One of the most commonly used components in electronics is the resistor. They are found in every electronic device where their main purpose is to resist or limit the flow of current in the circuit. They are also widely used as voltage dividers or to complement other components like transistors, microcontrollers, and integrated circuits.
The resistance of any object is the measurement of its opposition to the flow of electric current. All objects contain some amount of electrical resistance which typically depends on the material the object is made of. Objects made of an insulating material such as rubber have very high resistance while objects made of a conductive material such as metals have very low resistance. The unit of the measurement of electrical resistance is the ohm and uses the symbol of Ω.
In a calibration laboratory, resistance is used in many different forms and is used in the calibration of many diverse types of electronic equipment such as in Digital Multimeters, Insulation Testers, Conductivity Meters, ESD/Ground Strap Checkers, and RTD Measurement Devices.
Resistance Calibration Sources
In a calibration laboratory, there are two main types of resistance sources, Electrical resistance from Multifunction Calibrators and Decade Resistance boxes.
Multifunction Calibrators perform a wide array of calibration functions and are typically used in the calibration of Multimeters and other resistance measuring devices.
Decade Resistance boxes contain high accuracy resistors that are arranged inside a box to form a set of resistance ranges which is increased incrementally by use of a dial. They usually have several dials of various decade steps to encompass a wide range of resistance values. Boxes with highly accurate single resistances are also used.
Resistance Measurement Methods
Multimeters are the most common type of device to measure resistance. Nearly all Analog or Digital Multimeters on the market have a resistance measurement function to measure the resistance of objects. Typical handheld Digital Multimeters can measure resistances to accuracies of 0.1 Ohm where highly accurate bench multimeters can measure down into the micro-Ohms. Measuring resistance with a Digital Multimeter is quick and easy and is widely used for measurements with standard accuracies.
Another widely used method for determining the resistance of an object is by using Ohms Law. According to Ohms Law, R=V/I which means that the resistance of an object is equal to the voltage measured across the object divided by the value of the current running through the object. Ohms law is used to calculate resistance by either introducing a current or voltage into the object and measuring the other factor. Depending on the accuracies of the equipment used for the voltage and current measurements, the resistance can be calculated to a very high nominal accuracy.
The current method sources a known current through the object and the voltage across the object is measured. This method is typically used for measuring the resistance of resistors below 100 ohms and for measuring the resistance value of current shunts. Most Digital Multimeters use this method to determine the resistance measurement.
The voltage method sources a known voltage across the object and the current through the object is measured. This method is typically used for high resistance measurement applications. Most Insulation Testers or Meggers use this technique by using various preset test voltage values.
High Accuracy Resistance Calibrations
Using two wires to measure resistance is the simplest and most common method used. One wire is placed on each side of the resistor and the measurement is read on the meter. However, this creates an error in the measurement because the measurement includes the resistance of the measuring leads as the current through the leads causes an additional voltage drop that is measured by the meter. For measurements above 10 kOhms, this error is negligible, however for lower resistance values, the leads can add significant resistance which can greatly affect the accuracy of the measurement.
For precision measurements with resistances below 10 kOhms, the error can be reduced by using a 4-wire measurement. By using 4 wires, one set of leads carries the sourced current to the resistor and the second set of leads measures the voltage directly across the resistor. This method provides a much more accurate result by getting around the additional voltage drop caused by the current source through the leads.
Measuring very high resistances also contains challenges to get the most accurate measurement results. For measurements above 100 MOhms, electrostatic noise becomes a major concern from sources such as power lines, fluorescent lights, RF interference and even changes in electrostatic fields caused by nearby movement of laboratory personnel.
To prevent these problems when measuring higher resistances proper shielding is critical. Shielded cables should always be used when measuring high resistances. If shielded cables are not available, twisting the measurement leads around each other will assist in reducing the effects of any noise in the measurement.
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