DC Measurements: Voltage and Current

  　 People usually prefer voltage measurement to current measurement because it is safer for the equipment when set up properly. The advantage of voltage measurement is more obvious when the instrument is far away from the measurement point and has to use long lead wires.

　　When using long test leads (longer than 6 feet), there are several things to consider:
　　■ Lead resistance, which affects frequency
　　■ Transmission line effects, including lead inductance, which show up at high frequencies
　　■ Electromagnetic interference (EMI), which shows up in the extremely low frequency (ELF) band below 30 Hz.
　　The basic DC measurement circuit consists of a power supply and three resistors: the sensor output resistor, the transmission line (lead) resistance, and the instrument resistance. The only thing that connects these together is the current in the circuit. When you make a measurement, what you are really recording is the voltage drop in the loop current caused by the instrument resistance.
        

　　Voltage sources have low impedance, and we generally define an "ideal" voltage source as having zero impedance. For example, a thermocouple is a Thevenin equivalent circuit consisting of an excitation source and a sensor resistor. The excitation source produces a voltage in the millivolt range proportional to the hot/cold temperature difference at the junction, and the sensor impedance is well below 1 ohm. The excitation source has a major influence on the loop current.
　　Another example is a thermistor, which requires an external excitation source and has an internal sensing element resistance of about 100 ohms. Its Thevenin equivalent circuit also consists of an excitation source and a sensor resistor. The difference is that the sensor resistance has a major influence on the loop current.
　　Typical test leads are made of #22 copper wire and have a resistance of 0.019 ohms per foot. Two #22 test leads, each 6 feet long, have a resistance of 0.228 ohms. This is negligible compared to the impedance of a thermistor, but many times greater than the impedance of a thermocouple. If the test leads are close to 60 feet from the sensor, the test lead resistance will have a significant impact on the accuracy of the thermocouple (approximately 2%). The distance between the test lead and the sensor is the distance along the wire to the test instrument.
　　When it comes to instrument impedance, always use a high impedance instrument to measure voltage and a low impedance instrument to measure current. This helps reduce the interference of the instrument impedance on the source impedance. Whether measuring current or voltage, always use a wire (transmission line) with low impedance relative to other components. The
　　input impedance of a DMM is about 100 kilo-ohms, while the impedance of an oscilloscope is several orders of magnitude higher. When using the above equipment, even if the wire is hundreds of meters long, the effect is minimal.
　　If you want to use a high impedance instrument with a thermocouple, the impedance of the instrument will dominate the effect on the loop current. No matter how large the temperature difference is, the instrument impedance will mask the effect of the excitation voltage. You must test the thermocouple as a current measurement, even though the sensor impedance is only about 100 ohms. This means using a low impedance instrument and being careful about the wire resistance.
　　This explains why voltage measurement is a more common method for long distance applications. This effectively ignores the effect of wire resistance. For example, if you wanted to measure the current through a DC motor from a separate control room, you would need a way to convert the current measurement into a voltage measurement.