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Posted By Topic: Pressure Switch System

Dec 20 2017 23:19

Hi team,

I am building a pressure switch control system for a Vacuum pump.

It is designed to disengage a contractor to a 7.5kw 3phase load.

The problem I am facing is the nature of the Pressure switch.

It is designed to be used in a PLC interface, however the secondary output is an analogue 1-5VDC voltage.

I have acquired a 5VDC relay to switch the 24VDC coil on the contactor but I do not know how to switch the coil on this relay without the negative polarity for the coil.

Is there a less costly method than just purchasing a din-rail mounted 5VDC power supply and using the negative voltage to match the pressure switches positive 5VDC output?



Dec 21 2017 11:06

A USB or old phone charger may work but from your vague description it may not even be required. Are you a sparky or have you spoken with one?

Dec 21 2017 19:10

Hi jasont,

I suggest you get an electrical person involved.

If the output from the pressure switch is 1 to 5Vdc analog then it is not an On/Off signal (e.g. scale is 0-100psi so 0psi=1Vdc and at 100psi=5Vdc and usually linearly proportional inbetween).

The analog output will have a negative rail it is the negative rail of the (usually) 24Vdc supply to the presure switch.


Dec 21 2017 20:21

Are you sure that you are not dealing with a 4-20mA pressure transducer? either way a you will require a device with an analogue interface to process the signal.

Dec 21 2017 21:59

how about make and model of pressure sensor?

Dec 31 2017 11:18

What are you actually trying to do - pull the contactor ON when the pressure (or vacuum) reaches a defined point or turn the contactor OFF at that point?

Why not get one of those adjustable (mechanical) pressure switches with a two-way switch - they'll carry 10A+ and cost <$50.