A 4–20 mA signal conditioner powers a bridge transducer, amplifies the few thousandths of a volt it returns, and sends the result out as a current rather than a voltage. Four milliamps means nothing applied; twenty milliamps means full scale; everything in between is proportional.
It is tempting to read that as the same measurement expressed in different units. It is not. A current loop is a different way of moving a signal from one place to another, and it solves three problems that a voltage output cannot.
First, current does not weaken along a wire. Everything else in this list follows from it. In a series circuit the same current flows at every point — through the cable going out, through the receiving device, through the cable coming back. Cable resistance changes the voltages around that circuit, but it does not change the current. So the value that arrives at the far end is the value that left.
A voltage output has no such protection. Send ten volts down a long cable into a receiving input and what arrives is slightly less than ten volts, because the cable's resistance and the receiver's input resistance form a divider. The reading comes out low by a consistent percentage — steady, plausible and wrong, which is the worst way for a measurement to fail. A current loop simply does not do this.
Second, zero is not zero. The bottom of the range is four milliamps, not none, and that deliberate offset carries information. If a receiver sees no current at all, that is not a reading of nothing — it is outside the valid range entirely, and it means a broken wire, a disconnected terminal, a failed instrument or a dead loop.
A voltage output cannot make that distinction. Zero volts is exactly what a correctly working system produces when nothing is applied, and it is also exactly what a severed cable produces. The two are indistinguishable, so a system that has quietly stopped measuring keeps reporting a perfectly believable zero. On anything that matters — an overload trip, a batch weight, an alarm — that is a real hazard, and the live zero removes it.
Third, a current loop is hard to disturb. Electrical interference works by inducing a voltage into a circuit. In a low impedance current loop, an induced voltage produces very little change in current, so the interference has almost nothing to act on. The same induced voltage arriving at a high impedance voltage input lands directly on the reading. That is why current loops survive routes through cable trays, past motors and alongside drive wiring that a millivolt or a low voltage signal could not tolerate.
And it is the language every controller already speaks. Analog input cards, chart recorders, distributed control systems, indicators and data loggers have accepted 4–20 mA for decades. Sending a measurement that way means the receiving end needs no special card, no scaling module and no conversation about what a particular voltage range means.
What this particular instrument adds is that it is complete. It has its own power supply and plugs into a wall socket, so there is no separate supply to specify. Balance and span are set with precision ten-turn potentiometers rather than software. And it carries the same floating shunt calibration applied at the transducer that the rest of this family uses — a check that travels the whole cable, so it verifies your installation rather than only our electronics.
Excitation goes out to the bridge, a differential amplifier takes the difference between the signal wires, a filter decides what passes, and the result is converted into a current the receiving equipment can read. The part worth planning is the loop itself.
A current loop has a budget, and it is the thing installations get wrong. The instrument can only push its current through so much resistance before it runs out of voltage to do it with. That limit is called the compliance, and everything in the circuit counts against it: the cable out, the cable back, the receiving device's input resistance, and anything else placed in series such as an indicator, an isolator or a barrier.
The arithmetic is simple and worth doing on paper before anything is installed. Add up every resistance in the loop. Compare the total against the compliance. If the total is comfortably below, the loop will reach full scale. If it is close or above, the current will run out before it reaches twenty milliamps — and the symptom is distinctive: everything looks fine at low readings and the output flattens out near the top of the range, as though the measurement had hit a ceiling. It has.
That figure is not on this page, so please ask us for it. We would much rather give you the compliance and the maximum loop resistance for your arrangement than have you discover the limit during commissioning. Send us the cable type, the run length, what is receiving the signal and whether anything else sits in the loop, and we will confirm it works before you buy the cable.
The same is true of the wiring arrangement. Some current outputs source the current themselves; others expect the loop to be powered externally. This instrument has its own mains supply, so it does not depend on the loop for power the way a two-wire transmitter does — but the exact terminal arrangement is worth confirming with us before a panel drawing is issued, because getting it wrong costs a revision rather than a component.
Balance and span still set zero and scale, and here they set the ends of the current range. Balance decides what condition produces four milliamps and span decides what produces twenty. That means the two adjustments interact more than they do on a voltage instrument, so set them iteratively: zero, then span, then check zero again, and repeat until both hold. Ten-turn potentiometers make that a matter of feel rather than of counting.
