Introduction to Two-Wire Transmitters and 4-20mA Current Loops

The Basics of 4-20mA Current Loops

In two-wire 4-20mA control loops, we use 2-wire transmitters to convert various process signals representing flow, speed, position, level, temperature, pressure, strain, pH, etc., to 4-20mA DC for the purpose of transmitting the signal over some distance with little or no loss of signal. This paper reviews the operation of this transmission standard and its advantages; in particular as it relates to two-wire transmitters and the associated 4-20mA current loop.

What makes 4-20mA signal transmission so attractive?

Probably the greatest advantage of using a current loop for signal transmission is the current loop’s low sensitivity to electrical noise. This is very important for long distance transmission in harsh industrial environments. As a generally low impedance system, it is much less sensitive to induced noise, than perhaps the high impedance input of a voltage amplifier. The currents injected by typical noise sources are generally no more than a few hundred microamps, usually insignificant to the 16mA span. The use of a “Live Zero” also improves the signal to noise ratio at low levels, allowing us to accurately discern low signal levels without added noise or interference.

Another advantage to the 4-20mA current loop is that it is essentially lossless with respect to the transmission media (wire) and the interconnections (connectors). That is, the accuracy of the signal is not affected by the voltage drop in the interconnecting wiring. This allows the signal transmission to occur over long distances, with varying conductors. Compare this to voltage signals, which will always have an associated signal loss related to the length of the wires—the 4-20mA signal current does not exhibit any signal losses under this same scenario. Kirchoff’s Current Law teaches us that the current in a loop is equivalent at any point in the loop. That is, if you happen to be reading 12mA at your receiver input, you can be certain that 12mA is passing through your transmitter.

What is the 250T Two-wire RTD Transmitter Failsafe Hook-Up Approach?

Two-wire Transmitters and Their “Hook-up” to Achieve a Failsafe Configuration

The attached analysis references Acromag’s 250T series RTD (100 ohm platinum)

(250T Two-wire RTD Transmitters Fail-safe Hook-up Approach)

This type of hook-up is typically not used because the effects of lead-wire resistance inaccuracies are additive to the RTD measurement. However, when addressing the problem of FAILSAFE, this configuration offers the most predictable output behavior. Also, H the RTD hook-up is properly done, the inaccuracies due to lead wire resistance can be minimized. The following is a list of most probable failures that could happen to the input sensor. An analysis will be performed to address each failure and predict the output of the transmitter.

1. Lead wires +/-could break open
2. The RTD sensor could fail open up
3. The input could become grounded
4. The input +/-could become shorted

Conditions/Assumptions for Failsafe Hook-up Approach:

1. RTD type: 100 ohm platinum
2. Lead-wire length: 50ft (or shorter)
3. “-” & “L” connection: internal
4. Max. temp. change for lead wires: 50°C
5. Lead wire gauge: 17 AWG (0.005 ohms/ft)
6. RTD break detection: UP

Based on these conditions, we can derive accuracies for this type of approach.

Analysis

1. Total lead wire resistance:

·         2 * 50 ft * (.005 ohms/ft) = 0.5 ohms (static value)

·        NOTE: This lead wire resistance is a static value which means that at 25°C, R’s effect can be calibrated out of the transmitter. The value of the most concern, however, is the dynamic resistance change of the lead wires over temperature

       2. The following equation approximates copper wire resistance with temperature changes of up to 50°C.

·         Rt= R(25°C) * [1 + (6 T)(0.004)]

·         Rt= 0.5 * (1 + (50)(0.004)]

·         Rt= 0.6 ohms

·         Thus, the dynamic resistance change of the lead wires is:

Dynamic change= 0.1 ohms (maximum temperature change= 50°C)

 To download click here What is the 250T Two-Wire RTD Transmitter Failsafe Hook-up Approach?

Why Do You Need USB Isolation for Industrial I/O?

USB Isolation and Industrial I/O

The USB port has become the most popular method for connecting virtually anything to a computer; from simple flash drives to complex industrial I/O equipment. Most computers built in the last five to ten years have at least one USB port; with newer computers having upwards of six or more. For most home users, having an isolated USB connection is not an issue. However, isolating a USB connection can be a necessity for applications such as; industrial/remote I/O, data acquisition, IT or medical equipment. This paper examines the advantages of using USB; what isolation means, types of isolation, and when USB connections should be isolated.

What is Isolation?

