Digital multimeter operation and use strategy

With the development of science and technology, there are more and more varieties and specifications of digital instruments, and their circuit structure and technical performance are also constantly improving and innovating, especially multi-purpose instruments and high-precision instruments. In addition, the use environment, the objects being measured and the quality of the users are also different. Therefore, in addition to mastering the above-mentioned essentials, it is also necessary to adopt corresponding technical strategies according to the technical characteristics and use requirements of the instruments used, targeting the specific measurement objects, measurement environments and measurement requirements, and to analyze and treat specific problems specifically, and never generalize.

1. First, you need to understand the basic performance of the instrument

There are hundreds of brands and models of digital multimeters on the market in my country, and there are dozens of common domestic and foreign models. Therefore, before using a digital multimeter that you are not familiar with, you should read the accompanying technical manual in detail or consult experienced personnel, focusing on understanding the characteristics, main performance, technical indicators, panel functions, measurement method requirements and special precautions of the instrument. Figure 1 shows the appearance structure of two commonly used digital multimeters, and other instruments are similar.

figure 1

2. Environmental conditions must be met

The internal circuit of a digital multimeter is sensitive to temperature and humidity, so it has certain requirements for the use environment. For example, the temperature and relative humidity in the workplace must not exceed the limits specified by the instrument's technical conditions, otherwise the accuracy of the instrument will not be guaranteed. For example, the dt1000 digital multimeter guarantees accuracy at a test temperature of 23°C ± 5°C and relative humidity < 75% rh, but it also specifies a temperature range, including an operating temperature of 0°C to 40°C (32°F to 104°F) and a storage temperature of -10°C to 50°C (14 to 122°F).

During the use and storage of digital multimeters, try to avoid direct sunlight, dust and various corrosive media to damage the instrument. Also, prevent all forms of mechanical vibration, impact and falling to keep it clean, dry and intact.

3. Inspection items before use

Digital multimeters are mandatory instruments, so before using them, you should confirm whether they have been periodically inspected by an organization approved by the quality and technical supervision department and whether they are within the inspection period. The main inspection items include: Check whether the appearance of the instrument is seriously damaged; Check whether the range selection switch, key switch and other operating mechanisms are flexible and whether the switching is accurate; Check whether the battery energy is normal; Check whether the display content on the display is complete and whether there are any missing strokes; Check whether the insulation of the test leads and their wires is intact and whether the conductivity is good (generally, it is judged by short-circuiting the test leads at the 200ω resistance block).

4. Key points for using selector switches

If the voltage or current to be measured is unknown in advance, the selector switch must be set to the highest range position first, and then adjusted down appropriately according to the measurement results. If it is an automatic range conversion digital multimeter (such as dt-840, dt-860, dt-910, etc.), it will be much easier, but the measurement process of this instrument is longer. For example, if the amount of electricity to be measured is very small, it must also follow the established procedure rules. Most of them start measuring and displaying the results from the highest range, and then automatically gradually reduce the range until it is appropriate. In contrast, the dt-960t type automatically switches from the lowest range to the highest range.

When measuring higher voltages and larger currents, do not switch the selector switch, otherwise it is easy to burn the switch contacts and damage other parts inside the meter. If the situation is special and switching is necessary, the test lead should be removed from the measured circuit before switching.

5. Make sure the test leads are electrically charged

When the digital multimeter is in the test position, buzzer position and resistance position, the red test lead is connected to the high potential inside the meter and is positively charged, while the black test lead is connected to the virtual ground and is negatively charged. This is completely opposite to the polarity of the two test leads in the resistance position of the analog multimeter. When using a digital multimeter to detect electronic components with polarity (such as semiconductor devices, electrolyzers, etc.) or related circuits, full attention must be paid.

6. Key points for measuring large currents

When the measured current is greater than or equal to 200ma, the test lead plug should be switched to a high current dedicated jack (such as 10a or 20a, depending on the meter). Since most meters do not have overcurrent protection measures for high current ranges, it is necessary to prevent the occurrence of measurement overload to avoid damage to the meter. In addition, a digital multimeter in high current range should not be connected to the measured circuit for a long time as a high current display device. Usually, the measurement time under high current should be controlled within 30 seconds.

7. When measuring low-value resistors, the contact resistance component cannot be ignored

If the resistance to be measured is below 10ω, the contact resistance between the test lead and the two test leads and the measured part must be considered. Therefore, before measuring, the resistance of the test leads and their test lead wires should be checked first. The method is to short-circuit the contact pins of the two test leads and write down the displayed value on the screen. This value is used as the base number. After measuring the resistance, subtract the base number from the measured value. The difference is the actual value of the resistance to be measured.

8. The functional attributes of the AC voltage range must be clearly defined

The AC voltage range of a common digital multimeter is an average value meter, and is designed according to the characteristics of a sine wave, so it cannot be used directly to measure non-sinusoidal voltages such as sawtooth waves, triangle waves, and rectangular waves. Even if the measured voltage is a sine wave, if the waveform distortion is large, the correct measurement result cannot be obtained. This must be highly valued for measurement objects such as pulse circuits, digital electronic systems, video and audio circuits in radio and television equipment, and non-electrical electrical measurement signals in instrumentation. If you need to accurately measure the effective value of voltages of various non-sinusoidal waveforms, you should use a true effective value digital multimeter (such as the dt980 model) for measurement.

9. Isolation rule of the tested part

When measuring resistance, no matter whether the object to be measured is a complex system or a single electronic component, the measured part must not have any current components other than the test current of the digital multimeter, and the fingers of the measurer and any other part of the body must not touch the probes of the two test leads and the conductive parts of the measured object, so as to avoid shunting the measured part or introducing interference components, thereby causing large measurement errors. Therefore, before measuring resistance, the working power supply of the measured object or the circuit connected to the power supply must be cut off, otherwise, not only the measurement result cannot be guaranteed, but also the instrument may be damaged or pose a threat to personal safety. Even when measuring voltage and current parameters, it is necessary to consider the isolation rule of the measured part, because the accuracy of the measurement result under weak parameters is the key, and the safety of personnel and instruments under strong parameters becomes the theme.

10. The flow characteristics of the resistor must be clearly defined

The internal resistance of the digital multimeter's resistance block is very high, and the measurement current it provides is of course very weak (such as 20kω block: 75μA for DT-830 type; 60μA for DT-840D type). If this resistance block is used to measure the resistance between the electrodes of a semiconductor device, in fact, the effect of the measured current inside the semiconductor device cannot overcome the dead zone voltage of its pn junction at all. The resistance value measured is much higher than the resistance value measured by an analog multimeter, and there is no linear proportional relationship between the two, and there is no comparability, so it cannot constitute a reliable basis for judging whether it is good or bad. Therefore, the resistance block of a digital multimeter cannot be used to judge the performance of a semiconductor device. It should be switched to the diode test block for judgment, and this is specially designed for measuring semiconductor devices.

11. The applicable scope of the hfe test block must be clearly defined

The HFE range of a digital multimeter measures the crystal common emitter DC current amplification factor . Since it can directly display the HFE value in digital form , it is very convenient for identifying and selecting crystal transistors . However , it is little known that this range is only suitable for measuring the HFE of low - power crystal transistors , but not for measuring the HFE of large and medium - power crystal transistors . The reasons are analyzed as follows