Filter bandwidth is fixed before the unit ships. You are offered three, priced alike, separated only by one component. Anything that settles over seconds belongs on the narrowest; machinery in motion belongs on one of the wider two. Note which one arrives by default if nobody specifies — it is the narrow one, and it is the least forgiving choice for a customer who turns out to be measuring something quick.
There is also a route to far higher speed. A single capacitor is what holds the response down; take it out and the bandwidth opens up to ten kilohertz. Because that means opening the unit rather than moving a control, raise it with us first — but it does mean the range of measurements one of these can serve is much wider than the three catalog versions suggest.
Everything else is shared with the rest of the family. Any full bridge transducer from 120 to 1000 ohms and roughly one to ten millivolts per volt. Constant eight volt excitation. Common mode rejection of 110 decibels. Nonlinearity of a hundredth of one percent and overall accuracy within five hundredths of one percent of full scale, which puts the conditioner well clear of being the limiting element in anybody's measurement.
The decision here is usually settled by where the signal has to arrive and how far it has to travel. Talk it through with our applications engineers before you buy cable, because that is the part that is expensive to change.
Choose a current output when distance, interference or safety are involved. A long run across a plant, a route that shares a duct or a tray with power wiring, a destination expecting a process signal, or an application where a severed cable must not look like a valid reading — any one of those points at a current loop. If the receiving equipment sits on the same bench and expects volts, a voltage output instrument is simpler and there is one in this family.
Work out the loop before you commit to a route. Cable length, conductor size, what is receiving the signal and anything else in series all count against the compliance. Ask us for the figure, do the addition, and leave margin — loops have a way of gaining an extra device a year after they are built, and a loop designed with nothing to spare is a loop that will fail when somebody adds an indicator to it.
Decide the filter from the speed of what you are measuring, not from habit. Write down a number in hertz first. Weighing, load holding and slow proof testing take the narrow version. The middle version is aimed at working machinery — a press closing, an actuator stroking, a cycle repeating. Faster than that takes the widest, or a conversation about extending it. The capacitor is fitted before the unit ships, so a mistake here is a return rather than an adjustment.
Give some thought to the instrument's own surroundings. It is a small enclosed box for a bench, a shelf or a rack, and it plugs into line power. It has no ingress rating and its temperature range is modest, so a laboratory, a test cell, a control room or an equipment rack suits it and a hot sealed cabinet or a wash-down area does not. If the conditioner needs to live out on the machine, that is a different product in the range and we will point you at it.
Plan how you will verify the installation later. The shunt calibration on this family checks the sensor, the cable and the electronics together, which makes it genuinely useful — but only if you know what a healthy result looks like. Press it when the system is newly commissioned, record what the loop reads, and keep that figure with the instrument. Checking a current loop against a recorded milliamp value is one of the fastest diagnostics available to a maintenance technician.
Count your sensors against the excitation supply. One standard bridge is straightforward and two in parallel are usually workable; more than that, or a low resistance bridge, needs checking against the supply's current limit before the mechanical design is finished rather than after.
Tell us the transducer, the distance to the receiving equipment, what that equipment is, how fast the measurement changes and what else sits in the loop, and we will confirm the version and the loop arrangement before you order.
TMO-2A 4-20 mA Signal Conditioner Applications.
The Transducer Techniques TMO-2A is a complete differential amplifier and signal conditioner with self-contained power, providing excitation, balance and span adjustment on precision ten-turn potentiometers plus shunt calibration, for coupling bridge type transducers to indicating instruments with a 4–20 mA output.
- PLC and Controller Interfacing: A current output arrives at any standard analog input card without a scaling module or a special interface.
- Long Distance Signal Transmission: Current is unchanged by cable resistance, so a measurement arrives at the far end as the value that left.
- Electrically Noisy Plant: A low impedance loop gives induced interference very little to act on, allowing routes that a low level signal could not survive.
- Safety and Alarm Circuits: The live zero at four milliamps distinguishes a broken cable from a genuine reading of nothing.
- Process Weighing: The narrow standard filter delivers a steady current for tank, hopper and vessel measurement.
- Press and Machine Monitoring: Wider filter versions follow a working cycle rather than averaging its peak away.
- Dynamic Force Measurement: Frequency response can be extended for impact and fast transient work.
- Chart Recorders and Data Loggers: Recording equipment expecting a process signal connects directly with no intermediate conversion.
- Laboratory and Test Benches: A complete instrument with its own power supply, needing nothing bought alongside it to begin work.