Isolation is achieved by removing any direct electrical connection between two devices or portions of a circuit, but having them work as if they were still electrically connected. Isolation is generally rated by how high a low-current AC voltage can be set before there’s arcing across the isolation barrier. Some common isolation terminologies are:

1. Isolation Rating: The transient overvoltage that an isolation barrier can withstand. Usually expressed in terms of AC voltage. Some common ratings are:

·         500 Vrms at 1 minute for industrial equipment

·         2500 Vrms (IEC 60950-1) at 1 minute for IT equipment

·         5000 Vrms (IEC 60601-1) at 1 minute for medical equipment

     2. Working Voltage: Continuous voltage that can be applied across an isolation barrier, for the life of the device, without breaking down. Like the isolation rating, it’s usually expressed as an AC voltage.

     3. Reinforced Isolation: Usually required for medical systems. This isolation rating equals two separate isolation barriers. In medical systems, this means the isolation barrier would have to withstand a short duration of 10,000 Vrms.

      4. Creepage: The shortest distance between two lines on a surface layer of a PCB; on either side of the isolation barrier.

      5. Clearance: The shortest air gap between two lines on a PCB.

Methods of Isolation

There are several methods of isolation. The method used depends largely on; what is being isolated and the bandwidth of the signals being isolated; as well as whether or not power must be transferred between the two sides of the isolation barrier. Some of the common methods are:

1. Optoisolators: Combine an LED and a phototransistor in the same package. As the LED emits light, the phototransistor will change states. These are well-suited for isolating digital signals, but cannot transfer power across the barrier.
2. Transformers: Use magnetic coupling. Additionally, transformers are very good for passing power and fast switching signals across the isolation barrier.
3. Differential Capacitor Coupling (DCC): This method makes use of a capacitor's ability to pass AC signals while blocking DC. DCC can be used for high-speed digital data.

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(Courtesy of Acromag)

Why Do I Need Signal Isolation?

 

Examining the important aspects of signal isolation; what it does, why we need it, and how to test for it.

Acromag is in the business of signal conditioning. We manufacture circuits that amplify, isolate, filter, and convert one signal form to another. Most of these circuits also provide electrical isolation. However, added isolation has a cost. Sometimes customers question their need for isolation or fail to recognize the need for adding isolation in their application. This paper covers the basic aspects of electrical isolation.

Briefly, electric current refers to the conceptual flow of atomic particles or electrons through wires and electrical devices. Conductive materials like metal and water allow electric current to easily pass through them. The force that drives electric current to flow through a conductive medium is potential difference or voltage. The opposing force that curbs or limits this current flow is resistance. Comprised of 60% water, the human body is an excellent conductor; except that electric current from a source allowed to pass through the body can induce injury via electric shock. Materials that are weak conductors of electricity have high resistance to current flow; these materials are often used to add isolation or insulate circuits. In general, greater force (voltage) along a conductive path will drive higher current flow; if poorly controlled in the absence of resistance (insulation/isolation), it may result in circuit damage, personal injury, or even death. 

Table of Contents

1. What is Isolation?

2. Common Methods of Signal Isolation

·         Transformer or Inductive Coupling

·         Galvanic Isolation vs Galvanic Isolator

·         Optical Isolator, Optical Coupler, or Fiber Optic Link

·         Capacitor

·         Magnetoresistance

3. Why Do I Need Isolation?

·         Block High or Hazardous Voltage

·         Protection from Electric Shock

·         Reject High Common-Mode Voltages

4. More Resources

Click here to download the complete white paper Why Do I Need Signal Isolation?

What is Isolation?

With respect to electric circuits and electronic instruments; “isolation” means the deliberate introduction of a non-conductive separation to inhibit current flow. Galvanic Isolation is the process of blocking current flow to prevent a direct conduction path between circuits is called. This term sometimes causes confusion because “galvanic” refers to metal and the electrochemical process in which one metal corrodes to another when both metals are in electrical contact and in the presence of an electrolyte. But galvanic isolation refers to the absence of metal or a conduction path.

Galvanic isolation is accomplished by physically adding distance, clearance, or insulating material around a circuit to block unwanted current flow. But how do we preserve a circuit signal and allow it to be transmitted across an isolation barrier? We can additionally isolate the signal by transmitting it magnetically using transformers or magnetoresistance. We could transmit it optically using optical couplers, optical isolators, or fiber-optic media. Or we could capacitively couple the signal across an isolation barrier using capacitive isolators.

Signal isolation is usually accomplished by a combination of actions; physical separation and insulating material, combined with a method of isolated signal transmission (magnetic, optical, or capacitive). The important thing is that regardless of our isolation method, isolation prevents the electrical conduction of unwanted current between circuits, while still allowing our wanted signal to cross an isolation barrier without providing a conductive metal path.

