Understanding IO Sourcing versus Sinking.

Issue

What are the sourcing and sinking specifications of the analog voltage outputs on the different controllers.

Product Line

Andover Continuum, SmartStruxure Solution

Environment

i2

b3

XP

IO modules

Cause

Documentation

Resolution

Sourcing and sinking refer to the ability of a DC circuit to PUSH or PULL a direct current thru the circuit.

In a controller's  VOLTAGE SOURCING ouput, the output provides a voltage source that PUSHES a direct current thru the load as indicated in the illustration below, the current flows from the output's positive (V/+) terminal thu the load to the output's negative terminal (-/ RTN).

CURRENT SINKING

In the illustration below a controller's voltage output is sinking a current from the load, note the voltage source in the load's circuit which is at 15 VDC, because current always flows from positive to negative the voltage coming from the controller's output (0 - 10 VDC) will ALWAYS be more negative than the load's source of 15VDC, thus a current will flow from the load's voltage source thru the controller's output circuit to reach the negative (RTN) side of the output's circuit.

The lower the voltage coming out of the controller's output the more negative it will be as compared to the load's voltage source, thus the output will be sinking the most current when it has a value of 0 volts.

It is extremely important when using a controller's voltage output in a way that will make it sink current to know the maximum amount of current the output will be sinking and to ensure it does not exceed the current sinking capacity of the controller's output.

The following table shows the sourcing and sinking capabilities of the voltage outputs of the different controllers.

CONTROLLER VOLTAGE SOURCING CURRENT SINKING COMMENT
i2/b3 814 Voltage output 0-10 V @5 ma max 1 ma max  Exceeding the 1 ma sink limit will cause the IO readings to be incorrect. ie, inputs may read 999.99