- Torque and Pressure Measurement: Conditions any full bridge transducer from 120 to 1000 ohms, not only load cells.
The TMO-2A works with our full range of load cells, torque sensors and pressure transducers, and is available with three standard filter frequencies to suit static or dynamic measurement.
Frequently Asked Questions
What does the TMO-2A do that a voltage output conditioner does not?
It sends the measurement as a current rather than a voltage, which changes how well the signal survives the journey. Current is the same at every point in a series loop, so cable resistance cannot reduce it — the value arriving is the value that left. A low impedance loop also gives electrical interference very little to act on. And the range starts at four milliamps rather than zero, so a dead loop is distinguishable from a genuine reading of nothing. Everything else about the instrument — the self-contained power, the ten-turn adjustments, the shunt calibration at the transducer, the filter options — matches the voltage output version in this family.
Why does a current loop survive distance when a voltage output does not?
Because of how a series circuit behaves. The same current flows through every part of the loop, so although cable resistance changes the voltages around the circuit, it cannot change the current. A voltage output has no such protection: cable resistance and the receiving device's input resistance form a divider, so what arrives is slightly less than what was sent, and the reading comes out low by a consistent percentage. That kind of error is steady and believable, which makes it far more dangerous than an obvious fault, and it gets worse the further the signal travels. A current loop simply does not have the failure mode.
What is a live zero, and why does the range start at 4 mA?
It is the deliberate offset that makes a broken system detectable. Because a healthy loop never produces less than four milliamps, a receiver seeing zero current knows something is wrong — a cut cable, a loose terminal, a failed instrument, a loop that is not powered. Compare that with a zero to ten volt output, where zero volts is what a correctly working system produces with nothing applied and also what a severed cable produces. The two are indistinguishable, so a system that has silently stopped measuring goes on reporting an entirely plausible zero. Wherever a reading feeds an alarm, a trip or a batch record, that difference matters a great deal.
What do I need to know before designing the loop?
The total resistance the loop will present, and the instrument's compliance — the voltage it has available to drive current through that resistance. Add up everything in series: the cable out, the cable back, the receiving device's input resistance, and any indicator, isolator or barrier sitting in the circuit. That total has to stay comfortably inside the compliance. Please ask us for the compliance figure and the maximum loop resistance for your arrangement rather than estimating them, and send us the cable type, the run length and what is in the loop. It is a five minute conversation that prevents the one failure mode current loops actually have.
What transducers will it work with?
Any full bridge transducer between 120 and 1000 ohms with a sensitivity of roughly one to ten millivolts per volt — load cells, torque sensors and pressure transducers alike. Excitation is a constant eight volts. Gain adjustment covers the sensors most people connect, with the caveat that the extremes of the sensitivity range are worth checking before ordering: a very low output transducer asks more of the gain range than a typical one does. Send us the transducer's sensitivity and bridge resistance and we will confirm the arrangement.
How do I pick between the three filter versions?
Three are available at one price and the choice is permanent, because the filter is a component rather than a switch. The standard version is the narrowest and suits weighing, load holding and slow proof testing, where its job is to remove vibration and machinery noise so the current sits still. The middle version follows presses, actuators and machine cycles. The widest suits faster work. Pick with some margin above the speed of what you are actually measuring, and if you are unsure how fast your event is, tell us what it is and we will help you estimate it.
Does it have the same shunt calibration as the rest of the family?
Yes — a floating circuit applied at the transducer rather than at the instrument. That matters more on a current loop installation than almost anywhere else, because these are exactly the arrangements where the sensor is a long way from everything else and difficult to load by hand. Since the simulated signal is generated at the sensor, it travels the whole cable exactly as a real measurement does, so the check covers the wiring as well as the electronics. Record what the loop reads when the button is pressed on a newly commissioned system, and you have a complete health check you can repeat in seconds for the rest of the installation's life.
What adjustments does the instrument have?
Balance and span, on precision ten-turn potentiometers. On a current output instrument these set the two ends of the range: balance decides what condition produces four milliamps and span decides what produces twenty. They interact more than they would on a voltage instrument, so set them iteratively — zero, then span, then check zero again, and repeat until both hold. Ten turns from end to end means a precise setting is a matter of feel rather than of counting clicks, and once set there is no configuration file to lose and nothing that needs a computer to restore.
What power does it need, and how big is it?
