Measuring Voltage and Current With a Data Logger

A very common application for data loggers is measuring voltage or current flowing into our out of a piece of equipment. The measurements can be broadly classified into 2 groups, AC and DC. Depending on the levels involved, data loggers can often measure DC voltage and current directly. For example, the DT80 family of data loggers can accept DC voltages up to +/- 30 volts and DC currents up to 30 mA directly. With few exceptions most data loggers can not measure AC voltage or current directly and so require external transducers to convert the parameter of interest into a signal that the data logger can measure.

DC Voltage Measurements

• Attenators - The simplest method of measuring a DC voltage that is outside the measurement range of the data logger is to use an attenuator which is just a fancy name for a few resistors wired together to divide down the incoming voltage to a range compatible with the data logger. Two issues with using attenuators are the common mode voltage and the potential effects of resistive loading on the measurement accuracy.
• DC voltage transducers - A number of companies offer packaged DC voltage transducers that convert the incoming voltage to a range that is compatible with the data logger. These units offer the advantages of being able to measure very small (< 0.1) and very high (>1000) volt inputs, input to output isolation and minimal loading effects. They can also provide an output either as a voltage or as a 4-20 mA signal which is advantageous if there is a long distance between the measurement point and the data logger.
• Signal conditioner modules - Standard signal conditioner modules such as the ubiqutous 5B series provide up to 1500 volts of isolation and amplification or attenuation in compact packages that are suitable for multi-channel systems. They are available in a wide range of input voltages and provide a DC voltage output. Because of their small size and the ability to mix and match input types and ranges, these are very useful in high channel count systems.

DC Current Measurements

• Current Shunts - A current shunt consists of a conductor with a very small, but known resistance. The current flowing through the shunt creates a voltage drop than can be measured with the data logger. These are available in ranges to handle 5 to 1000 amps and provide an output from 0-1 volts. Like the attenuators for DC voltage measurements, these suffer from the issue of common mode voltage.
• DC Current Transducers - DC current transducer often utilize a hall effect sensor to allow current measurement without direct contact with the conductor. The disadvantage of these sensors is that they typically have limited resolution for lower currents but they work very well for higher currents.

AC Voltage Measurements

• AC Voltage Transducers - If your application involves tracking incoming line voltage, there are a number of vendors that provide standard AC voltage transducers. They are available in both single and multi-phase versions. Also they are available with standard or True RMS calibrations.
• AC Voltage signal conditioner modules - If the applications requires the measurement of small AC voltages or a large number of channels, 5B series signal conditioner modules are available with 100 mV to 300V inputs.

AC Current Measurements

• AC Current transducers - A standard method of measuring AC current for a power line connected device is to use an AC current transducer which converts an AC current to a DC voltage or 4-20 mA signal that can be measured with the data logger. As with AC voltage sensors, current transducers are available in single phase and multi-phase models. The current transducers can utilize an internal current sensing element for small currents up to about 20 AAC or an external current transformer or sensor for currents up to thousands of amps.
• Clamp-on current sensors - Clamp on current sensors are available in a variety of models and current ranges with either DC or AC voltage outputs. Clamp on sensor offer the advantage of ease or use - simply open the clamp and place it around one of the current carrying conductors. They are ideal for temporary installations and can be moved from site to site very easily although they are somewhat more expensive than fixed current transducers.
• Split core transformers - Split core transformers are very similar to the clamp-on current sensors. The are intended for more semi-permanent installations. They consist of a transformer where one of the leg's can be opened or removed to place around the conductor and they secured with a latch or some other type of fastner. Some models provide a AC current output that must be used with a current transducer to provide a signal for the data logger while other models have built in signal conditioning to provide a dc voltage or current that can be measured with the data logger.
• Rogowski coil - A Rogowski coil is a specialy wound torridial coil that can be opened up and placed around a condutor carrying an AC current. The alternating magnetic field generated by the AC current induces a voltage in the coil. This voltage is proportional to the rate of change of current in the conductor. This voltage is then electronically integrated to provide and output voltage that mimics the current waveform in the conductor. These coils are suitable for measurment of currents up to thousands of amps, are not sensitive to positioning around the conductor and can provide accurate phase response.