General Sales Questions

Which products have a five (5) year Warranty and which products have a three (3) year or one (1) year Warranty?

All of the ASIC based products carry the five year Warranty. They are clearly marked as being ASIC based on their data sheet.

Where and to whom do I place orders for Action Instruments equipment? 

You have several options for placing an order with Action Instruments. You may contact you local Action distributor. A list of distributors can be found here. You may also contact Action Instruments direct by phone: 703-669-1305, fax: 703-669-1357 or via email at

How long does it usually take before my order can be shipped?

Using Lean Enterprise techniques, Action Instruments has many products that are built for stock and can ship within 48 hours. Others take somewhat longer.

Thermocouple/RTD Input

What does “cold junction compensation” mean? 

Cold junction compensation allows accurate temperature measurement when using a thermocouple. What’s a thermocouple? Well, a thermocouple is created when wires made from two different types of metal are connected together, (such as Iron and Copper-Nickel). Thermocouples generate a small Voltage, which increases when the thermocouple junction gets hotter. When the thermocouple wires are connected to an instrument, two more thermocouple junctions are created, because the terminals are made of a different material than the thermocouple wires. These “extra” junctions, (called cold junctions), create their own Voltage, which alters the Voltage generated by the actual thermocouple. Cold junction compensation negates the voltage created by these cold junctions, allowing only the Voltage created by the thermocouple to be sensed by the instrument.

Why does an RTD need an excitation source? 

Usually an electrical reference or excitation source is required for sensors whose electrical properties are measured. A thermocouple produces a potential difference between the two dissimilar metals that varies with temperature and is repeatable. The measurement is therefore a voltage. The resistance of an RTD changes with temperature, therefore, a constant DC current excitation source is used as a reference to produce a proportionally changing voltage. Thus a signal conditioner which measures an RTD input provides a current reference as excitation and measures the voltage produced.


What is the maximum load I can connect to the current output of a 4-wire transmitter? 

The maximum load that can be connected to a current output depends on the output compliance. Each product has an output compliance specification that can usually be found on the last page of the data sheet. We can determine the maximum allowable load by using Ohm’s Law. For example, if the compliance of a 4-20 mA output signal is 15 V, then the maximum load is 750 ohms. (15 V / 0.02 A)=750 ohms.

What is the maximum load I can connect to the output of a 2-wire transmitter? 

Unlike a 4-wire transmitter, the maximum load that can be connected to a 2-wire transmitter is dependent on the power supply used to power the output current loop and the loop voltage drop of the 2-wire transmitter. The 2-wire transmitter has a minimum source voltage and a maximum source voltage. The maximum output load would be realized using the maximum output source voltage. A 24 Vdc supply is most often used. The general formula is:
Max RL=(Vss – LVD) / 0.02

A Where RL is the maximum load, VSS is the DC power supply voltage, and LVD is the loop voltage drop. All 2-wire transmitters have a Loop Voltage Drop specification. For example, the T713 RTD input transmitter has a loop voltage drop of 12 V. If a 24 Vdc power supply is used, what is the maximum load that can be connected to the output loop? Using the above formula the maximum load is 600 ohms.

What is the difference between a Two-Wire and a Three-Wire Transmitter? 

The three-wire transmitter is a blend of the four-wire and the two wire versions. The four-wire transmitter has two wires for power and two wires for the output signal. The power for a four-wire transmitter can be either AC or DC and the output signal can be either voltage or current. The two-wire transmitter has two wires for both power and the output signal. A two-wire transmitter is always DC powered and the output can only be a current signal, typically 4-20mA. The three-wire transmitter uses two wires for power and the third wire is used for the output signal (+) positive terminal. The power (-) negative terminal is used as a common reference for power and the signal (-) negative reference. This allows the best of both transmitter features to be utilized. There is one less wire required than a four-wire transmitter and powered outputs are provided for both 4-20mA signals and 0-10V signals. These transmitters can be lower in cost than four-wire transmitters because they are DC powered and do not incorporate an isolating power supply. However, designers must be aware of grounding especially since several transmitters are usually connected to one power supply, and the negative (-) terminal is common to all signals.

On frequency input devices, what is the sensitivity adjustment? 

The sensitivity adjustment actually changes the level of an input filter; Switching the unit to “Low” sensitivity range filters all input signals up to 1 Volt peak amplitude. “High” sensitivity range blocks input signals of up to 10 Volt peak amplitude. The sensitivity potentiometer allows fine adjustment of the filter threshold within the chosen range. The purpose of the input filter is to block low-level electrical noise from the input of the device. The presence of noise makes it harder for the unit to recognize the input frequency. Ideally, the filter is adjusted to block the low-level electrical noise, allowing only the desired input signal to be sensed by the signal conditioner.


What are the dimension differences between the older Visipaks, i.e. the V508, and the newer Visipaks?

The newer Visipak models, the V43x Series and the V108 and V408 are 1/8th DIN sized. This makes the difference three millimeters. The V50x series cutout dimensions are 92 mm by 42 mm. The 1/8th DIN cutout is 92 mm by 45 mm.

What is a loop powered indicator?