It plugs into a line socket and draws only a few watts, with its own internal fuse. That is the point of the design — there is no separate supply to specify, no enclosure to buy and nothing to build around it. Physically it is a small enclosed box of roughly three by four by six inches weighing under two pounds, intended for a bench, a shelf, a rack or a test cell. It carries no ingress rating and its operating temperature range is modest, so a dry temperate location is what it wants.
Can I get a voltage output instead?
There is a voltage output version of this same instrument, and if your receiving equipment sits nearby and expects volts, it is the simpler choice. Choose the current output when the signal has to travel, when its route is electrically unfriendly, when the destination expects a process signal, or when a severed cable must not be able to masquerade as a valid reading. If you are genuinely unsure which you need, describe where the signal starts and where it has to arrive and we will tell you — the answer is usually obvious once the distance and the destination are known.
Questions From The Field
My controller reads 4 mA no matter what load I apply.
Four milliamps is the bottom of the range, so the loop is alive and the instrument is producing its zero output — which narrows things usefully. Check the span setting first, since a span wound fully down produces almost no change for a large input. Then check that the transducer is actually being excited and that its signal is reaching the amplifier, by measuring the excitation at the sensor terminals and using the shunt calibration to see whether anything moves. If the shunt check produces no change either, the problem is between the sensor and the amplifier. If the shunt check works but real load does not, the problem is mechanical — the load is not reaching the transducer.
My reading is lower at the controller than at the instrument.
That should not happen on a current loop, and the fact that it does is informative. Current is the same everywhere in a series circuit, so cable resistance cannot account for it — which means the difference is somewhere other than the wire. The usual causes are scaling rather than signal: the controller configured for a different span than the instrument was set to, or an input configured for zero to twenty milliamps when the instrument is producing four to twenty, which produces a smooth, believable and entirely wrong number. Check what each end thinks four milliamps and twenty milliamps represent before looking at anything physical.
The output climbs normally and then flattens out near the top.
Classic compliance limit. The instrument has run out of voltage to push current through the total resistance of your loop, so it can reach part of the range but not all of it — and the symptom is exactly this, correct at low readings and capped at high ones. Add up everything in series: cable there and back, the receiving device, and anything else in the circuit. Reducing the total usually means heavier cable, a shorter route, a lower input resistance at the receiver, or removing something from the loop. Send us the arrangement and we will confirm the figure you have to stay inside.
Can I read the same loop in two places?
In principle yes — devices in a current loop go in series, so a second indicator or logger can be added to the same circuit, and each sees the same current. In practice the limit is the compliance, because every device you add contributes resistance and eats into the budget. That is why a loop designed with no margin causes trouble later: somebody adds a panel indicator two years afterward and the top of the range disappears. If you know now that a second device may be added, tell us at the design stage and we will leave room for it.
I need to reach a controller several hundred feet away.
Distance is what this output is for, and several hundred feet is ordinary for a current loop — but the arithmetic still has to be done rather than assumed. Cable resistance depends on conductor size as well as length, so a heavier conductor buys you distance directly. Send us the run length, the cable you intend to use or the size you have available, and what is receiving the signal, and we will confirm it before you pull anything. Keep the route away from motor and drive wiring where you can, even though a current loop tolerates it far better than a low level signal would.
My loop reads zero milliamps.
Zero is outside the valid range, which is the whole point of the live zero — it is telling you the circuit is broken rather than that the measurement is nothing. Work around the loop in order: confirm the instrument has power, then check the loop wiring for an open circuit at every terminal and junction, then check any device in series in case one has failed open, and finally confirm the polarity, since a loop connected backward passes no current. If the circuit is confirmed continuous and there is still nothing, contact us. A reading of zero is a fault report, and it is worth treating as one.
Do I need a separate power supply for the loop?
This instrument has its own line-powered supply, so unlike a two-wire transmitter it does not depend on the loop for its own power. Whether the output actively sources the current or expects the loop to be powered elsewhere is worth confirming with us before a panel drawing is issued, because it determines the terminal arrangement and it is much cheaper to establish now than to correct on a revision. Tell us what the receiving device expects and we will confirm exactly how the two should be wired together.
Can my existing voltage output unit be converted to a current output?
Ask us with the model number to hand rather than assuming either way. The two are separate products rather than one with an option fitted, so what is possible depends on the unit you have. Where conversion is not practical there are usually good alternatives — a separate voltage to current converter after your existing instrument will do the job, and is often the sensible route where the unit is already installed and working. If you are equipping something new, it is cleaner to start with the current output version than to add a stage.