V+ - 2 V and V- + 2 V.)
The 150-, 1% tolerance resistor across the input pins gives
an output of 0.6 V at 4 mA input to 3 V at 20 mA input plus the
2-V offset for an output range of 2.6 V to 5 V (Fig. 2). This is the
input to an analog-to-digital converter (ADC) in my application.
The ADC output is interpreted by a small microcontroller
that controls the process.
Note that a 0- to 20-mA input would result in a 2.0-V to 5.0-V
output range. For other applications, designers can adjust the
output range and offset by simple changes in resistor values.
The single supply voltage can be up to 36 V dc. If dual supplies
are used (up to ±18 V dc), the offset isn’t necessary, and the
INA134 single amp can be used with one input resistor, reducing
the cost to less than $1.60.