Loop powered indicators were designed for field (outdoor) use and have operating characteristics similar to two-wire transmitters. Like two-wire transmitters, they use a 4-20mA signal for power. Therefore, they are very low power devices which are ideal for hazardous environments as well. They typically use a liquid crystal display (LCD) for indication and are very easy to use since they can easily be included in a 4-20mA loop, requiring only a few volts (1 to 4V) of loop drive.

General Signal Conditioning

What is the difference between “unipolar” and “bipolar”?

A unipolar signal is zero based, such as: 0-10 Volts, 0-5 Volts, and 0-20 milliamps. A bipolar signal is negative as well as positive, such as: -10 to 10 Volts, -5 to 5 volts and -20 to 20 milliamps.

How do I connect several devices to the same signal?

If the signal is a voltage signal the devices must be connected in parallel. It is desirable to know the input impedance of each device. Connecting devices in parallel decreases the total impedance. The current drive of the device providing the voltage signal cannot be exceeded if accurate signals are expected. Most 0/10 V output sourcing devices have a drive of 10 mA which translates (using Ohm’s Law: V=IR) to a minimum load of 1000 Ohms. It may become necessary to use an isolator as a voltage repeater. Voltage input devices should have a high input impedance, >100,000 Ohms.

If the signal is a current signal the devices are connected in series. It is critical to know the total input impedance of all the devices. Connecting devices in series increases the total impedance. The voltage drive of the current loop is another critical piece of information. Most current loops installed today are 4/20 mA. A typical current loop will have a voltage drive of 12 V which translates (using Ohm’s Law) to a maximum loop resistance of 600 Ohms. Current input devices typically have low input impedances, the lower the better. Many controller input devices have 250 Ohm input impedance. Action Instruments current input devices are even lower, typically 10-20 Ohms.

How do I convert a 0-5A AC signal coming from a current transformer (C/T) to a 4-20mA signal, None of your products accept a 5A AC input?

A shunt resistor must be used. Action offers a a 0.1 Ohm shunt resistor, model C006. Connecting the shunt resistor in series with the C/T’s output, the 5 A signal is converted to a 500 mV AC signal via Ohm’s Law. This 500 mV AC signal becomes the input to an AC input signal conditioner.

What are standard output signals for signal conditioners?

First 10-50mA, then 4-20mA became the industry standard signal for process control. The primary advantages of the 4-20mA signal is the “live zero” which refers to the 4mA minimum (0% of full scale) and the fact that current signals have a high immunity to induced noise. The live zero is an advantage in the case where signal wires might be damaged. If there were as open circuit no current would flow (e.g. 0mA or -24% of full scale) and an operator would be sure to recognize a problem, versus the case where a 0-10V signal is used and an open circuit would produce 0V (or an intermediate value) which might be mistaken for 0% of full scale.

Regarding noise, the physical principals of electromagnetics prove that voltage signals and high impedance voltage input instruments are much more susceptible to noise generated by radio transmitters or electric motors and power lines than current signals and their low input impedance instruments. Other popular signal levels are 1-5V and 2-10V which are the result of 4-20mA current signals and 250 Ohm and 500 Ohm load resistors, respectively.

Model Specific

Why would I use a non-regulated power supply for Ultra SlimPaks or the DC Action IQ?

The Ultra SlimPaks and the DC Action IQ are powered from 9-30 Vdc (Except the bridge input models Q448, Q446, G448 and WV448 which are 18-30 Vdc). They all regulate the power internally and do not require a regulated power supply. One nice feature of an unregulated power supply is the ability to provide extra current in case it should be needed for whatever reason, usually a fault condition of some kind. Let’s say we have a 13 A, 24 Vdc unregulated power supply. The power supply will provide more than 13 A. When the current demand increases the voltage level decreases. The Action Instruments’ products will continue to operate down to 9 Vdc!

Why is the IQ Rail required when using the AC Action IQ products? 

To achieve the size restraints of the Action IQ housing a switching power supply is used. Switching power supplies emit a high-frequency noise. The AC IQ Rail prevents that noise from feeding back onto the AC power line.

What are the differences between a single channel Q406 and a dual channel Q406?

There are two main differences between a signal channel Q406 and a dual channel Q406. First, only the single channel modules have a reverse / direct switch. The reverse mode allows the Q406 to have an increasing output with a decreasing input. Second, only the single channel module has a 0-20 mA output range. Both single and dual channel units have the 0-1 mA, 4-20 mA, 0-5 V and 0-10 V output ranges.

Which product should I use for split range inputs where a 4/12 mA or a 12/20 mA input signal will provide an isolated 4/20 mA output? 

The best product to use is the Q406 for AC powered modules or a Q408 for DC powered modules. The Q406 and Q408 have up to 90% input and output offset allowing the input and/or output to compress for almost an input / output configuration.

My sensor is sourcing 4-20 mA and I want to connect it to a PLC that also sources the current loop. How do I interface the two devices? 

This common problem is easily resolved with a 2-wire transmitter with an input and output of 4-20 mA. The sensor’s sourced 4-20 mA signal is connected to the passive input of the 2-wire transmitter. The PLC supplies power for the 2-wire transmitter’s output loop, typically with a 24 Vdc source. Action Instruments’ dc input 2-wire transmitter products are the Q501 (available as a single or a dual channel), and the T703.