turck Articles

Sensor Characteristics

TURCK 8/1/2018

Maximum Switching Frequency

Minimum parameters for measuring at maximum switching frequency are shown in Figure 8. Using a smaller target or space may result in a reduction of a specific sensor’s maximum switching frequency and decrease sensor to target air gap tolerance. See page M7 for determining dimension "A" of standard target.

TURCK Sensor Characteristics

Weld Field Immunity

Many critical applications for proximity sensors involve their use in weld field environments. AC and DC resistance welders used in assembly equipment and other construction machines often require in excess of 20 kA to perform their weld function. Magnetic fields generated by these currents can cause false outputs in standard sensors.

TURCK has pioneered the design and development of inductive proximity sensors that not only survive such environments, but remain fully operative in them.

The limit of the weld field immunity depends on the kind of field (AC or DC), the housing size of the sensor and its location in the field. For example, in an AC or DC weld field, the "/S34" inductive sensors can be positioned one inch from a 20 kA current carrying bus. See Section H for a list of weld field immune sensors.

Reference values for magnetic induction:
Distance [mm]
I [kA] 12.5 25 50 100
80 mT
160 mT
320 mT
800 mT
1600 mT
40 mT
80 mT
160 mT
400 mT
800 mT
20 mT
40 mT
80 mT
200 mT
400 mT
10 mT
20 mT
40 mT
100 mT
200 mT

Gauss = 10 x mT


TURCK inductive proximity sensors are manufactured with a shielded coil, designated by "Bi" in the part number, and a nonshielded coil, designated by "Ni" in the part number. Embeddable (shielded) units may be safely flush-mounted in metal. Nonembeddable (nonshielded) units require a metal free area around the sensing face. Because of possible interference of the electromagnetic fields generated by the oscillators, minimum spacing is required between adjacent or opposing sensors.

It is good engineering practice to mount sensors horizontally or with the sensing face looking down. Avoid sensors that look up wherever possible, especially if metal filings and chips are present.

Maximum Locknut Torque Specifications

The locknut torque should be considered for all threaded sensors to prevent the housing from being over stressed. The values below pertain to the locknut provided with each sensor. Liquid thread sealants of an anaerobic base, such as Loctite, are recommended if strong vibrations are likely.

Caution: Sensor barrels are typically brass. Consider break torque when selecting grade of thread sealant.
Barrel Size Metal Barrel Plastic Barrel
5 mm
8 mm
12 mm
18 mm
30 mm
47 mm
5 Nm (3.7 ft-lb)
10 Nm (7.4 ft-lb)
10 Nm (11 ft-lb)
25 Nm (18 ft-lb)
90 Nm (66 ft-lb)
90 Nm (66 ft-lb)
1 Nm (0.7 ft-lb)
2 Nm (1.4 ft-lb)
5 Nm (3.7 ft-lb)
Drill Hole Sizes for Metric Threads
Thread Size Pitch Thru Hole (mm) Tap Hole Dia. (mm) Thru Hole (in) Tap Hole Dia. (in)
M5 x 0.5 0.5 5 4.5 13/64 5/32
M8 x 1 1 8 7 21/64 1/4
M12 x 1 1 12 11 31/64 13/32
M18 x 1 1 18 17 23/32 41/64
M30 x 1.5 1.5 30 28 1-3/16 1-5/64
PG 9 1.41 15.2 14 5/8 1/2
PG 13.5 1.41 20.4 19 13/16 23/32
PG 36 1.59 47 45.5 1-7/8 1-47/64
DC Outputs

Two-, three-, or four-wire proximity sensors contain a transistor oscillator and a snap-action amplifier. This provides exceedingly high accuracy to a set switching point, even with very slowly approaching targets. Switching characteristics are unaffected by supply voltage fluctuations within the specified limits.

The sensors can drive electromechanical relays, counters, solenoids, or electronic modules, and interface directly with logic systems or programmable controllers without additional interface circuitry. They are available with either NPN output transistors (current sinking) or PNP output transistors (current sourcing).

Load current ratings vary from 100 mA to 200 mA depending on physical size. Standard voltage range is 10-30 VDC with certain types available for 10-65 VDC. All models incorporate wire-break, transient and reverse polarity protection. Power-On false pulse suppression is also standard.

