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FLOW SENSORS: CONNECTING FLOW SENSORS TO PROGRAMMABLE LOGIC CONTROLLERS
There are many applications in which it is desirable to connect one or more flow sensors to a PLC (programmable logic controller), and the flow sensors are well suited for this.  In many cases the digital pulse of a flow sensor can be connected directly, or with a minimum of components.

The flow sensors most commonly chosen for use with a PLC are EX series and TX series insertion flow sensors.  These flow sensors, which have three wires, use Hall-effect technology.  They need 5-24 Volts DC and up to 8 mA current, and they are current sinking (NPN) devices.  The flow sensors can connect directly to the PLC if:

  • The flow sensor power on the PLC is 5 - 24 VDC (VDC is typical.)
  • The flow sensor power supply can provide at least 6 mA (100 mA is typical.)
  • The flow sensor input on the PLC can accept a current sinking device.

flow sensors
Flow sensor connection diagram.

If the PLC input only accepts current sourcing devices, a pull-up resistor must be added to allow the flow sensor to work correctly.  Connection of this resistor is shown below.  Typically, on a 24 VDC input a 2.2 K Ohm resistor will be effective.

flow sensors Since the three-wire, Hall-effect flow sensors are solid state, they do not exhibit switch bounce and can be used at relatively high frequencies.  Any input module will have certain frequency limitations, and the table on the right gives maximum frequency that a particular flow sensor or meter could be expected to produce.

If the PLC is equipped with a 4-20 mA analog input module, it is necessary to order the flow sensor with some form of 4-20 mA transmitter.  Two options are the AO55 blind transmitter and FT420 flow sensor computer.  Follow the connection diagrams for these products to connect the flow sensor output to the analog input.

flow sensors connection diagram

Please visit our flow sensor pages for more information on these products.
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