Commonly used resistance measurement methods (voltammetry\multimeter)

Measurement of resistance (1)-Basic knowledge

The measurement of resistance is a very important measurement in electrical measurement and is needed in many places. Such as judging the continuity of the circuit, accurately measuring the resistance of the resistor under test, understanding whether the value of the insulation resistance meets the requirements, and mastering the resistance of the grounding resistance, etc. Correct and convenient selection of appropriate measuring instruments and equipment is something that power workers must master.

1. Understand the classification of resistors

The resistance value measured in engineering is generally in the range of 1×10-6Ω) to 1×1012Ω.

In order to choose a suitable method for measuring resistance and reduce the measurement error, resistors are usually divided into three categories according to their resistance values:

Below 1Ω is a small resistance; between 1Ω and 100kΩ is a medium resistance; above 100kΩ is a large resistance.

2. Common resistance measurement methods

1) Direct method: The method of measuring resistance using a direct-reading instrument such as the ohm range of a multimeter is called the direct method.

2) Comparison method: The method of measuring resistance using a comparison instrument such as a DC bridge is called the comparison method.

3) Indirect method: The method of first measuring the quantity related to the resistance and then calculating the measured resistance through the relevant formula is called the indirect method. A common example is voltammetric measurement of resistance.

3. Commonly used resistance measuring instruments

According to the classification of the instruments used, it is divided into: multimeter method, voltammetry method, megohmmeter method, single-arm bridge method, double-arm bridge method, ground resistance meter method, etc.

Measurement of resistance (2) - Measurement of resistance by voltammetry

1. Method of measuring resistance by voltammetry

Connect the resistor to be measured to a DC power supply, then use a voltmeter and ammeter to measure the voltage across the resistor and the current through the resistor respectively, and then calculate the resistance to be measured according to Ohm's law. Because a voltmeter and an ammeter are required during the measurement process, voltammetry is an indirect method of measuring resistance. We know that voltmeters are often connected in parallel and used in circuits, and ammeters are often connected in series and used in circuits. They can both be operated with electricity, so voltammetry can be used to measure resistance with electricity.

2. Wiring method for measuring resistance by voltammetry

1) Circuit in front of the voltmeter: suitable for situations where the resistance to be measured is very large (much larger than the internal resistance of the ammeter). The principle is the same as the internal resistance requirement of the voltmeter, the larger the better.

2) Circuit connected behind the voltmeter: suitable for situations where the resistance to be measured is very small (much smaller than the internal resistance of the voltmeter). The principle is the same as that of the ammeter. The smaller the internal resistance requirement, the better.

The circuit connected before the voltmeter The circuit connected after the voltmeter

3. Measurement steps for measuring resistance by voltammetry

(1) Adjust the pointer of the ammeter and voltmeter to 0, and perform wiring as shown below. Adjust the sliding rheostat to maximum value.

(2) Close the switch, adjust the sliding rheostat to the appropriate position, and read the readings of the voltmeter and ammeter respectively.

(3) Calculate the value of the resistance to be measured according to Ohm’s law.

4. Things to note

(1) Zero adjustment of voltmeter and ammeter

(2) The range of voltmeter and ammeter must be selected appropriately.

(3) Pay attention to safety when operating under power

Measurement of resistance (3) - Measuring resistance with a multimeter

Multimeters, also known as multiplex meters, multimeters, multimeters, etc., are indispensable measuring instruments in electric power and other departments. They are generally used to measure voltage, current and resistance. Multimeters are divided into analog multimeters and digital multimeters according to their display modes. It is a multi-functional, multi-range measuring instrument. Generally, a multimeter can measure DC current, DC voltage, AC current, AC voltage, resistance and audio level, etc. Using a multimeter to measure resistance is simple and convenient.

1. Preparation before use

1) Install the battery (pay attention to the positive and negative poles of the battery)   

2) Insert the test leads. "-" black; "+" red  

3) Mechanical zero adjustment: Before measuring the multimeter, you should pay attention to whether the pointer of the meter head is on the zero scale line of the AC and DC scale when placed horizontally, otherwise the reading will have a large error. If it is not at zero, the pointer should be returned to zero by mechanical zeroing (that is, using a small screwdriver to adjust the mechanical zeroing knob under the meter).

4) Selection of measuring range:

Step one: test. First roughly estimate the resistance of the resistor being measured, and then select the appropriate range.　

Step 2: Choose the correct gear. When measuring, the pointer stops in the middle or near it!

5) Ohm zero adjustment: After the range is selected correctly, it must be zeroed before formal measurement, otherwise there will be errors in the measured values.

Method: Short-circuit the red and black pens and see if the pointer points to the zero-scale position. If not, adjust the ohm zero-adjustment knob so that it points to the zero-scale position. Note: If you change gears again, you must also zero-adjust once before formal measurement.

2. Measurement of the resistance to be measured

The two test leads of the multimeter are connected to both ends of the resistance to be measured for measurement.

Note: 1. It cannot be measured with power on; 2. The resistance being measured cannot have parallel branches. 3. Do not use two hands to hold the metal part of the test lead to measure resistance, otherwise the resistance of the human body will be connected to the resistance being measured, causing measurement errors.

3. Reading

Select the first scale when reading! Read from right to left. Resistance = scale value ╳ magnification

3. Gear reset:

Turn the gear switch to the OFF position or the highest AC voltage level.

5. Measurement precautions

1) The resistance of the resistor to be measured should be measured by throwing the range switch to the minimum range, watching the swing amplitude of the test pen, and then adjusting the range switch from small to large, so that the meter needle points to the center range of the dial, and the range is appropriate.

2) Once the resistance value is known, the range switch can be moved to the appropriate range for measurement.

3) When measuring, do not touch the two leads of the resistor with both hands at the same time to prevent measurement errors.

4) Correctly read the resistance value according to the needle indication.

5) If the pointer points to "zero" or close to "zero" during measurement, it means that the gear selection is too large.

6) If the pointer points to "infinity" or is close to "infinity" during measurement, it means that the gear selection is too small.

7) Pay attention to the polarity of the test leads of the multimeter. The red test lead is connected to the negative pole of the battery in the meter, and the black test lead is connected to the positive pole of the battery in the meter.

The important thing is to choose the right range. When the pointer indicates 1/3 to 2/3 of the full scale, the measurement accuracy is the highest and the reading is the most accurate. It should be noted that when using the R×10k resistor range to measure a large megohm resistance resistor, do not pinch your fingers at both ends of the resistor, as this will cause the measurement result to be smaller due to human body resistance.

6. Tips for using multimeters

Take a look: Pick up the test pen and check the gear position

Second lever: the corresponding power lever is in place

Third test: Instantaneous deflection to test the gear position

Four measurements: Measure stable and record readings

Five reset: Put down the test leads and reset