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Connection and Circuit Technology


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Connection and Circuit Technology

Types of connection

Two-wire DC and AC technology

Proximity sensors in two-wire technology have only two connecting wires. They are connected in series to the load to be switched and thus receive their supply voltage via the load. This has the effect of a certain amount of residual current flowing via the load even if the output is closed, and that of a voltage drop over the proximity sensors in the switched through status.

Proximity sensors are designed with either 'normally closed' contacts (N/C) or 'normally open' (N/O) contacts, but designs are also available which incorporate the two functions.

Fig. 1 Connection diagrams for two-wire technology (DC, AC and DC/AC [universal current] designs) V = Operating voltage,
L = Load

A potential protective grounding terminal is identified by green-yellow.

In the case of designs for AC or AC/DC (universal current), the connection cables may be identified in any colour other than green-yellow. Generally, however brown or blue is selected, as for direct current designs.

Voltage supply e.g.: 15 V to 250 V DC

20 V to 250 V DC

In the case of two-wire sensors it should be noted that in the unactuated status, a residual current must flow to provide a current supply for the proximity sensor. The residual current also flows via the load. In the acknowledged status, a minimum load current must flow to guarantee the reliable operation of the proximity sensor.

Three-wire DC technology

Proximity sensors in three-wire technology have three connecting wires. As a rule, the colours of the connecting wires comply with European standard EN 50 044. Two wires are for the purpose of voltage supply (brown +, blue -). The third wire (black) represents the signal output of the proximity sensor.

Fig. 2 Connection diagrams for three-wire technology (DC), L = load

Four-wire DC technology

Proximity sensors designed in four-wire technology are further divided into proximity sensors with PNP outputs (positive switching) and NPN outputs (negative switching). Unlike proximity sensors in three-wire technology, proximity sensors in four-wire technology are equipped with antivalent switching function, i.e. they possess both a normally open as well as a normally closed output.

Fig. 3 Connection diagram for four-wire technology (DC), L = load

Terminal designation




Positive supply voltage (+)



Negative supply voltage (-)



Switch output



Antivalent switch output



Terminal designation is in accordance with European standard EN 50044. The colour short code is laid down in the international standard IEC 757.

Positive and negative switching outputs

Generally, two proximity sensor designs are distinguished. PNP (positive switching) and NPN (negative switching). Other designation are P-switching or positive switching as well as N-switching or negative switching. Positive switching proximity sensors usually have a PNP transistor output. However, positive switching proximity sensors with an NPN transistor output are also possible. The designations PNP and NPN output are nevertheless widely used.


In the case of direct current proximity sensors with PNP output, the output is connected to positive potential in the switched state. This means that is a load is connected (display, relay, ), one connection must be connected to the proximity sensor output and the other connection to 0V.

Fig. 4 PNP output

(The purpose of the diodes is to provide a protective circuit)
L = load)

PNP-proximity sensors can be differentiated as being 'normally closed' or 'normally open'.

Fig. 5 PNP normally open contact (L = load)

Fig. 6 PNP normally closed contact (L = load)

NPN - output

In the case of proximity sensors with NPN-output, the output is connected to the negative potential in the switched state. This means that if a load is connected (display, relay, ), one connection is connected to the proximity sensors output and the other connection to the positive potential.

Fig. 7 NPN output

(The purpose of the diodes is to provide a protective circuit)
L = load

In the same way, one differentiated between 'normally closed' and 'normally open' with NPN proximity sensors.

Fig. 8 NPN normally open contact (L = load)

Fig. 9 NPN normally closed contact (L = load)

Circuit technology

Usually, logic operations of the proximity sensors are carried out by the controller. By means of series or parallel connections it is possible to achieve the logic operation of several sensors.

Parallel and series connection of proximity sensors

With parallel connection, it is possible to effect a logic (Boolean) OR-connection and with series connection, a logic AND-connection.

The advantages of this type of connections are:

Logic operations can be achieved without using an electrical controller.

With the use of electrical controllers, logic operations can be carried out immediately on the spot so that only the logic operation result is signalled to the controller using a minimum amount of cabling.

The disadvantages are:

The design and construction of logic operations required experience, as the mutual influences of proximity sensors, increases response and drop-off times and a limit in the number of proximity sensors connected must be taken into account.

Maintenance becomes more difficult.

If however an electrical controller is used for signal processing, then it is more straightforward to carry out all logic operations in the controller.

Parallel connection of proximity sensors using two-wire technology

With parallel connection of proximity sensors in two-wire technology, the following points must be observed:

Because the sum of all possible quiescent currents of parallel connected proximity sensors flows via the load in the unswitched status, steps must be taken to ensure that this does not lead to a malfunction of controller connected downstream.

If a proximity sensor has switched through, then it 'withdraws' the supply voltage from the other parallel connected proximity sensors. This has the effect, that the remaining proximity sensors can no longer indicate their actual switching status. If the first proximity sensor now returns to it's unswitched status, then a second already activated proximity sensor can only indicate it's switching status correctly after the ready delay time of the actual proximity sensor. This can lead to incorrect signals.

Parallel connection is not possible with NAMUR-technology.

Fig. 10 Parallel connection in two-wire technology (L = load)

Parallel connection of proximity sensors using three-wire technology

Parallel connection of proximity sensors in three-wire technology can be achieved without any problems. The following points must be observed:

In the unswitched status, the low residual currents of the parallel connected proximity sensors accumulate (simultaneous use of mechanical contacts and proximity sensors is possible).

