Voltage Transducer Circuit: How 110V AC Is Converted to 4-20mA Signal and Sensing Explained

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#1 21665857 10 Jan 2013 03:54

ARJ ARJ

Anonymous

Post #1
21665857 10 Jan 2013 03:54

I want to know how a voltage transducer works..any exact circuit to show that how a voltage like 110V AC become to 4-20mA signal..and how sensing side of transducer works?
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#2 21665858 10 Jan 2013 08:40

Mark Harrington Mark Harrington

Anonymous

Post #2
21665858 10 Jan 2013 08:40

The first question is what? Voltage transducer

So what do you do here

Use google, Look up Voltage transducer

Thats the first step
Read it, make some notes, decide what other questions you may have

Write those down

Come back to the forum and discuss what you have read and understood
Followed by other questions you may then have and we will then guide and assist you further

ALSO Good english is as such

Ask politely, example "HI I wonder if someone could assist me please

Im having difficulty understanding what a voltage transducer is

Could someone explain this to me or provide possible links as to where I might gain more information "

ITS VERY RUDE TO ASK LIKE THIS, "I WANT..
."

LOTS OF PEOPLE WANT, BUT NOT EVERYONE GETS WHAT THEY WANT BESIDES WHICH YOU NEED TO ASK NOT DEMAND

Adopt this attitude on first request you will imiediatley allianate yourself and NO ONE WILL HELP YOU
#3 21665859 10 Jan 2013 13:01

Steve Lawson Steve Lawson

Anonymous

Post #3
21665859 10 Jan 2013 13:01

Just so you have a diverse first experience posting on this forum, let me say that I didn't hear any of the "negative tone" that Mark is chastising you for (i.e. your tone doesn't sound demanding to me ;)

I'm not familiar with voltage transducers but judging by the data sheet for the LV25-P, it uses a hall-effect device to sense the magnetic field generated by a current passing through the transducer -- so, it's really a current transducer. Voltage is measured by passing a current proportional to the measured voltage (set by an external resistor). The resistor, in series with the transducer, is applied across the voltage to be measured.

Here's the datasheet:

http://www.europowercomponents.com/media/uploads/LV25-P-SP2.pdf

BTW, I Googled _Voltage Transducer_ (and _How does a Voltage Transducer work_) and found very little information.

Now, regarding the 4-20ma signal and how 110VAC is _converted_ to a 4-20ma loop, are you looking for a circuit that _measures_ (or _monitors_) 110VAC and sends that information via a 4-20ma loop?

There is a wealth of information at the end of a Google search on "4-20ma circuit". Give that a try, and then, as Mark suggested, ask a more specific question in a fresh post (so it gets more exposure than it would if asked here).
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#4 21665860 10 Jan 2013 13:25

Chuck Sydlo Chuck Sydlo

Anonymous

Post #4
21665860 10 Jan 2013 13:25

There are many devices that are considered transducers.The question cannot be answered as asked! Now if there was a specific tranducer mentioned that portion of the question could be addressed. Then attention could be brought to the second queston asked and that is; Can this device be used with 110v AC?

*DICTIONARY*
transducer
Definition
trans·duc·er[ tranz dssər ]trans·duc·ers Plural

NOUN
1. device that converts energy: a device that transforms one type of energy into another, e.g. a microphone, a photoelectric cell, or an automobile horn
2. biological converter of energy: a biological entity that converts energy in one form to another, e.g. the rods and cones of the eye or the hair cells of the ear
[ Early 20th century. < Latin transducere "lead across, transfer" (see traduce) ]
#5 21665861 10 Jan 2013 13:26

Earl Albin Earl Albin

Anonymous

Post #5
21665861 10 Jan 2013 13:26

There are many types of transducers. You don't ask what kind. Having said that there are many things written on Wiki pages that discuss transducers of varying types. Pick one.
#6 21665862 10 Jan 2013 15:24

Mark Harrington Mark Harrington

Anonymous

Post #6
21665862 10 Jan 2013 15:24

Just for reference this is not being negative at all Its commonly known as basic mannors and it isnt "I want at all", neither has he bothered to look anything up

You obviously havent had the pleasure of workng with "I wants, or the obnocious phone calls from so called Directors etc nevemind some of the less distastefull influences in life"

Basic mannors cost nothing in life and retrieve far greater results Its no wonder we have appauling attitudes such as this

So with all due respects Im quite right to point this out

You may tolerate this I dont You may talk to other people like this but with me If you want help then you ask nicely
#7 21665863 11 Jan 2013 13:17

Shrikant Kamble Shrikant Kamble

Anonymous

Post #7
21665863 11 Jan 2013 13:17

Take a ferrite rod and wound it with 3 to 4 hundred turns of 36 gauge enamelled copper wire preferably evenly so that loses are reduced. connect coil terminals to transistor DC amplifier. Connect a low impedance device in collector circuit through which you want to flow 4 to 20 mA of current.

