I.Q.Instruments - Using 4-20mA current loops

WHY DO WE CONVERT SIGNALS INTO 4-20mA.?

There are a wide variety of sensors available to provide us with electrical signals representing many different parameters which we wish to measure. For example we can measure temperature with thermocouples and resistance thermometers, pH and conductivity with suitable electrodes, and mass with strain gauges.

All these devices have different characteristics of signal type, amplitude and linearity.

To make use of them in an industrial environment it is useful to convert their signals into a standard signal which we can connect to our measuring and control equipment. We are then able to use the same measuring and control equipment to process many different physical parameters. This simplifies design and is more economical especially when spare units have to be kept for backup. Instead of having five different units and five spares we can have five identical units with only one spare.

Over the years several standards have developed, some of which are unique to certain countries and many which have gained popularity around the world.

The most popular standards worldwide are (not in any particular order):-

0-5V 0-10V 1-5V 2-10V 1-5mA 0-20mA 4-20mA 10-50mA The first four are DC voltage signals whose uses are fairly obvious. 0-5V is especially popular with microprocessor applications as 5V is the popular supply voltage for these products. It does however suffer from a drawback that it is very difficult to tell when you are at zero.

Take the example where a thermometer is calibrated so that 0°C = 0V and 100°C = 5V. Typically the circuit is powered by a single 5V supply so that the output cannot go negative. If we are reading 0V are we at 0°C or even lower. Could the output voltage have failed or perhaps a faulty sensor is giving us a false reading?

ELEVATED ZERO

The advantage of a voltage signal is simplicity. Almost everyone understands the concept. The signal is on two wires, one positive the other negative. Power must be provided to the electronics which drives the voltage signal and this may be provided as two extra wires, or a single extra wire using one of the signal wires (usually the negative) as a common.

The disadvantages of voltage signals are primarily loss of accuracy caused by the input impedance of the measuring instrument and electrical interference from nearby power cables and radio transmitters.

To overcome both of the above problems a current signal is more effective. Assume a situation where 0-20mA is used to represent 0-100°C. In our measuring instrument we have a 250ohm resistor through which the current passes and we measure the voltage across it.

At 100°C we have 20mA passing through the resistor giving us a voltage of 5V. Ah-ha you say. This is just a 0-5V signal. Not so. This 5V signal remains the same even if the resistance of the wires carrying it changes, whereas with a voltage signal a change in wire resistance causes a change in signal at the measuring instrument unless it has an infinite impedance.

This means that a current signal may be transmitted long distances through conductors of varying resistance with no loss of accuracy. Neat!

The other advantage of the current signal is improved noise immunity. Spurious signals are superimposed on the signal we wish to measure by varying magnetic fields (maybe generated by AC power cables) which cut the conductors and induce currents in the same fashion as a generator. If our conductors are small and close together (theoretically in the same place) these fields generate equal and opposite currents in the two conductors which cancel. In practice a twisted pair of conductors of telephone cable sized wire works well.

The story about elevated zeros also works the same as with voltage signals but has one extra plus feature.

If we are using a 0-20mA signal we have to supply power to our electronics in a similar way as to power a voltage signal using one or two extra power supply lines.

With a 4-20mA signal we always have at least 4mA flowing in the loop. This means that if we can power our electronics from the residual 4mA we can have our power supply AND signal on the same pair of conductors. This technique simplifies installation, especially in large plants, as only a twisted pair is needed to transmit a signal from a sensing device to a measuring or control instrument.

LOOP POWERED TRANSMITTER

The principle of operation is not however immediately obvious to engineers used to dealing with voltage signals.

The transmitter is a current sinking circuit, which means that it will attempt to draw a current from an external power supply. This is usually a 4-20mA signal powered from 24V DC which is often an integral part of the measuring instrument to which the transmitter is connected.

Unlike voltage transducers which are wired in parallel to measuring instruments, the current transmitter is wired in a series circuit.

Conventional current flows from the positive terminal of the power supply, through the transmitter, through the internal load resistor of the measuring instrument and back to the negative terminal of the power supply. The voltage generated across the internal load resistor is the signal which the measuring instrument processes.

These days it is common to use a PLC or computer based system with multiple input channels. Several 4-20mA transmitters can be used with a single power supply as shown here.

Transmitters can drive above 20mA, particularly under fault conditions. The power supply should be rated to ensure adequate available current. If the maximum output of the transmitter is 30mA the power supply for three units should be rated at least 90mA.

Of course if the power supply goes down, or gets shorted, we will lose the signals from all the transmitters it powers. Sometimes it is a good idea to use smaller individual power supplies for each loop. That way problems with one loop do not affect the others.

Fred Philpott B.Sc. (Hons. Lond.)
IQ Instruments CC
South Africa

MORE TO COME

Return to Technical Links Page