If proximity sensors with an output stage in the form of an open-collector circuit are used, then there is no mutual effect. In the case of proximity sensors with different switch outputs, decoupling diodes are necessary (see Fig. 11). The diodes are usually integrated in the sensor for the purpose of reverse polarity protection.

Fig. 11 Parallel connection in three-wire technology (DC), L = load

Direct current three-wire proximity sensors can be parallel connected without major limitations, if the residual currents of the signal outputs are sufficiently small in the non-switched status. This is the case with most proximity sensors so that for instance up to 20 or 30 proximity sensors can be parallel connected. Also, a combination of proximity sensors and mechanical switches is possible. The decoupling diodes illustrated in the sketch are provided in order to prevent the activated sensors from being loaded with the output operating resistances of other parallel connected sensors. Moreover, this avoids all LED's illuminating in the case of sensors with LED displays. If the diodes are an integral part of the sensors protection circuitry, no additional external diodes are necessary.

Parallel connection of AC sensors is not recommended, as malfunction can occur during oscillator start-up.

Series connection of proximity sensors using two-wire technology

As a rule, series connection of proximity sensors using two-wire technology is to be avoided. If it is unavoidable, the following points must be observed.

The supply voltage is distributed to each series connected sensor. If identical proximity sensors are used, the following applies in respect of the voltage for each proximity sensor (in activated status):

V Supply voltage

V Proximity sensor = n

(n = Number of proximity sensors)

In the switched through status, a voltage drop occurs through each proximity sensors (approximately 0.7 2.5 V per sensor). When calculating the load, it should be taken into account that the voltage through the load is full supply voltage reduced by the individual voltage drops through the in series connected proximity sensors.

Fig. 12 Series connection in two-wire technology (L = load)

Series connection of proximity sensors using three-wire technology

Series connection of proximity sensors using three-wire technology is possible, as shown in Fig. 13, where by the following points must be observed:

Series connection of the individual series connected proximity sensors are loaded additionally: Added to the current consumed by the load is the current consumption of each individual proximity sensor connected in series.

In the switched through status, a voltage drop occurs with each proximity sensor (approximately 0.7 2.5V per sensor). As a result of this, the supply voltage available for the load is reduced by the sum total of the individual voltage drops.

As in the case of series connected three-wire sensors, it is always the supply voltage of the proximity sensor connected downstream which is switched, the actual time delay before availability must be taken into account. If a 'detection process' falls within the period of proximity sensors with operating status display (LED, ), correct indication of the operating status cannot be guaranteed.

The decoupling diodes may be included in the sensor circuitry

If the sensors include power supply bypassing capacitors and if they have a short circuit protection, series connection of the sensors may lead to the following problem:

If the preceding sensors switched through, the short circuit protection of this sensors will become effective due to the high dynamic load of the capacitor in the succeeding sensor. As a result, the preceding sensor is not able to supply the succeeding sensor, which in turn, is not able to switch through.

Fig. 13 Series connection in three-wire technology (L = Load)

Connection technology under conditions of strong electro-magnetic influence

As far as connection is concerned, it should be ensured that proximity sensor cables are installed separately from supply lines to motors, switching valves etc.

If proximity sensor connection cables run over long distances in cable ducting or cable trays parallel to other cables which conduct alternating currents or strong current pulses, this can lead to interference with the proximity sensor via the connection cable.

If the proximity sensors are used in areas of high interference (welding equipment, motors, magnetic couplings, ), the following steps are to be taken:

Keep the connection cables of proximity sensors short

Screen the sensors connection cables

If possible, error signal to be limited at source

Install interference voltage filter into the voltage supply

Connection of controller, relay and display elements

If the output of a proximity sensor is loaded as a result of a downstream connected device, the following must be observed:

Current consumption of the connected load should not exceed the permissible load current of the proximity sensor. Typical values for proximity sensor load currents range between 50 500 mA.

In order to guarantee reliable operation of the proximity sensors in the switched state, the resistance of the connected load should not be too high such as to impair the flow of the minimum load current.

Proximity sensors can emit irregular switching signals if the supply voltage is switched on or off, depending on whether the proximity sensor is attenuated or unattenuated. These stray pulses can lead to malfunctions in controllers downstream and must therefore be suppressed by using additional hardware or taken into account in the software programming of the controller.

If lamps are used by way of display elements it should be noted that the switch-on current of lamps with a cold spiral-wound filament is considerably higher than the nominal current. It is therefore possible for the switch-on current to be reduced as a result of preheating the spiral-wound filament by means of a by-pass resistor which is connected in parallel to the proximity sensor.

If a relay (a valve or an other high-inductance device) is to be actuated by proximity sensors, they should be checked for built-in protection against voltage peaks. If not, additional protection diode circuitry is to be provided.

Required current supply

When switching on and off power supply units, care should be taken to ensure that there are no voltage peaks which may jeopardise the function of the connected proximity sensors. Power supply units with insufficient electronic control can create voltage spikes during switch-on, which can be above the permissible voltage supply of the proximity sensor and which, depending on the time constant, fade away relatively slowly. In the case of unfamiliar power supply units, it is recommended to check the voltage switch-on behaviour by means of storage oscilloscope.

Depending on the specification given in the data sheets for proximity sensors, the supply voltage ripple must not exceed a certain limit value.

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