Switch on transistor circuit. Take coil core near to AC 110 V line, you may find current as expected by you through your device or in collector circuit.

Ferrite core coil works like magnetic transducer, it picks up the magnetic fluctuations taking place in AC 110 Volt line due to current flow in line.

Coil produces weak fluctuations of AC voltage, which is then amplified by transistor circuit. Collector of the circuit can provide 4 to 20 mA of current through low impedance device connected at collector. low impedance device could be a 100 Ohm resistor.
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#8 21665864 11 Jan 2013 15:00

Steve Lawson Steve Lawson

Anonymous

Post #8
21665864 11 Jan 2013 15:00

You will, of course, need to include all the support circuitry to calibrate this and temperature stabilize it. Also, to get a consistent reading, the ferrite rod will need to be secured, somehow to the AC line, or rigged to achieve a repeatable orientation with the AC line. In other words, it's a bit more complicated than Kamble's description.

Unless this is merely to be some sort of "proof of concept" or demonstration.
#9 21665865 12 Jan 2013 02:00

ARJ ARJ

Anonymous

Post #9
21665865 12 Jan 2013 02:00

Hi mark
First I want to apologize for what you consider as "RUDE STUFF" and assure you that I`m not like that kind of man may appeared from my question.
then I tried so much to find some thing exact about a voltage transducer. you know that electronic producers do not give complete information about their gadgets! what I meant from my Q was that could any one help me to find exact circuit of a voltage transducer which senses 110 V AC and convert it to 4-20mA current signal..how it happens? and every thing like that.
thanks so much.
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#10 21665866 12 Jan 2013 02:40

ARJ ARJ

Anonymous

Post #10
21665866 12 Jan 2013 02:40

Dear Steve
First let me have my appreciate to your friendly manner then I want to say YES for this part of your post:

"Now, regarding the 4-20ma signal and how 110VAC is converted to a 4-20ma loop, are you looking for a circuit that measures (or monitors) 110VAC and sends that information via a 4-20ma loop?"

what exactly I have is a IME model TM3UL20 voltage transducer. this transducer senses 110 V AC and convert it to 4-20mA current signal. I have some problems with this kind of transducer. after a period of working like 1 or 2 years it sends 21 to 22 mA for 110 volt AC in its sensing line. this is common problem in instruments like a Voltage transducer that you can fix it by adjusting its Zero or Span potentiometer. but problem starts from here that by adjusting zero and span potentiometer in their minimum position current signal dose not reduce and you still have 21 to 22 mA! this makes a big error in what you measure and send back to control center.
now I wonder to find out more about this kind of voltage transducer in particular and common concept of all voltage transducers. like how exactly they sense voltage and then convert it to a 4-20 mA signal..
I also have to say that I don`t want or cant just replace with other brand or thing..
#11 21665867 12 Jan 2013 04:35

Steve Lawson Steve Lawson

Anonymous

Post #11
21665867 12 Jan 2013 04:35

Sounds like what you are saying is that your TM3UL20 has gone out of calibration and won't come back into calibration with adjustment of the span and zero controls.

Like you said, the manufacturer is unlikely to divulge the inner workings of their device. Perhaps you can locate a patent for it. Other than that, I have nothing for you, sorry. Perhaps someone else here does.

If you're adventurous, perhaps see if you can disassemble it and reverse engineer it. Or maybe someone here can recognize a key internal component (or two) and help you figure out how it works.

Also, if you _do_ disassemble it, perhaps you can find a resistor that has overheated and thus increased in value, or some other such easily identifiable failure.
#12 21665868 25 Sep 2016 02:01

Cactus jack Cactus jack

Anonymous

Post #12
21665868 25 Sep 2016 02:01

Hello.
I am also in the need of measuring voltage.
I was thinking in doing it with 2 resistors in series and this configuration in pararell with the load I want to measure the voltage from. The resistance of this arrange is much biguer than the one I am measuring from.
The proble I have, is with the ground of the micro that will aquire the voltage, does it have to be conected with ground of the load, is this dangerous for the Micro?.
This Micro is also conected to the Current transducer (that is working fine):

http://www.lem.com/docs/products/hlsr-p_sp33_series.pdf

So i havent try this voltage sensing configuration, for not burning anything yet.

Thanks a lot.

