How to choose a multimeter and what technical parameters should be paid attention to

As we all know, the multimeter is the most basic tool in the field of electronic testing and is also a widely used instrument. Due to its low price, simple operation, complete functions, and wide application, professional electronic engineers often ignore the following issues when using it:

1. Because the accuracy and resolution of the multimeter are not enough, judging by estimation often leads to "human" errors;

2. Because various multimeters have different test methods, multimeters often cause "errors" in the test of different signals and non-sinusoidal wave scale signals;

3. Safety, reliability and maintenance in operation: Since the maintenance of the multimeter itself is poor, the tester must not be careless during the test, otherwise it will cause unnecessary damage to the multimeter...

So how do you choose a multimeter?

1. Select the display digits and accuracy of the multimeter according to your needs.

The number of digits displayed and the accuracy are the two most basic and important indicators of a multimeter. The two are closely related. Generally speaking, the higher the number of digits displayed on a multimeter, the higher the accuracy, and vice versa. However, due to the different measurement principles and quality standards of each manufacturer, some multimeters have high accuracy and some have poor accuracy at the same number of digits. For example: for the same 41/2 multimeter, some models have an accuracy of up to 0.025% while others have an accuracy of only 0.8%. There are two ways to display the number of digits: count display and digit display. Count display is the actual expression of the range of digits displayed on a multimeter, but because of people's habits and the convenience of traditional names, it is generally expressed in digit display. For example, a 3000-digit count display means that the maximum display value of the multimeter can reach 3999, while a 1000-digit count display can only reach 1999. When measuring 220VAC voltage, it can be clearly seen that the 3000-digit display has one more decimal place than the 1000-digit display: this means that the resolution is one order of magnitude higher, which will play a greater role in the debugging and testing of highly sensitive micro-electric signals. At the same time, the count display and the digit display can be converted: first calculate how many zeros there are in the count display digit, then use the previous number as the denominator of the fraction, and then subtract 1 from the number to become the numerator, which becomes the digit display. For example, a 3000-digit count has 32/3 digits.

2. Select the measurement method and AC frequency response of the multimeter according to your needs

Generally speaking, the measurement method of a multimeter is mainly for measuring AC signals. As we all know, there are many types of AC signals and various complex situations. With the change of the frequency of the AC signal, various frequency responses appear, which affect the measurement of the multimeter. There are generally two methods for the multimeter to measure AC signals: average value and true effective value measurement. Average value measurement is generally for pure sine waves. It uses the estimated average method to measure AC signals, and there will be large errors for non-sinusoidal signals. At the same time, if the sine wave signal has harmonic interference, its measurement error will also change greatly. The true effective value measurement uses the instantaneous peak value of the waveform multiplied by 0.707 to calculate the current and voltage to ensure accurate readings in the distortion and noise system. In this way, if you need to detect ordinary digital data signals, using an average value multimeter to measure will not achieve the real measurement effect. At the same time, the frequency response of the AC signal is also very important, and some can be as high as 100KHz.

3. Select the function and measurement range of the multimeter according to your needs

Different multimeters have different function measurement ranges designed by their manufacturers. Generally speaking, ordinary multimeters can test AC and DC voltage, current, resistance, continuity, etc., but some multimeters do not set current functions in order to reduce costs. On this basis, some multimeters consider the convenience of use and add some other functions. For example: diode, "multimeter battery test", transistor hef, capacitance, frequency, temperature, etc. Now due to the development of electronic technology, some famous manufacturers, such as "Tektronix, Fluke, Hp", etc., have added more advanced functions based on traditional parameters and components, such as: duty cycle test, dBm value test, maximum and minimum value record and hold functions, etc. In short, the function of the multimeter will follow the needs of the test, and experienced manufacturers will create more and better functions. However, on the basis of pursuing the function of the multimeter, its measurement range cannot be ignored. For the same test current, some multimeters can reach 20A, while others are only 40mA or less.

4. Choose your multimeter according to your needs

Like most instruments, a multimeter has its own measurement stability, and the accuracy of its measurement results is related to its use time, ambient temperature and humidity, etc. If the stability of the multimeter is poor, after a period of use, the multimeter will sometimes "contradict itself" - the measurement results are inconsistent. Therefore, some manufacturers pay great attention to this issue: For example, Fluke in the United States recently promised lifetime traceable calibration and warranty in the sale of new products.

Most maintenance engineers are most worried about the poor protection of the multimeter when using it. They may accidentally insert the wrong probe or test the wrong file, which will cause unnecessary damage to the multimeter and affect their work. Therefore, the safety of the multimeter is very important. Some good multimeters have good self-protection. For example, some multimeters will automatically generate a buzzer alarm when the probe is inserted incorrectly... When buying a multimeter, don't blindly pursue cheapness, but make sure it is practical and easy to use.

5. Choose a multifunctional multimeter according to your needs

As mentioned in Section 3, with the development of electronic technology, many multimeters have added many functions.

1. Temperature measurement

During electronic maintenance, a multimeter with this function is convenient for you to check the fever level of electronic components. For example, when soldering and removing components, measuring the temperature can avoid damaging the components.

2. Measure AC and DC components simultaneously

In electronic testing, the signals we encounter are not very pure AC or DC signals. We need to observe the total true RMS value of the waveform (including AC and DC parts) in order to analyze the power consumption of the circuit and find out the cause of the burning of some parts - DC imbalance.

3.dBm and millivolt measurement

The so-called dBm value measurement is a low-level measurement - dB value measurement. dB is generally expressed by this formula: dB = 20LogVmeasured/Vreferenced. If the Vreference voltage is changed, we can measure the corresponding value through test comparison: for example, it can be used to analyze the voltage gain of a voltage amplifier.

4. Peak hold

Using the true effective value measurement of the multimeter, this function can measure the instantaneous peak voltage of irregular AC signals with a width greater than 0.25 milliseconds and automatically maintain it, which is helpful for finding the cause of component and equipment damage.

5. Relative value determination

This function allows for relative value determination, i.e. the difference between the test voltage or current and the reference voltage or current. The capacitance relative mode can eliminate stray capacitance from the readings.