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Voltage and current measurements are very common.
Voltage measurements can be split into several sections: high voltage grid measurements (in kilovolts), where we need voltage transducers, direct low voltage grid measurements (120/230 V) and low voltage measurements (up to 50 V). The high voltage converters convert kilovolts voltage signals to a measurable range - up to one kilovolt. If you have a good amplifier (Dewetron has a vast array of them), there is not much to be said about voltage measurements.
Current is similar - we can measure high currents with a Rogowsky coil or current clamps; or low current measurements which is often done with shunt resistors.
A Rogowsky coil can be used for AC current measurements. Directly it measures the derivative of current, therefore an integrator circuit or software filter module must be used. Current clamps work on the Hall effect principle, and it outputs the voltage proportional to the current. Both principles include a phase shift of the output.
The shunt resistors are very useful for the measurements of small currents. The theory behind their operation is simple - with known resistance and the a constant current flowing through the whole measurement chain, the voltage is directly proportional to the current. This will be explained a little bit later when calibrating the shunt resistor in the software.
In all cases, it is highly recommended to use isolated amplifiers. Even if the voltage itself is not too high for the not isolated card input, this voltage might have a high potential to ground, which might kill the card. A good example is measurement with shunt resistor, which is basically just the high precision high current resistor, on which the voltage drop is measured. Even though there might be only a few volts measured on the output, the potential might be as the high as grid line voltage. This would - if connected directly to AD card - surely kills the card.
Dewetron signal conditioning
Let’s take a look at one typical example of "low" voltage and current measurement. We will measure the grid voltage and the current consumed by a 40 W light bulb.
In this case it is more than enough to measure the voltage directly with a Dewetron amplifier. We will use a DAQP-DMM with 2kV isolation. We could also use a DAPQ-HV module with much higher bandwidth (for high frequency or transient measurements) or a PQL print (4 x voltage and 4 x current) to make the measurement.
The currents will be measured using two principles. We will use a current clamp, which can be easily mounted since it can be opened. The second principle is the shunt measurement, where we need to cut the wire to include the shunt in the series. One should also be very careful not to exceed the maximum current of the shunt; otherwise we can measure a nice fire. For the moment, let's focus on voltage and current.
The current converters have the voltage output, so this is measured with a DAQP-V, a DAQP-V-A or a MDAQ-V module.