I attach a picture of the configuration.
#13 21665869 25 Sep 2016 11:20

Steve Lawson Steve Lawson

Anonymous

Post #13
21665869 25 Sep 2016 11:20

Looks like you are trying to measure the voltage "V1" using a Voltage Divider composed of the resistors "R1" and the 250Ω resistor. The question of whether it's safe, or not, to connect the Microcontroller's [MCU] ground to the Test Point ground depends on what is powering the MCU. If it's battery powered, then no problem (unless high voltages are involved (50V or more), and there is the possibility for someone to touch it--i.e. electrocution potential). Otherwise, I suggest using something like an opto isolator to do this measurement.

Check this out for possible ideas on how to do this:

http://electronics.stackexchange.com/question...ct-way-to-measure-voltage-with-an-optocoupler

or this:

http://electronics.stackexchange.com/question...measuring-voltage-while-maintaining-isolation
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Topic summary

✨ The discussion centers on understanding how a voltage transducer converts 110V AC into a 4-20mA current signal and the sensing mechanism involved. Voltage transducers are devices that convert electrical energy from one form to another, often used to monitor voltage by producing a proportional current output. One example is the IME TM3UL20 voltage transducer, which senses 110V AC and outputs a 4-20mA signal; however, calibration issues such as inability to adjust zero and span potentiometers can cause output errors (e.g., outputting 21-22mA at nominal voltage). A practical sensing approach involves using a ferrite rod coil with hundreds of turns of fine wire to detect magnetic fluctuations from the AC line, amplified by a transistor circuit to generate a 4-20mA current loop. This method requires additional circuitry for calibration and temperature stabilization and a fixed coil orientation for consistent readings. Alternative measurement methods include voltage dividers and isolation techniques (e.g., optocouplers) to safely interface microcontrollers with high-voltage lines. The LV25-P transducer, based on Hall-effect sensing, is mentioned as a current transducer that can be adapted for voltage measurement by using an external resistor to convert voltage to current. Overall, detailed circuit schematics are scarce, and manufacturers often do not disclose internal designs, making reverse engineering or patent research necessary for in-depth understanding.
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FAQ

TL;DR: To turn 110 V AC into a 4–20 mA signal, you first sense voltage (e.g., Hall-effect core), then scale and condition it; “Voltage is measured by passing a current proportional to the measured voltage.” A ferrite pickup may use 300–400 turns. [Elektroda, Steve Lawson, post #21665859] Why it matters: This FAQ helps technicians pick, wire, and troubleshoot voltage-to-current transducers for controls and SCADA.

Quick Facts

Typical symptom: aged units can output 21–22 mA at 110 V even with zero/span at minimum (overrange/fault). [Elektroda, ARJ, post #21665866]
Hall-effect voltage transducers use an external resistor to set measured current and provide galvanic isolation. [Elektroda, Steve Lawson, post #21665859]
For proof-of-concept magnetic pickup, wind approx. 300–400 turns of 36 AWG on a ferrite rod, then amplify. [Elektroda, Shrikant Kamble, post #21665863]
Stable readings require mechanical fixation and temperature-stable, calibrated support circuitry. [Elektroda, Steve Lawson, post #21665864]
Keep MCU ground isolated from mains-referenced nodes; use opto/isolation when not battery-powered. [Elektroda, Steve Lawson, post #21665869]

What is a voltage transducer, in plain terms?

A voltage transducer converts electrical energy in the form of voltage into another signal form for measurement or control. In practice, it can transform sensed voltage into a proportional current, such as 4–20 mA, for robust transmission to controllers. The word “transducer” covers many devices that convert one energy form to another, including electrical and biological examples. Pick the specific type to match your range, isolation, and accuracy needs. [Elektroda, Chuck Sydlo, post #21665860]

How does a Hall‑effect voltage transducer work?

You pass a current proportional to the measured voltage through an internal conductor; a Hall sensor reads the magnetic field from that current. The device outputs a signal proportional to the original voltage, with galvanic isolation set by the core and insulation. An external resistor sets the sensing current and therefore the measurement range. As one expert put it, “Voltage is measured by passing a current proportional to the measured voltage.” [Elektroda, Steve Lawson, post #21665859]

How do I convert 110 V AC to a 4–20 mA loop signal?

Sense the voltage with an isolated transducer, scale it with an external resistor, then condition and map the reading to 4–20 mA. The loop current then represents your monitored AC level for PLC/SCADA. Conceptually, you measure voltage by creating a proportional current, then translate that into the standardized loop range for transmission. This approach keeps noise low and allows long cable runs to control rooms. [Elektroda, Steve Lawson, post #21665859]

My IME TM3UL20 now reads 21–22 mA at 110 V. What does that mean?