Acromag offers the industry’s best selection of process signal isolators. Click here to view them.

Common Methods of Signal Isolation

So, the real trick in isolation is not how to add insulation or separation to a circuit. The trick is adding electrical isolation to block unwanted signals, while still allowing the wanted signal to transmit through the circuit; and without providing a direct (galvanic) path for signal conduction. Below are some common ways to isolate a signal between two points without providing a direct conduction path between them.

Transformer or Inductive Coupling

The most common example of a galvanic isolator would be the transformer. The primary and secondary windings of a transformer are insulated from one another. They don’t connect to each other electrically, so there’s no metal to metal contact. Instead, they use magnetic field flux, generated by coils of wire overlapping a ferromagnetic material; signals are inductively coupled to/from the ferro-magnetic material using a varying magnetic field.

Transformers buffer or change voltages by stepping them up or down. They’re also used for isolating signals for safety, as well as isolating a circuit from AC line voltage. A transformer allows its secondary windings to be offset from a ground reference on the primary side. Thus, breaking potential ground loops between the primary and secondary circuits. Because it involves the mutual inductance of magnetic fields from coils, it can be more susceptible to magnetic interference. Further, unless properly shielded, it can also be a source of magnetic interference to adjacent circuitry (inductive and radiated emissions). Transformers are traditionally bulkier than optical or capacitive isolators. However, there’s newer technology that uses chip-scale transformers, encased in integrated circuit style packages, to magnetically isolate signals. (For one example of this technology, see Analog Devices isoPower® and iCoupler® technology.)

(Isolated Transmitters: Acromag 650Ts: Acromag 651T/652T with Transformer Isolation)

Galvanic Isolation vs Galvanic Isolator

“Galvanic isola-tion” should not be confused with “galvanic isola-tors.” A galvanic isolator is used to block low voltage DC currents from coming on board boats, via shore power ground wires. These DC currents can accelerate galvanic corrosion on underwater metals of boats and cause extensive damage; metal in hulls, zinc anodes, prop, drive-shaft, etc. Galvanic isolators are used because boats plugged into shore power at marinas each act like giant batteries; contributing DC voltage to the power signals via the ground wires. This produces corrosive electric currents through all the metals that contact the water. The metal and water form a giant battery, causing the metals to corrode in galvanic fashion; the way terminals and plates of a battery corrode as current passes through them. Zinc anode is a sacrificial metal added to a boat’s conductive metal surface; concentrating the resultant corrosion to itself.

How Galvanic Isolators are Used

Galvanic isolators are inserted in-line with the green safety ground as they enter the boat, between the shore-power inlet and the boat’s electrical panel. It allows AC fault current to pass through it while blocking DC current. Thus, AC faults are transmitted back to the power source, where they can safely trip a breaker or open a fuse. Simultaneously, destructive galvanic DC battery currents are blocked/minimized to reduce galvanic corrosion. This enables the zinc anodes of your boat to help protect its underwater metals and not those of other vessels that surround it; as they act to control the corrosion of the metal attached to your own boat. Most galvanic isolators are designed to be fail-safe; meaning that if they fail, they do not also open the path to ground for fault current.

Your first instinct might suggest, “Why not just remove the ground-wire?” However, this would be dangerous. The ground wire must be present to carry fault current back to the dock power source or transformer. Otherwise, if you accidentally contacted the shore power AC line by some type of wiring fault, you could become the medium to carry fault current back to the transformer; this could be fatal.

Learn more: Why You Need USB Isolation for Industrial I/O?

Optical Isolator, Optical Coupler, or Fiber Optic Link

Optical devices transmit information through their medium or across their barriers using varying levels of light intensity; with no direct electrical conduction path. A light source (transmitter, typically an LED) sends light waves to a photo-sensitive device (receiver, typically a photo-transistor). The combination is often held in place with insulating plastic, like that of an integrated circuit IC. Alternatively, transmit and receive functions are separated using a transmitter linked to a remote receiver via fiber optic cable. One major benefit of optical isolation is its inherent immunity to EMI (Electro-Magnetic Interference or electrical and magnetic noise).

Some comparative disadvantages to optical isolation are its:

• Generally higher power dissipation
• Susceptibility to temperature effects
• Traditionally slower speed (specifically optical couplers, not fiber optic links)
• Finite life of its transmitter (LEDs degrade over time)

(Acromag 612T DC Voltage/Current Input Dual-Channel DC-Powered Transmitter Drawing with Optical Isolation)

Capacitor

Remember that capacitors generally allow AC current to flow, but block DC current. Thus, they efficiently couple AC signals between circuits, at different DC voltages, via a varying electric field. There are many capacitive isolation devices available, and it is a common technology of digital isolators. Many modern devices will even use isolation-rated capacitors to connect between grounds on each side of an isolation barrier. This provides a conduction path for transient signals; perhaps to earth ground (also helpful in quelling radiated emissions). Capacitive isolation is faster than optical isolation.

