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4 to 20 mA Current Loops – Application Note

Designing 4 to 20 mA Current Loops – Application Note

This document describes how to manage the voltage drops in a 4 to 20 mA
current loop, and how to determine the proper power supply voltage for the
loop.

4 to 20 mA Current Loop Overview

The 4 to 20 mA current loop is a very robust sensor signaling standard.
Current loops are ideal for data transmission because of their inherent
insensitivity to electrical noise. Designing 4 to 20 mA current loops is just
managing the voltage drops around the loop. The voltage drops occur in the wire,
the transmitter and load resistor.

Wire
The voltage drop in
wire is directly proportional to the current flowing through it by Ohms law,
current x resistance equals voltage.  Table 1 shows the resistance of
common wire.

Transmitter
The
transmitter consumes 7 to 15 VDC of loop voltage, depending on model, to power
itself.

Load Resistor
Finally,
the controller’s load resistor converts the loop current to a voltage for easy
signal processing.  Most commonly a 250-Ohm resistor is used to convert the
loop current back to a voltage.  At 4 mA, a 250? resistor drops 1 volt; at
12 mA, a 250? resistor drops 3 volts and at 20 mA, a 250-Ohm resistor drops 5
volts.  The load resistor can be internal to the controller or
external.

While ANSI/ISA-50.1-1982 (R1992) Compatibility of Analog Signal for
Electronic Industrial Process Instruments says that the load resistor shall be
250?, resistor values of 50? to 1,000? may be used.  To properly design a
current loop you must know the value of the controller’s resistor.

2-Wire Transmitter with External Power & External Resistor

Many low cost HVAC/R controllers do not include a dedicated power supply or a
factory configured input for a 4 to 20 mA signal.  At the same time, many
of the best sensors in the industry are only available as a 4 to 20 mA
output.  Figure 2 on the next page shows how to create a 4 to 20 mA current
loop circuit for a controller without a dedicated power supply or an input for a
4 to 20 mA signal.

A 24 VAC transformer provides power to the controller and the BAPI VC350A
current loop power supply (The primary high voltage wiring to the transformer is
not shown).  The VC350A should be mounted as close as possible to the
controller. The positive output of the VC350A is connected through a cable to
the Red or positive terminal of the transmitter. The Black or signal terminal of
the transmitter is connected to the controller’s analog input.  A load
resistor is connected from the analog input to ground.  If the controller’s
analog input is configured for 0-5 VDC, then the load resistor should be
250?.  The signal voltage will be 1 volt at 4 mA and 5 volts at 20 mA. If
the controller’s analog input is configured for 0-10 VDC, then the load resistor
should be 500?.  The signal voltage will be 2 volt at 4 mA and 10 volts at
20 mA.

In the loop schematic above, a BAPI sensor is measuring the humidity of the
outside air.  The DDC controller is in the basement while the humidity
sensor is on the roof.  The two are connected by 1,600 feet of 22-gauge
twisted pair wire.
The controller does not have a peripheral power supply or
a dedicated 4 to 20 mA input.  An external power supply and load resistor
are therefore necessary.  The 4 to 20 mA transmitter is the BAPI sensor
referenced above while the current loop power supply is a BAPI VC350A voltage
converter set to 24 VDC.

Wire Resistance

1,600 feet of 22-gauge twisted pair wire is actually 3,200 feet long – 1,600
feet out and 1,600 feet back. The resistance chart on the previous page shows
that copper wire has a resistance of 16.14? per 1,000 feet at 20°C. Therefore
the resistance of the 3,200 feet of wire at 20°C is:

3,200 ft x 16.14 ?/1,000 ft = 51.65?.

The wire is run in equipment shafts that are not air-conditioned.  The
equipment shafts never fall below 32°F (0°C) but they may get as hot as 140°F
(60°C) in the summer.  The Temperature Coefficient of Copper is +0.393
percent per degree C. This means if the temperature increases 1°C, the
resistance will increase 0.393%. The wire in the schematic above has a total
resistance of 51.65? at 20°C, therefore the increase in resistance at 60°C would
equal:

(60°C-20°C) x  0.00393 per °C x 51.65? =
8.12?

