原创 Low-side versus high-side current shunt insertion

In this discussion low-side refers to the return path of the load. High-side refers to the supply path of the load. The decision to place a current shunt in either position has advantages and disadvantages that must be accounted for and assessed based on the particular application.

The primary difference between low- and high-side current shunt placements is that the former can eliminate common mode voltage, which appears simultaneously and in phase on either side of the current shunt. Since the presence of common mode voltage can create complications for the instrument used to measure shunt voltage, low-side current shunt insertion is often recommended, especially in high voltage situations. However, the low-side approach is not without drawbacks, which include the following:

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• The load is removed from a direct path to ground, which may create problems for control circuitry, result in unwanted emissions, or both.

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• Only current directly returned to the supply by the load is measured. Current leaking to ground through the load’s chassis, control circuitry, cabling, etc. are not measured, which can lead to faulty diagnostic results.

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• The only beneficial reasons to use a low-side shunt, the assumption that common mode voltages will be zero, may not prevail in applications where high in-rush currents create ground bounce. This effect is essentially a momentary potential difference (or common mode voltage) between the grounded side of the current shunt and the measuring instrument’s ground. This event may disrupt the measurement and even damage the instrument given sufficient common mode potential.

A current shunt placed in the high-side of a load resolves most of these problems, but common mode voltage is virtually guaranteed to be present with the high-side approach and will complicate the instrument used to make the measurement as a result. Failure to recognize this and make appropriate instrumentation adjustments, especially in high voltage applications, can have dire consequences that include explosive destruction of the instrument, and potential injury to nearby personnel. Novice technicians who have been victimized by this fiery event often lament that they attempted to measure only a 50mV current shunt signal. Of course, they completely overlooked that the millivolt signal was riding on top of a destructive common mode component.

http://en.wikipedia.org/wiki/Shunt_(electrical)