Short-Circuit and Overload Protection
TURCK DC sensors with a Voltage Range designation of "4", "6" or "8" in the part number are short-circuit and overload protected (automatic reset). These sensors incorporate a specially designed circuit which continuously monitors the ON state output current for a short-circuit or overload condition. If either of these fault conditions occurs, the output is turned OFF and pulse tested until the fault is removed. This added protection causes a ≤1.8 V drop across the output in the normal ON state. This may be a problem when interfacing with some logic low inputs (see TTL compatibility).

TTL Compatibility

Some solid-state loads requiring NPN (sinking) input signals need a ≤0.8 V signal to reliably turn ON. The output of these sensors will have a voltage drop of ≤0.7 V (0.3 V typical), which will ensure reliable operation. Do not use voltage ranges "4" and '6" when TTL compatibility is required. Contact the factory for a list of part numbers with this specification.

TURCK Sensor Characteristics
Voltage drop is measured from output wire black (BK) to ground wire blue (BU).

DC Outputs

"LIU" 4-Wire Linear Analog DC Output

TURCK Sensor Characteristics
Linear Analog Output; Current and Voltage

Series/Parallel Connection

Logic functions with DC proximity sensors:

Self-contained proximity sensors can be wired in series or parallel to perform such logic functions as AND, OR, NAND, NOR. The wiring diagrams show the hook-up of four sensors with NPN and PNP outputs. Take into account the accumulated no-load current and voltage drop per sensor added in the series string.


N.O. sensors: AND Function (target present, all sensors: load "on")
N.C. sensors: NOR Function (target present, any sensor: load "off")


N.O. sensors: OR Function (target present, any sensor: load "on")
N.C. sensors: NAND Function (target present, all sensors: load "off")


• To prevent the load from seeing the cumulative voltage drop of multiple 3-wire sensors in series, alternating polarity sensors can be used provided that the desired polarity is at the load.
• Wiring 3-wire sensors in series delays the load by the accumulated "time delay before availability" of all sensors in the string.

Short-Circuit and Overload Protection

TURCK AC sensors with the Voltage Range designation "30", "32" or "40" are short-circuit and overload protected (manual reset). These sensors incorporate a specially designed circuit which continuously monitors the ON state output current for a short-circuit or overload condition. If either of these fault conditions occurs, the output is latched OFF until the power has been cycled OFF and ON again.

Always select short-circuit and overload protected sensors whenever possible.

TURCK Sensor Characteristics

AC and AC/DC Outputs

TURCK Sensor Characteristics
SCP = Short-circuit Protected

These sensors are used as pilot devices for AC-operated loads such as relays, contactors, solenoids, etc. The solid-state output permits use of the sensors directly on the line in series with an appropriate load. They, therefore, replace mechanical limit switches without alteration of circuitry, where operating speed or environmental conditions require the application of solid-state sensors.

These sensors are typically available in a voltage range of 20-250 VAC. All models are available with either normally open (N.O.), normally closed (N.C.) or programmable outputs (from N.O. to N.C.). Careful consideration must be given to the voltage drop across AC/DC sensors when used at 24 VDC.

TURCK Sensor Characteristics

Since the sensors are connected in series with the load by means of only two leads, an off-state current flows through the load in the magnitude of approximately 1.7 mA.

This, however, does not affect the proper and reliable performance of most AC loads. Another characteristic of solid state sensors isa5 to7 volt drop developed across the sensor in the ON state.

All models contain a snubber network to protect against transients from inductive loads, which can cause false triggering.

Series Connection

TURCK Sensor Characteristics

The maximum number of sensors to be operated in series depends on the stability of the line voltage and the operating characteristics of the load in question. The supply voltage minus the accumulative on state voltage drop across the series connection (approximately 7 Vrms per sensor) must be ≥ the minimum required load voltage.

