We Ship Nearly Everywhere
Voltage to 4 - 20mA
Current Loop Translator
|The current loop translator turns any
analog voltage output sensor into a4
to 20mAcurrent loop sensor.
Many types of programmable logic controllers (PLCs) only have
current loop inputs, and are incompatible with voltage
We developed the current loop
translator,because we received many
requests to add current loop output capability to our soil
sensors so that they could be directly used in PLCs.
Current loops are useful because extremely long cables can be
used with no signal degradation. Cable lengths exceeding 1
kilometer can be run with no degradation or scaling to the
signal. Compare this to a voltage output sensor where the
resistance of the cable forms a voltage divider and scales down
the voltage as the cable length increases.
Current loops provide a sensor output
current that is proportional to the sensor's reading. The
standard for most current loops is 4mA to 20mA. So the
sensor's minimum value corresponds to 4mA and the maximum value
corresponds to 20mA. Because the loop never outputs less
than 4mA, broken wires or defective sensors can easily be
detected. (If the the current is less than 4mA then there
is an error.)
The translator can directly power sensors that consume under
4mA. For higher power sensors, an isolated external power
supply is needed. The translator has 2 solderable holes that
can be connected to external isolated power. The
translator has a jumper which can switch between loop power,
and external isolated power.
The current loop translators are factory calibrated with two
trim potentiometers, which adjust the 4mA and 20mA limits,
respectively. If desired, these can be re-calibrated in
the field with a simple calibration procedure which is
The current loop can read voltages up to 3.0V. Higher input voltages can be
read, if the sensor voltage is divided by a resistor network.
current loop translator can directly loop power the following
the VH400 consumes more than 4mA it must be powered from an
external isolated supply.
Voltage to 4 - 20mA Current Loop Translator
- Interface voltage sensors to PLCs.
- Remote placement of sensors
with long cables.
- Interface Vegetronix soil
moisture sensors to programmable logic controllers.
- Low Cost.
- Low Power.
- Screwless terminal blocks for quick
- Factory calibrated limits,
which can be re-calibrated in the field with 2 trim
- Interfaces with nearly any
voltage output sensor.
- Can power sensors from loop current if the
sensor consumes less than 4mA.
- Accepts isolated voltage supply for sensors
that consume more than 4mA.
- 4 mounting holes in each corner.
Pricing and Ordering Info
For volume pricing contact us.
||Voltage to current loop translator with 3V
|Maximum current loop
(Measured across pins of TB1)
|Minimum current loop Voltage
(Measured across pins of TB1)
|Maximum readable sensor input
|3V (This can be customized)
||2.54cm x 2.54cm (1in x
Other Vegetronix Products of Interest
Thee boards are
marked with the terminal definitions.
Terminal Block TB1 (CURRENT LOOP)
||V-, Current loop
||V+, Current loop
Terminal Block TB2 (SENSOR INPUT)
||VS+, Positive sensor
||OUT, Sensor input
||Vs-, Negative sensor Voltage
jumper selects between loop powered sensors, and external isolated
power. If the sensor is powered from the loop place the jumper on
the VLOOP setting. If the sensor is powered externally, select the VISO
setting on the jumper, and solder the power lines to +VISO, and
Figure 1. A current loop powered
sensor. The green wires show internal PCB connections. With the
jumper set to VLOOP the current loop supplies power to the sensor.
Note that the current consumed by the sensor must be less than
4mA. The current is measured by measuring the voltage across the
external current loop resistor, and dividing by its rated
Figure 2. Sensor powered by external
isolated voltage supply such as a battery. The green wires show
internal PCB connections. With the jumper set to VISO the
external battery supplies power to the sensor. Use this
configuration if the sensor consumes more than 4mA. Note that VS- and
V- must not be shorted directly or indirectly through a ground loop.
For this reason the sensor must be powered with an
isolated/floating voltage supply such as a battery so that there are no
ground loops. The current is measured by measuring the voltage
across the external current loop resistor, and dividing by its rated
The current loops are factory calibrated with two
trim potentiometers, which independently adjust the 4mA and 20mA
To perform the calibrations you will need to power the translator with
a voltage supply in series with a calibration resistor. A 1% 100
ohm resistor is a good choice. Place the leads of a multimeter
across the calibration resistor in parallel, and set the meter to
measure voltage. Independently, adjust the 2 trimmer pots with a small
screw driver to get the current adjusted correctly for both the 4mA and
20mA limits. For a 100 ohm resistor the multimeter will read 0.4V at
4.0mA and 2.0V at 20mA.
To adjust the 4mA limit, short the sensor input terminals ( OUT, and
Vs-) with a wire, and adjust the current across a calibration resistor
in the current loop to 4mA.
To adjust the 20mA limit, input 3V into the sensor input terminals
(OUT, and Vs-), and adjust the current across a calibrationresistor in the
current loop to 20mA.
Voltage to Current Equation
Current(mA) = 5.3333*Vin+4
Minimum Loop Voltage
The minimum loop voltage can be calculated as follows:
Vloop(min)= Vboard+ Vr+Vw
Vboard is the voltage needed by the current translator board.
Vr is the voltage across the current sensing resistor at 20mA.
Vw is the voltage across the wire at 20mA.
The current loop board requires 5V input voltage from the loop. The
voltage across the sensing resistor can be calculated as follows:
Likewise the voltage across the wire or cable can be calculated as:
Vw= (Resistance of wire) * 0.02A
For example, suppose the sensing resistor is 100 ohms, and the cable in the loop
is very long and has a resistance of 10 ohms. The minimum loop voltage is
calculated as follows:
Vr= 100* 0.02A= 2V
Vw= 10* 0.02A = 0.2V
Vloop(min)= 5V+ 2V + 0.2V= 7.2V
How to Get Started
The best way to get started is to order a few
sample current translators and try them out in your
application. Contact us now to