That behavior indicates an overrange or drift fault. The unit worked for 1–2 years, then produced 21–22 mA even with zero and span at minimum. This points to internal drift, component aging, or a failure that calibration can’t correct. Treat it as a miscalibrated or failing transmitter until inspected or repaired. This symptom is a practical field indicator of fault. [Elektroda, ARJ, post #21665866]

Zero and span won’t bring it back. What should I check next?

Assume the device is out of calibration. Open a non‑warranty unit for inspection if allowed. Look for overheated or drifted resistors and damaged parts, then consider reverse‑engineering the signal path to identify failure points. If you lack documentation, note board markings and search key components. “Sounds like it has gone out of calibration,” which adjustment alone may not fix. [Elektroda, Steve Lawson, post #21665867]

Can I sense mains with a ferrite rod and coil as a quick test?

Yes, as a proof‑of‑concept magnetic pickup. Wind about 300–400 turns of 36‑gauge wire on a ferrite rod, amplify the induced voltage with a transistor stage, and load the collector with a low‑impedance element to obtain 4–20 mA. This demonstrates magnetic coupling to the AC line’s field, not a calibrated measurement. Expect weak signals and the need for gain. [Elektroda, Shrikant Kamble, post #21665863]

What pitfalls exist with the ferrite‑rod method?

You must mechanically fix the rod near the conductor and control orientation for repeatability. Add temperature compensation and a calibrated conditioning chain, or readings drift. Without these, your output varies with placement and ambient changes. As one expert cautioned, the real build is “more complicated” than a simple coil‑plus‑transistor sketch suggests. [Elektroda, Steve Lawson, post #21665864]

Do I need to share ground between my MCU and the measured circuit?

Only if your MCU is battery‑powered and safe from hazardous voltages. Otherwise, keep grounds isolated to protect the MCU and users. Use opto‑isolators or isolated transducers to bridge measurement across different ground potentials. This avoids ground loops and shock risks when monitoring higher voltages. [Elektroda, Steve Lawson, post #21665869]

How can I measure voltage while maintaining isolation to my microcontroller?

Use an isolation strategy at the sensing interface. Options include an isolated voltage transducer feeding your MCU through analog scaling, or an optocoupler‑based front end that transfers a proportional signal across a safe barrier. Isolation is essential when the MCU is not floating on a battery supply. [Elektroda, Steve Lawson, post #21665869]

What is a 4–20 mA current loop, and why use it for voltage monitoring?

A 4–20 mA loop encodes the measured variable as loop current for noise‑immune transmission. You convert sensed voltage into loop current so long cables and EMI don’t corrupt readings. The scheme supports a live zero at 4 mA and scales to 20 mA for full‑scale. It integrates easily with PLC analog inputs via a loop receiver. [Elektroda, Steve Lawson, post #21665859]

Which components most often fail when a transducer drifts high?

Look for stressed resistors in the scaling network, aged electrolytics, or damaged semiconductors in the conditioning stage. Visual signs include discoloration or cracking. Replace suspect parts and recalibrate. If the device still pegs high, the sensing element or reference may be compromised and require replacement. [Elektroda, Steve Lawson, post #21665867]

Can you give me a simple 3‑step check for a misreading unit?

Verify applied voltage and wiring, then set zero/span to midscale.
Measure loop current with a trusted meter and note drift.
Inspect internally for overheated parts; repair and recalibrate, or replace. [Elektroda, Steve Lawson, post #21665867]

What is the LEM LV 25‑P that was mentioned?

It’s a Hall‑effect voltage transducer that senses voltage by driving a proportional current and reading the resulting magnetic field. An external resistor sets the measurement range, and the design provides galvanic isolation between primary and secondary circuits. It’s widely used for isolated voltage monitoring. [Elektroda, Steve Lawson, post #21665859]

What is the LEM HLSR‑P current transducer used by another poster?

HLSR‑P is a closed‑loop Hall‑effect current sensor family with galvanic isolation and an analog output proportional to primary current. It’s suitable for MCU interfacing and keeps measurement isolated from power circuits. Use it when you need accurate current feedback without sharing grounds. [LEM, HLSR‑P SP33 Datasheet]

Is there a quick way to prototype a voltage‑to‑current transmitter?

Pair an isolated voltage transducer with a loop‑powered 4–20 mA stage. Sense voltage via a Hall‑effect module, scale to a control voltage, and drive a 4–20 mA loop element. This splits isolation and loop generation, easing calibration and safety. [Elektroda, Steve Lawson, post #21665859]

Who is this FAQ for, and what problem does it solve?

For technicians and engineers who must monitor 110 V AC and feed PLC/SCADA via 4–20 mA. It explains sensing options, isolation practices, calibration pitfalls, and fault symptoms like persistent 21–22 mA overrange. Use it to choose, wire, and fix voltage transducers confidently. [Elektroda, ARJ, post #21665866]

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