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Series / Parallel connections of Proximity Switches

The series or parallel connection of proximity switches enables the creation of "hard-wired logic circuits". The latter are particularly useful in applications where logic controllers are not available and yet some simple logic requirements have to be met. Even where logic controllers are available, in large factories, cabling requirements can be greatly reduced by interconnecting switches in a localised area into a logic.

Two wire and three wire switches may be either connected in series or in parallel. They can thus be inter-connected in four ways, each of which is discussed below.

Series connection of two wire switches

A series connection enables an "AND" logic i.e. only when all the switches connected in series are simultaneously damped is the load switched.




This combination is not recommended. Where it seems unavoidable we recommend the connection of only two switches in series. It should however be explored whether the desired logic can be realised by an equivalent connection of switches in parallel eg. A series connection of two Normally Open switches can be replaced by an equivalent connection of two Normally Closed switches in parallel, a combination that poses no problems.

Where the series connection is nonetheless preferred it should be remembered that there is a voltage drop of 5-12 volts across each switch. The voltage available to switch the load is correspondingly reduced. For example when connecting two AC switches in series in an 110VAC application, the voltage available to switch the load (a contactor coil, say) is only 86V (110 - 2x12). Whether this is adequate to energise the load must be determined first.

 

Parallel connection of two wire switches

This is as illustrated below. A parallel connection enables an "OR" logic i.e. if atleast one of several switches connected in parallel is damped, the load is switched.



This connection poses no problem barring one limiting factor: the sum of all quiescent currents flowing through the load may cause a faulty signal where the load is a PLC. Under ideal conditions, however, upto 10 switches may be connected in parallel. It is however advised that only one of these switches remain ON at any given moment. This switch will remove the operating voltage from the other switches connected in parallel; if a second switch now has a target in front this will result in a brief load drop-out when the first switch turns OFF (when this happens the supply voltage becomes available to the second switch which turns ON after a small delay).

 

Series connection of three wire switches

This is as illustrated below.



This connection poses no problem provided the following points are considered:

  • There is a voltage drop of 1 to 2.5V across each switch. The voltage available to switch a load is correspondingly reduced. For example in a 24V DC application where three switches are connected in series only 16.5V (24 - 3x2.5) are available to the load. The adequacy of this must be considered.

  • The first switch must be capable of switching not only the desired load current but also the currents consumed by all the other switches.

  • From the above figure it can be seen that the operating voltage of switch 3 is supplied by switch 2 and that of switch 2 by switch 1. After switch 1 turns ON and thereby supplies power to switch 2, the latter turns ON after a small delay (the Power-On delay which is typical for a proximity switch); likewise switch 3 will turn ON with another delay. At the end of the chain these delays can add up to a sizable figure (a few hundred milliseconds); the load is switched ON after this figure.

Provided the above are considered upto 10 switches can be connected in series.

 
Parallel connection of three wire switches

This is as illustrated below. Please note the addition of isolating diodes; not incorporating these diodes will cause the LEDs of all the switches to light up when any one switch is damped.


This connection basically poses no problem. It should however be considered that the quiescent currents of all undamped switches also flowing through the load should not cause a faulty signal say where the load is a PLC. Where this is no problem upto 30 switches can safely be connected in parallel.