Linear position sensor with 4-20 mA output for PLC: how to choose it

Technical guide · GUEMISA

Why 4…20 mA and not a voltage

Distance

A current signal is independent of the voltage drop in the cable: the transmitter forces the same current through the entire loop, whether the cable is 5 m or 200 m long. With a voltage signal of 0–10 V, the cable resistance and the PLC input impedance form a voltage divider that introduces error. As an order of magnitude, 100 m of 0.5 mm² cable adds up to about 7 Ω of round-trip resistance, which is negligible compared to a 250 Ω load.

Electrical noise

The power input has low impedance, so voltages induced by drives, contactors, or motors generate very small currents and have less of an impact on the measurement than with a high-impedance power input. Even so, a shielded cable with the braid connected according to the manufacturer's recommendations and kept separate from power cables is recommended.

Cable break detection

The starting point for the scale is 4 mA, not 0 mA («live zero»). If the cable is cut, the current drops to 0 mA, an off-scale value that the PLC may interpret as a fault. With a 0–10 V range, a broken cable and a position at the starting point both give 0 V and are indistinguishable.

Some transducers on the chips add their own error signal. The TF1 outputs less than 3.5 mA with a broken wire. The TM1 outputs 24 mA or more if the marker is out of range, and 3.5 mA or less if the GND wire is broken.

Two wires, three wires and maximum load

Two and three threads

In a two-wire loop sensor, the same conductors power the sensor and carry the signal; therefore, the signal never drops below 4 mA, as this current powers the electronics. In a three-wire loop, power, ground, and signal are carried on separate conductors. The Novotechnik transducers with a 4–20 mA output mentioned in this article have independent power supplies (24 V, or 8–32 V in the TM1) and do not function as a two-wire loop. If your installation only has a two-wire loop, this must be verified before selecting a sensor.

Maximum load

The load is the sum of the PLC's input resistance (for example, 250 Ω on a card with a shunt resistor) and the cable resistance. The connectors clearly indicate the maximum: no more than 500 Ω for TH1, TP1, TF1, LS1, FTI, and MUW. For TM1, the maximum is 500 Ω with a 24 V power supply and 250 Ω with a 12 V supply. With a 250 Ω input and a 20 Ω cable, the load is 270 Ω: it meets the 500 Ω limit, but not the 250 Ω limit for TM1 powered at 12 V.

Alternatives: 0…10 V and ratiometric output

• 0–10 V: suitable for short cables within the cabinet. Many transducers offer both outputs, such as the TE1, LS1, TF1, and TM1. The connectors require a minimum load of 5 kΩ or 10 kΩ on the voltage output.
• Ratiometric: the output is a percentage of the supply voltage (e.g., 10–90%). This is used when the same 5V supply powers both the sensor and the converter's reference voltage, and is common in vehicles. It is not suitable if the supply voltage varies and the converter has a different reference voltage.
• Digital (SSI, CANopen, IO-Link): The TH1 and TP1 are also offered with these interfaces, on their own separate cards. They avoid analog conversion, but require a compatible input module.

Integrated output or potentiometer with conditioner

A potentiometric transducer is a voltage divider: it delivers a voltage proportional to its position and requires a signal conditioner to convert it into a normalized signal. A transducer with an integrated output has the electronics inside the housing.

When is each option useful?

• Integrated output: a single component, less wiring, and a single cable to the PLC. This is common in long-running or high-pressure environments, such as magnetostrictive circuits.
• Potentiometer with conditioner: allows you to choose between very different stroke transducers and formats (LWH, TLH, or TEX) and change the output (0…10 V, ±10 V, 0…20 mA, 4…20 mA) without changing the sensor. The conditioner supplies the potentiometer with a stable reference and detects the cursor with almost no load.

Novotechnik Conditioners

• MUW-200/250: fits inside the DIN 43650 connector housing of LWH and TLH transducers. Output: 0…10 V, 0…20 mA or 4…20 mA; power supply: 18…30 VDC; IP65. The 200 series is fixed range; the 250 series is adjustable.
• MUK-350: IP66 die-cast aluminum housing, output 0…10 V, ±10 V, 0…20 mA or 4…20 mA, with adjustable zero and range, and 18…30 VDC power supply.
• MUP-080: DIN rail, output 0…10 V or 4…20 mA, load up to 500 Ω, without galvanic isolation.
• MUP-100/150 and MUP-110/160: DIN rail; the MUP-150 and MUP-160 models add galvanic isolation between power and output.
• MUP-410: DIN rail, microprocessor controlled, output 4…20 mA, 0…10 V or ±10 V.

Table of options with actual products

Which GUEMISA products are a good fit?

To see more models, check the category Linear transducers and the category Resistance converters and potentiometers.

Documentation and related cases

• Potentiometers: Technical Definitions by Novotechnik (PDF, English)
• Novotechnik sensors with IO-Link interface (PDF, English)
• Position and displacement sensors (report, PDF)
• All technical information about GUEMISA

Frequently Asked Questions

How do you convert from 4…20 mA to millimeters in the PLC?

It scales linearly: position = stroke × (I − 4 mA) / 16 mA. At an input with a resistance of 250 Ω, the board sees from 1 V to 5 V, and the scaling is configured with those values.

What does a reading below 4 mA mean?

This should not occur in a healthy sensor. It indicates a broken cable, power failure, or sensor error. Some transducers, such as the TF1 and TM1, explicitly indicate this with a current below 3.5 mA.

Can I use a 4…20 mA transducer with a 12 V power supply?

Only if the connector allows it. The TM1 accepts 8…32 VDC, but the maximum load drops to 250 Ω at 12 V. The others in the table are powered by 16 to 32 V, depending on the model.

What cable length does a 4…20 mA output support?

The signal itself supports long distances because the current is independent of length. The limit is determined by the total load, which must not exceed the maximum rating of the connector, and by noise. Shielded cable, kept separate from power cables, is recommended.

When is a potentiometer with a conditioner advisable?

When you need a type of stroke or mounting that doesn't exist with an integrated output, or when you want to choose the output signal later. With LWH or TLH, the MUW fits in the connector box and leaves the signal at 4…20 mA next to the sensor.