Mechanical Switches in Series

Mechanical switches in series with proximity sensors should always be avoided because they can create an open circuit, leaving the proximity sensor without power. In order to operate properly, a proximity sensor should be powered continuously. A typical problem encountered when the mechanical contact closes while the target is present is a short time delay that is experienced before the load energizes (time delay before availability).
TURCK Sensor Characteristics

A 33 kΩ, 1W by-pass resistor can be added across the mechanical contact to eliminate the time delay before availability. This will allow enough leakage current to keep the sensor ready for instantaneous operation.
TURCK Sensor Characteristics

Parallel Connection

TURCK Sensor Characteristics

Wiring AC proximity sensors in parallel can result in inconsistent operation and should generally be avoided.

On-state voltage drop: With any sensor ON, the voltage across all other sensors is typically 7 Vrms. Since the minimum rated voltage for AC sensors is 20 Vrms, no other sensor with a target present can turn ON until the first sensor turns OFF. This transition is not instantaneous due to the time delay before availability, during which the load may drop out.

Leakage current through the load: This is equal to the total leakage of all sensors wired in parallel. Too much leakage into a solid state load can cause the input to turn ON and not turn OFF. Small relays may not drop out if the leakage current exceeds the relay’s holding current.

Mechanical Switches in Parallel

TURCK Sensor Characteristics

As previously discussed, proximity sensors should be powered continuously to avoid the time delay before availability during power-up.

With mechanical switches in parallel, the sensor is shorted out every time the contact is closed, leaving it without power. If the target is present when the mechanical contact is opened, a small delay will be experienced during which the load may drop out.

TURCK Sensor Characteristics

This delay can be avoided by adding a resistor in series with the mechanical contact. The voltage drop developed across the resistor with the contact closed will be enough to keep the sensor active. Use the formula below to determine the value and wattage.

TURCK Sensor Characteristics

NAMUR (Y0 and Y1) Output

NAMUR sensors are 2-wire sensing devices used with switching amplifiers. Because of the small amount of energy needed to operate NAMUR sensors, they can be used in intrinsically safe applications.

The operation of this sensor is similar to that of a variable resistor with a change in impedance as a target approaches the sensor. When no metal is being sensed, the inductive sensor is in a low impedance state and draws a current of more than 2.2 mA. When a metal target enters the high-frequency field radiated from the sensor head, the impedance increases as the target approaches. When fully damped, the sensor draws less than 1.0 mA. Note: For capacitive and inductive magnet operated sensors, the current change characteristics are opposite.

The current differential from the undamped to the damped (metal present) state is used to trigger an amplifier at a defined switching point. These sensors contain a relatively small number of components, which allows the construction of small devices and also assures a high degree of reliability.

In the undamped and damped state, the devices have fairly low impedance and are therefore, unaffected by most transients. NAMUR sensor circuits operate on direct current. Therefore, cable runs of several sensors may be run parallel to one another without mutual interference.

The NAMUR (Y0 and Y1) sensor behaves like a variable resistor when a target approaches. The impedence increases or decreases between 1 kΩ and8 kΩ.

TURCK Sensor Characteristics

Typical Output Curves

Note: The typical curve of current versus sensing distance with 8.2 V DC supply and 1 kΩ source impedence. All NAMUR (Y0 and Y1) sensors are calibrated to pass through 1.55 mA at nominal sensing range ±10%.

TURCK Sensor Characteristics

Typical Intrinsically Safe Installation

TURCK Sensor Characteristics

For guidance on installation of TURCK intrinsically safe systems, refer to the Instrument Society of America publication ISA-RP12.6-1995, "Wiring Practices for Hazardous (Classified) Locations Instrumentation".

The complete line of Intrinsically Safe and Associated Apparatus is featured in the TURCK "Isolated Barriers and Amplifiers" catalog.

Custom Interface Circuits

TURCK Sensor Characteristics

NAMUR sensors can operate outside the nominal operating values when the sensor is used in a nonhazardous area.

The supply voltage limits are: Vmin = 5 VDC; Vmax = 30 VDC

Within this voltage range the load resistance Ri must be adjusted for the supply voltage. The following table gives typical values:
Vsupply (DC) Ri (k) Isn (mA) I (mA)
If these values are used, the current Isn corresponds to the rated operating distance (Sn) of the sensor. NAMUR sensors are short-circuit protected up to 15 VDC and reverse polarity protected up to 10 VDC.