Adding this increase to the original resistance equals:

8.12? + 51.65? = 59.77?

The resistance of the 3,200 feet of wire at 140°F increased by 16% from
51.65? to 59.77?.

Voltage Drops Around the Loop

To properly design a 4 to 20 mA current loop, the voltage drops around the
loop need to be calculated at the maximum resistance and maximum signal current
of 20 mA, and calculated again at the minimum resistance and minimum signal
current of 4 mA.

The maximum resistance of the wire (60°C) is 59.77?. Multiply this by the
maximum signal current of 20 mA and the voltage drop of the wire equals:

59.77? x 0.02A = 1.195 volts

The BAPI humidity transmitter drops a minimum of 15 volts, while the 250 ?
controller resistor drops 5 volts at 20 mA. Adding all the voltage drops
together equals:

1.195 + 15 + 5 = 21.195 volts

The power supply is set to 24 volts.  Where did the extra 2.805 volts
go?  It’s dropped across the transmitter.  All the voltage that isn’t
dropped across the wires or the controller resistor is dropped across the
transmitter itself. Therefore, at a maximum signal current of 20 mA, the total
drop across the transmitter is 17.802 volts (24 volts – 1.195 volts – 5 volts =
17.802 volts).  Having the transmitter drop a couple of extra volts above
its minimum rating (15 volts) is just good engineering practice. It protects the
integrity of the current loop as the connections oxidize and age.

The voltage drops must also be calculated at the minimum resistance and
minimum signal current  of 4 mA to be sure that the transmitter is not
being asked to drop more voltage than its maximum rating.

The equipment shafts where the copper wire is run
may get as low as 0°C.  The Temperature Coefficient of Copper is +0.393
percent per degree C, therefore a drop of 1°C will decrease the resistance by
0.393%. The wire in the schematic of Figure 2 has a total resistance of 51.65?
at 20°C, therefore the decrease in resistance at 0°C would equal:

(0°C-20°C) x  0.00393 per
°C x 51.65? = -4.06?

Subtracting this from the original resistance equals;

51.65? – 4.06?  = 47.59?

The minimum resistance of the wire (0°C) is 47.59?. Multiply this by the
minimum signal current of 4 mA and the voltage drop of the wire equals:

47.59? x 0.004A = 0.19 volts

The BAPI humidity transmitter drops a minimum of 15 volts, while the 250 ?
controller resistor drops 1 volts at 4 mA. Adding all the voltage drops together
equals:

0.19 + 15 + 1 = 16.19 volts

With the power supply set to 24 volts, the transmitter drops 22.81 volts (24
volts – 0.19 volts – 1 volt = 22.81 volts).  The BAPI humidity transmitter
has a maximum voltage drop rating of 24 volts. Therefore the voltage dropped
across the transmitter is close to its maximum specification, but still within
tolerance at the minimum resistance and minimum signal current.

Some Programmable Logic Controllers have power supplies of up to 36VDC,
allowing really long current loops.  A 36VDC supply in the above example
would create a voltage drop across the transmitter which is above its maximum
rating. The only way to use the 36VDC supply would be to put a resistor or two
into the loop to eat up the extra voltage.  Just be sure not to go
overboard on the low end, or you will not be able to reach the 20 mA signaling
point.  A 50? resistor will drop 1 volt at 20 mA and 0.2 volts at 4 mA.

3-Wire Transmitter Current Loop

Some of BAPI’s current transmitters are three-wire devices, rather than
two-wire devices.  A three-wire transmitter is significantly different from
a two-wire transmitter.  A three-wire transmitter will consume more current
from the system power supply than the 20 mA signaling current because the
transmitter itself requires some current to operate.

In the loop schematic above, a BAPI-Stat room sensor is meauring the
temperature of a room. The BAPI-Stat itself requires 10 mA of current to
operate. Therefore when the signal current (the white wire) is at 4 mA, the
total current from the power supply is 14 mA.  When the signal current is
at 20 mA, the total current from the power supply is 30 mA.

