Common Devices for Multimeters
Digital multimeter is currently the most commonly used digital instrument. Its main features are high accuracy, strong resolution, perfect test function, fast measurement speed, intuitive display, strong filtering ability, low power consumption, and easy to carry. Since the 1990s, digital multimeters have been rapidly popularized and widely used in my country, and have become necessary instruments for modern electronic measurement and maintenance work, and are gradually replacing traditional analog (ie, pointer) multimeters.
Digital multimeters are also known as digital multimeters (DMMs), and there are many types and models. Every electronic worker hopes to have an ideal digital multimeter. There are many principles for choosing a digital multimeter, and sometimes they even vary from person to person. However, for a handheld (pocket) digital multimeter, it should generally have the following characteristics: clear display, high accuracy, strong resolution, wide test range, complete test functions, strong anti-interference ability, relatively complete protection circuit, and beautiful appearance , generous, easy to operate, flexible, good reliability, low power consumption, easy to carry, moderate price and so on.
The main indicators, display digits and display characteristics of the digital multimeter
The display digits of a digital multimeter are usually 3 1/2 to 8 1/2 digits. There are two principles for judging the display digits of digital instruments: one is that the digits that can display all numbers from 0 to 9 are integer digits; The numerator is the numerator, and the count value is 2000 when the full scale is used, which indicates that the instrument has 3 integer digits, and the numerator of the fractional digit is 1, and the denominator is 2, so it is called 3 1/2 digits, read as "three and a half digits ", the highest bit can only display 0 or 1 (0 is usually not displayed). 3 2/3 digits (pronounced "three and two-thirds digit"), the highest digit of the digital multimeter can only display numbers from 0 to 2, so the maximum display value is ±2999. Under the same conditions, it is 50% higher than the limit of a 3 1/2 digit digital multimeter, which is especially valuable when measuring 380V AC voltage.
Popular digital multimeters generally belong to handheld multimeters with 3 1/2 digits display, and 4 1/2, 5 1/2 digits (less than 6 digits) digital multimeters are divided into two types: handheld and desktop. More than 6 1/2 digits mostly belong to desktop digital multimeters.
The digital multimeter adopts advanced digital display technology, with clear and intuitive display and accurate reading. It not only ensures the objectivity of the reading, but also conforms to people's reading habits, and can shorten the reading or recording time. These advantages are not available in traditional analog (ie pointer) multimeters.
Accuracy (precision)
The accuracy of a digital multimeter is the combination of systematic errors and random errors in the measurement results. It indicates the degree of agreement between the measured value and the true value, and also reflects the size of the measurement error. Generally speaking, the higher the accuracy, the smaller the measurement error, and vice versa.
The accuracy of digital multimeters is much better than that of analog analog multimeters. The accuracy of the multimeter is a very important indicator. It reflects the quality and process capability of the multimeter. It is difficult for a multimeter with poor accuracy to express the real value, which may easily cause misjudgment in measurement.
Resolution (resolution)
The voltage value corresponding to the last digit of the digital multimeter on the lowest voltage range is called resolution, which reflects the sensitivity of the meter. The resolution of digital digital instruments increases with the increase of display digits. The highest resolution indicators that digital multimeters with different digits can achieve are different.
The resolution index of the digital multimeter can also be displayed by resolution. Resolution is the percentage of the smallest number (other than zero) that the meter can display to the largest number.
It should be pointed out that resolution and accuracy are two different concepts. The former characterizes the "sensitivity" of the instrument, that is, the ability to "recognize" tiny voltages; the latter reflects the "accuracy" of measurement, that is, the degree of consistency between the measurement result and the true value. There is no necessary connection between the two, so they cannot be confused, and the resolution (or resolution) should not be mistaken for similarity. Accuracy depends on the comprehensive error and quantization error of the internal A/D converter and functional converter of the instrument. From the perspective of measurement, resolution is a "virtual" indicator (which has nothing to do with measurement error), and accuracy is a "real" indicator (it determines the size of measurement error). Therefore, it is not possible to arbitrarily increase the number of display digits to improve the resolution of the instrument.
Measuring range
In a multi-function digital multimeter, different functions have their corresponding maximum and minimum values that can be measured.
Measuring rate
The number of times a digital multimeter measures the measured electricity per second is called the measurement rate, and its unit is "times/s". It mainly depends on the conversion rate of the A/D converter. Some handheld digital multimeters use the measurement period to indicate the speed of measurement. The time required to complete a measurement process is called the measurement cycle.
There is a contradiction between the measurement rate and the accuracy index. Usually, the higher the accuracy is, the lower the measurement rate is, and it is difficult to balance the two. To solve this contradiction, you can set different display digits or set the measurement speed conversion switch in the same multimeter: add a fast measurement file, which is used for the A/D converter with a faster measurement rate; To increase the measurement rate, this method is relatively common and can meet the needs of different users for the measurement rate.
input resistance
When measuring voltage, the instrument should have a very high input impedance, so that the current drawn from the circuit under test is very small during the measurement process, which will not affect the working status of the circuit under test or the signal source, and can reduce measurement errors.
When measuring current, the instrument should have a very low input impedance, so that the influence of the instrument on the circuit under test can be reduced as much as possible after being connected to the circuit under test. Burn out the meter, please pay attention when using it.
