What Is a Low Resistance Measuring Instrument?
A low resistance measuring instrument is a device that can measure minute resistances in units of milliohms, which is less than 1 ohm.
General resistance measuring instruments cannot measure resistance of less than 1 ohm with high accuracy because they are affected by the contact resistance between the measuring instrument and the resistance and the wiring resistance of the measuring leads. A low resistance measuring instrument, on the other hand, is less affected by contact resistance and wiring resistance, and can measure resistances of less than 1 Ohm with high accuracy.
Uses of Low Resistance Measuring Instruments
Low resistance measuring instruments are used to measure the resistance of various samples. Specifically, they are used for measuring the winding resistance of motors, solenoids, and choke coils. They are also used to measure the contact resistance of wire harnesses, connectors, and switch contacts; and wiring resistance on printed circuit boards.
Low resistance measuring instrument should also be used to measure the resistance of current fuses and shunt resistors, and the DC resistance of conductive rubber.
Principle of Low Resistance Measuring Instruments
General resistance measuring instruments measure resistance using the 2-terminal method, while the measurement method used by the Low resistance measuring instrument to measure resistance is the 4-terminal method.
1. 2-Terminal Method
The two-terminal method uses a common lead wire for the current source and a common lead wire for the voltmeter. The current from the current source also flows through the voltmeter leads during resistance measurement, causing a voltage drop in the leads and in the contact resistance, resulting in a measurement error during resistance measurement.
2. 4-Terminal Method
The 4-terminal method uses the current source and voltmeter leads independently. Since no current flows in the voltage measurement leads and no voltage drop in the leads and contact resistance occurs, resistance can be measured with high accuracy.
Other Information on Low Resistance Measuring Instruments
1. Setting of Resistance Range
Set the resistance range of the low resistance measuring instrument according to the resistance value. There are two ways to set the resistance range: manual mode and auto mode. Use manual mode if you want to set the resistance range arbitrarily, and select auto mode if you want to set the resistance range automatically.
If you select auto mode, you have the advantage of not having to change the resistance range setting yourself. However, in the case of wire-wound resistance of motors, solenoids, choke coils, etc., the measured value may not be stable in the auto range. In such cases, it is better to set the resistance range manually and then measure the resistance value.
2. Setting of Measurement Current Range
Increasing the measurement current as much as possible increases the voltage at both ends of the resistor, which improves measurement accuracy. However, increasing the measurement current increases the current consumption of the resistor and causes the resistor to heat up.
As the resistor heats up, it is prone to problems such as resistance breakage, changes in resistance value, and magnetization of the resistor. These problems may result in incorrect resistance measurement. Conversely, if the measurement current is reduced, the voltage at both ends of the resistor becomes smaller, making it easier to be affected by external noise during measurement and making it more difficult to stabilize the measured value.
3. Setting the Measurement Speed
Slow down the measurement speed as much as possible, this will produce better measurement accuracy and the less the influence of extraneous noise. Faster measurement speeds shorten the measurement time, but are more susceptible to extraneous noise and make it more difficult to stabilize the measured value.
4. Method of Reducing External Noise
If the resistance measurement is affected by external noise, the measured value will not be stable and correct measurement will not be possible. Adequate shielding or twisting of lead wires can reduce the influence of external noise.