Calculating the voltage drops for this three-wire transmitter loop is
different than with a two-wire loop.  In the three-wire loop, the power
supply positive (+) wire has the total current flowing through it while the 4 to
20 mA signal wire has only the signaling current flowing in it.  Therefore,
there are two one-way voltage drops to calculate; one is the power supply line
voltage drop and the second is the signaling line voltage drop.  Because
there are different currents in each wire, the voltage drops are determined
separately and then added together.

Since BAPI-Stats measure occupied spaces, it is not necessary to calculate
wiring resistance due to large temperature swings.  Using the 20°C (68°F)
resistance is all that’s necessary. If the BAPI-Stat is 250 feet from the
controller utilizing 22 gauge wire, and the signal current is at 20 mA, the
voltage drop from the power supply wire to the BAPI-Stat is:

16.14? x (250Ft/1000Ft) x 30mA = 0.12 volts

The voltage drop on the signal wire at 20 mA is:

16.14? x (250Ft/1000Ft) x 20mA = 0.08 volts

The 250-Ohm receiver resistor drops 5 volts at 20 mA and the BAPI-Stat’s
minimum voltage is 15 VDC, so the voltage around the loop is:

0.12 + 0.08 + 5 + 15 = 20.2 VDC

Any system power supply of greater than 20.2 VDC will allow full
signaling.

Since the voltage on the power pin of a 3-wire transmitter is essentially
constant and is not affected by the signal current level, you do not have to
calculate the voltage drops at a signal current of 4 mA.  As long as the
voltage of the power supply is greater than the voltage drops around the loop at
20 mA signal current, and below the maximum voltage rating of the transmitter
itself, then the power supply voltage is designed correctly.

3-Wire Transmitter with Two Independent 4 to 20 mA Signals

The BAPI-Stat is also capable of supplying two independent 4 to 20 mA
signals. Calculating the voltage drops and properly sizing the power supply in
this case is very similar to the previous example. The BAPI-Stat itself requires
10 mA of current to operate. Therefore, when each signal is at 4 mA output, the
BAPI-Stat consumes 18 mA from the system power supply.  When each signal is
at 20 mA output, the BAPI-Stat consumes 50 mA from the system power supply.

In the example above, the power supply positive (+) wire has the total
current flowing through it while the 4 to 20 mA signal wires have only the
signaling current flowing them. As in the previous example, there are two
one-way voltage drops to calculate; one is the power supply wire voltage drop
and the second is the signaling wire voltage drop.  Because there are
different currents in each wire, the voltage drops are determined separately and
then added together.

If the BAPI-Stat is 250 feet from the controller utilizing 22 gauge wire, and
both signal current are at 20 mA, the voltage drop from the power supply wire to
the BAPI-Stat is:

16.14? x (250Ft/1000Ft) x 50mA = 0.20 volts

The voltage drop due to one channel’s 20 mA signal current is:

16.14? x (250Ft/1000Ft) x 20mA = 0.08 volts

The second signal channel is in parallel with the first channel, so it does
not need to be added to the voltage drops around the loop. The 250-Ohm receiver
resistor drops 5 volts at 20 mA and the BAPI-Stat’s minimum voltage is 15 VDC,
so the voltage around the loop is:

0.20 + 0.08 + 5 + 15 = 20.28 VDC

Any system power supply of greater than 22.28 VDC will allow full
signaling.

Since the voltage on the power pin of a 3-wire transmitter is essentially
constant and is not affected by the signal current levels, you do not have to
calculate the voltage drops at a signal current of 4 mA.  As long as the
voltage of the power supply is greater than the voltage drops around the loop at
20 mA signal currents, and below the maximum voltage rating of the transmitter
itself, then the power supply voltage is designed correctly.

If you would like more information about 4 to 20 mA current loops, call your
BAPI representative or see the BAPI Application Notes: 4
to 20 mA Current Loop Configurations or Designing
4 to 20 mA Current Loops on this website.

Reference

ANSI/ISA-50.1-1982 (R1992) Compatibility of Analog Signal for Electronic
Industrial Process Instruments.  (This document is the industry
specification for 4 to 20mA current loop signaling.)

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