Shunt measurement

Current measurements (AC and DC) can optionally be carried out with a shunt. A resistor of known (and ideally very temperature-stable) size is introduced into the current path; according to Ohm's law, the voltage drop ΔU is a direct linear representation of the current value in [A]. The shunt can be mounted against + (high-side shunt) or - (low-side shunt) - or "in between".

Fig.67: Low-side shunt

The voltage ΔU must be determined with a measuring device (here in focus: a Beckhoff I/O product).

To keep the heating of the shunt within limits and the influence of the shunt on the source as negligible as possible, very small ohm values are common [mΩ] and, as a result, usually also small voltage values in the higher mV range; good line shielding is important.

See also a comparative overview of shunt measurement in chapter Shunt measurement.

Instructions for use

• Recommended potential reference (see chapter Ground reference: single-ended/differential typification) of the Beckhoff I/O device:
• with low-side shunt
  - SingleEnded: possible
  - Differential: limited possibility (see following note)
  - Differential/channels galvanically isolated: possible (recommended)
  - Cascaded: not possible
• with high-side shunt or "in between"
  - Single-ended: generally not possible
  - Differential: very limited possibility (see following note)
  - Differential/channels galvanically isolated: possible (recommended)
  - Cascaded: not possible
• Dynamics: the ΔU measuring device must be able to follow the sometimes very dynamic (kHz to MHz) potential changes
• When using non-galvanically isolated differential channels, the common-mode voltage Ucm (common mode) between the channels or earth must be observed:
• For multi-channel devices, U_{cm, max} (see specifications in the technical data) must not be exceeded between the channels.
• For example, with a 24 V supply to the loads, it may not be permissible to use a high-side shunt at 24 V potential on one channel and a low-side shunt at 0 V potential on another channel. The internal reference ground AGND (-Uv) would be set in the middle so that Ucm is exceeded
  → Only high-side or only low-side shunts should therefore be used for each device.
• High-side shunt and dynamic processes with pulsed current, fast/steep current changes: Depending on the inductance in the load circuit, this leads to sudden current changes and therefore voltage changes over the shunt on both sides. Ucm at the differential inputs changes accordingly. A differential analog input channel (this therefore also applies to 1-channel devices) is usually LC-coupled to the internal reference ground AGND - the sudden increase in Ucm at the inputs therefore results in an increase in AGND. During this transient (several ms), measurement errors may occur when exceeding U_{cm, max}.
  → A very large measuring range should be selected or a device with galvanically isolated channels is better.
• If a shunt is to be determined with several voltage measuring devices, a separate shunt R1, R2 should be installed for each measuring device.
  
  Otherwise, the impedance of the measuring devices could influence the other measurement, e.g. if the measuring device impedance changes when the device is switched off.