Unfortunately, capacitors are more prone to failure when stressed by voltages above their voltage rating. And for some capacitors, this failure mode can result in a short circuit condition; abruptly ending its isolation-ability, as well as possibly rendering its circuit unsafe or hazardous. Safety rated Y-type capacitors are used in line to ground applications and are designed to fail open; while X-types are used in line-to-line filtering applications and may fail short. Also bothersome when used to isolate digital signals; often the first bit transmitted after power-up using capacitive digital isolators is used to setup the data stream, and must be ignored (only the trailing bits contain useful data).

Magnetoresistance

Magnetocouplers use Giant Magneto Resistance (GMR) to couple from AC to DC. An explanation of GMR isolation is beyond the scope of this paper. Briefly: GMR refers to an isolation scheme that relies on the property of a material to change the value of its electrical resistance, when an external magnetic field is applied to it. It’s important to remember that GMR operates like a transformer; it uses the variable magnetic field of an AC coil. However, it does this to linearly alter the DC resistance of a physically isolated sensing element.

Click here to download Why Do I Need Electrical Isolation?

Why Do I Need Electrical Isolation?

We have two principal reasons for introducing isolation into an electric circuit:

1. To block the transfer of high or hazardous voltagesTo break ground loops
2. Block High or Hazardous Voltage

We use isolation to prevent the transfer of high or hazardous voltages between circuits. We typically block these voltages using isolation for safety reasons and protection from electric shock; but also to block high common mode voltage present in our signals, which can prevent its measurement and damage equipment. Isolation can also block transient voltages for the same reasons. High voltage may drive injury via electric shock and the unintended flow of electric current through the body. Additionally, it may also drive damage to an electrical circuit because of unintended electric current flowing between conductive circuits.

Protection from Electric Shock

One reason we isolate a circuit is to help prevent electrical shock. That is, by introducing isolation between conductive bodies, we minimize or eliminate the potential for unintended current flow. With no shared common reference or conductive path between two conductors or circuits you cannot complete a circuit for current to flow. This is because of potential differences between them sufficient to produce electric shock; the sudden and rapid flow of electricity between potentials when crossed with a conductor. Shock currents in the body can be felt at about 0.5mA; they can drive an erratic heartbeat and potentially be fatal above 10mA; and they can stop a human heart at 2A. Isolation blocks voltage potentials that could drive dangerous current levels through a body if contacted/crossed.

Learn more: How to Select the Right Isolator?

Reject High Common-Mode Voltages

Isolation blocks the dangerous transmission of high voltages between circuits which can drive electric shock to personnel or equipment. Another key use of isolation is to enable the measurement of a signal with a high common-mode voltage that prevents valid measurement and could damage equipment. The reality is that most instruments will have a common-mode input range inside of ±10V; unless specifically designed to reject high common-mode voltage. Thus, signals with an offset from measurement ground greater than 10V cannot be converted properly and could damage the instrument. Isolation rejects the unwanted high common-mode voltage present in some signals, allowing the real signal of interest to be discerned.

Remember that electromagnetic noise is ever-present in most environments because of nearby machinery and electric motors, relays, fluorescent lighting, etc. As a result, common-mode noise can be capacitive-coupled, inductively coupled, or radiated into the measurement system. And it will typically take the form of a DC offset, combined with a continuously variable 50-60Hz component (and even higher frequency harmonics of 50-60Hz) that can mix with and obscure your measurement. Isolation blocks the transmission of this error through our system (see Ground Loops below). But some applications will naturally contain a greater offset voltage than this.

Example

If an input is restricted to voltage potentials in the ±10V range; how would you measure one cell of a large array of solar cells connected in series? Or measure the individual cell of a large hybrid battery? Since these signals are offset from circuit common by larger amounts (they have high common-mode voltage potentials), certainly greater than ±10V; this makes their measurement difficult and potentially dangerous to your equipment. Note: the common-mode portion of an input signal is normally computed as the sum of the voltage potential of the positive lead, with respect to measurement return or common; and the voltage potential at the negative lead, with respect to measurement return or common, divided by two (Vcm = [Vin+ + Vin-]/2). Signal isolation blocks the high common-mode portion of input signals like these; which otherwise make our measurement difficult and can damage our equipment.

(Courtesy of Acromag)

Electrical Ground Rules Best Practices

 

(Courtesy of Acromag)

Why is Circuit Grounding Important?

This whitepaper is Part 1 of a three-part Grounding Electrical Equipment Best Practices series that looks at grounding and its role in protecting personnel, equipment, and ensuring the integrity of electrical signals. In this part, we will review circuit grounding and its importance to you, as well as the US AC power system and its use of earth ground.

Download PDF white paper here

Topics include the following electrical ground rules principals:

1. What is Ground?
2. The Importance of Ground
3. AC Powered Systems
4. The Ground Loop

Firstly, the two principal purposes of connecting your system to earth ground:

1. Stabilize the voltage to earth during normal operation (think of earth like an anchor to the system voltage)
2. Limit the voltage rise created by lightning, line surges, and unintentional contact with higher voltages

Secondly, the three main purposes for correctly applying ground:

1. Safety: connect to ground to provide an alternative path for fault current to flow
2. Limit Voltage and its Variation: connect signals to ground to stabilize them & keep them from floating
3. Limit the Induced Voltage Magnitude: connect to ground to limit the voltage-rise induced on powered circuits

To, another very important side-benefit to providing a good connection to earth ground: grounding our circuits to gain electromagnetic compatibility (EMC) benefits that result in lower noise and radiated emissions.

How do ground fault circuit interrupters work?

This Electrical Ground Rules white paper is Part 2 of a three-part series that takes a look at grounding and its role in protecting personnel and equipment and ensuring the integrity of electrical signals. In this part, we will examine our use of the ground to protect from ground faults, and how ground fault circuit interrupter (GFCI) devices operate to protect us from severe shock. We also answer the below questions:

1. What does wired earth growly protect you from?
2. Does it protect you from electrocution?
3. What if your appliance does not include a connection to earth ground and you become the path of least resistance for some sort of ground fault?

We begin with a discussion on the use of the ground to protect from ground faults. Additionally, we cover how ground fault circuit interrupter (GFCI) devices operate to protect us from severe shock.

What is the role of ground as a voltage stabilizer and transient limiter?

This Electrical Ground Rules whitepaper is Part 3 of a three-part series that takes a look at grounding and its role in protecting personnel and equipment, and ensuring the integrity of electrical signals. Consideration of ground can be very complex and application specific. However, in many of these applications, there are a few rules of thumb that are helpful when we make wired connections to ground and electrical equipment. This whitepaper not only looks at ground and its role as a voltage stabilizer and transient limiter; it also offers tips on how to improve your connection to ground and realize benefits to safety and signal integrity.

Download PDF white paper here

How to Add Protection when Switching Inductive Loads?

The importance of adding protection to the load terminals in inductive load switching: a review of important inductor behaviours.

Acromag manufactures products that are often used to switch inductive loads. For example, our digital outputs and alarm relay commonly drive inductive loads that include motor windings, solenoids, interposing relay coils, physical inductors, and even the load wiring itself. In all cases, we routinely recommend that protection be placed local to the inductive load being switched. This usually takes the form of an added reverse-biased shunt diode placed right across the load terminals for DC inductive loads. We make this recommendation even when our switches may already incorporate similar protection inside the device. Often, we’re asked to justify the “inconvenience” of adding protection to the load by customers and some have even inferred that our switching devices must be weak because we make this recommendation. However, this is not an “Acromag thing,” but good practice for all inductive load-switching applications.

This application note aims to explain the importance of this added protection and how it works.

Topics Discussed:

1. Inductor Behaviour Using Simplified Low-Side Switched Inductor or Relay Coil (Sinking) Examples

•     RL Charging (Storage) Cycle
•     RL Steady State (Fully Charged)
•     Traditional RL Discharge Cycle without Protection but with a Constrained Voltage Spike
•     Common RL Discharge Cycle without Protection Where the Reverse VL Spike is not Constrained to Vs
•     Common RL Discharge Cycle with Added Protection

2. AC Inductive Load Switching

Inductor Behaviour

To help you understand why you should add this protection, we need to review some points about inductor behaviour. You might have been told things such as; “…the current through an inductor cannot change instantaneously,” or “…inductors impede changes in current flow,” and “…inductors store energy.” The current in an inductor cannot change instantly; thus inductors will impede changes in current, and do indeed store energy. The stored energy in a charged inductor is not indefinite; its eventual discharge is precisely why you need to add protection to switched inductive load circuits. This is illustrated by the RL charging and discharging circuits shown in the tech note.

Click here to download How to Add Protection when Switching Inductive Loads?

 

Adding Protection When Switching Inductive Loads