Q8: How do I know if a mA signal is active or passive?
Example 1: Your device outputs a 4…20 mA signal which is measured by a PR module.
- Disconnect the two current loop wires from the PR module input terminal.
- Using a voltmeter, measure for DC voltage present on the wires.
- If no voltage is present, then your device is creating a passive current signal. This means a loop excitation source, (typically 12...24 VDC), must be used to power the current loop and measure the signal. Many PR modules have an input excitation source which powers a passive current loop, allowing our module to both measure and energize your device.
- If you measure voltage on the wires, (typically 12...24 VDC), then the current signal created by your device is active. Do NOT use our loop excitation source when measuring an active current signal.
Example 2: A PR module outputs a 4…20 mA signal which is measured by your device.
- Disconnect the two current loop wires from our output terminal.
- Using a voltmeter, measure for DC voltage present on the wires.
- If no voltage is present, then your device has a passive input. Therefore, the PR module must provide the excitation source needed to power and regulate current flow on the loop.
- If you measure voltage on the wires, (typically 12...24 VDC), then your device has an active input. Therefore, the PR module should only regulate the level of current flowing through the loop, it should NOT power the loop.
List of PR devices by active/passive input and output measurement type
A PR module that measures:
- an active input signal, simply measures the active input current. It does not excite the loop.
- a passive input signal, excites the current loop, excites other loop powered devices wired in the loop, and measures the current.
A PR module the creates:
- an active output signal, regulates the current level, excites the current loop, and excites other loop powered devices wired in the loop (e.g. a loop powered display). As an example, the output from the PR module can be connected directly to a passive input card on a PLC.
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a passive output signal, only regulates the loop current level. The loop is powered by an external excitation source. As an example the output from the PR module can be connected directly to an active (powering) input card on a PLC.
You can find more information about the mentioned devices here:
