The article "Characteristics of Closed Loop Current Transducers" describes the measurement of current with galvanic isolation, but it is also possible to measure a primary voltage with galvanic isolation using the same technology. Such voltage transducers are based on sensitive and accurate current measurement technologies, such as closed loop Hall effect designs. The main difference when compared to a current transducer is the addition of an internal primary winding with a large number of turns, giving the transducer the ampere-turns necessary to measure the small primary current.
Transducer construction and principle of operation
Figure 1. Basic principle for voltage measurement
The operating principle of closed loop Hall effect voltage transducers is to measure a small current that is directly proportional to the voltage of interest. Dividing the voltage to be measured by a large resistance, R1, creates a small current, Imes, that can be measured by an ‘optimized’ transducer (figure. 1), enabling it to accurately measure the small Imes current, while also having controlled insertion impedance to maintain suitable accuracy and measurement bandwidth. The R1 resistance is added in series with the transducer primary coil to obtain the optimal Imes current value for the nominal voltage level. This resistance is often split into two resistors, placed on both sides of the primary coil, to improve common mode rejection (right side of figure 1).
Figure 2. Equivalent diagram for primary part of voltage transducer
The equivalent electric diagram of the primary part of a voltage transducer is shown in figure 2, including the serial resistance R1, the primary coil resistance RP, and the primary coil inductance LP. The latter generally creates an insertion reactance ωLP, which can be neglected in most cases.
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