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Entry type: FAQ Entry ID: 25204813, Entry date: 07/17/2007

How through external switching can you use a 0-10V analog input on a makeshift basis also for 0-20mA?

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The two analog inputs of the S7-224 XP are factory-set for voltage measurement (0..10V). In order to be able to use the inputs as current inputs as well you must incorporate a 500 Ohm resistor between terminals A+ and M (or B+ and M).

Figure 1 shows the connection of a sensor to a voltage source as a 2-conductor connection.

Fig 01

Figure 2 shows the connection of a sensor to a voltage source as a 3-conductor connection.

Fig 02

Figure 3 shows the connection of a sensor to a voltage source as a 4-conductor connection.

Fig 03

Figure 4 shows the connection of a measuring transducer with voltage output to a current source as a 4-conductor connection.

Fig 04

To avoid damage you must ensure that the external current source has short-circuit protection in all conductor cases.

The conductor connections shown above can also be used for the analog inputs of the LOGO! Basic versions (LOGO! 24 and LOGO! 12/24).

The external resistor is an error source because it is not sufficiently insulated.
In order to obtain measuring results that are as precise as possible it is recommended to use resistors with tolerances that are as small as possible.

With a 500 Ohm resistor you must make sure that when applying the maximum voltage of 28.8V you have a power of 1.66W. Commercially available resistors have a power dissipation of 0.25W to 0.5W.

The following expansion modules permit you to measure current as an alternative to voltage:

  • S7-200: Expansion module EM 231 (4 analog inputs, voltage/current setting via DIP switches)
  • S7-200: Expansion module EM 235 (4 analog inputs / 1 analog output, voltage/current setting via DIP switches)
  • LOGO!: Expansion module AM2 (2 analog inputs, separate terminal for voltage or current)

Analog input, Current measuring, Measuring transducer

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