Introduction to the basics of super smart multimeter

Automatic shutdown time setting:

Generally, digital multimeters use RC time-delay discharge method to achieve automatic shutdown. The main disadvantage of this method is that the time delay shutdown is affected by RC uncertainty and the timing is inaccurate. In addition, no matter whether you are measuring or not, it will shut down at that time. Although some timings are changed to CPU control, the shutdown time cannot be set.

Added automatic shutdown time setting function. The shutdown time can be selected. Scan the range before shutting down, and if it is in use, it will automatically delay the set time to shut down; if it is detected that the machine has not been used or pressed for a continuous period of time within the set time, it will automatically shut down after a buzzer prompt.

Backlight display time setting:

In order to solve the problem of difficulty in observing the measurement results when the natural light in the test environment is relatively weak, an LED light-emitting panel, commonly known as "backlight", is added to the back of the LCD. Since the backlight device consumes more power, the backlight is generally turned off with a delay after it is turned on. However, once the turn-off time is determined, it is often inconsistent with the observation time and cannot be changed.

The backlight delay shutoff time of this machine can be set. There are two state options: one is to set a time between 5 and 30 seconds; the other is to keep the backlight on as long as you press the backlight button, and turn it off again.

LCD display contrast setting:

This is a function that no digital multimeter currently has. The digital multimeter uses LCD display and is assembled according to the same specifications before leaving the factory. When the user observes the test results, the observation angle will not be exactly the same in different occasions and between the multimeter's LCD display. At this time, the user must change his standing position to meet the LCD observation angle, otherwise the LCD display will appear very dim or all symbols will be fully displayed from a certain angle.

The only way to solve it is to make the LCD contrast adjustable, which brings considerable difficulty to LCD manufacturing. This machine adds a contrast setting function to change the LCD display contrast by changing the LCD driving voltage method.

Timing measurement time setting:

As a multimeter with a timing measurement function, it is the dream of many users. In practical applications, too many timing measurements are used to detect battery charging and discharging, load characteristics, power supply stability, etc. Manual measurement is inefficient and time-consuming. With the timing measurement function, automatic measurement and recording are achieved to some extent.

Enter the timing setting state, first set the measurement interval. During the timing measurement, the LCD sub-display shows the time from minutes to hours. Every 6 seconds before the timing interval, a beep will sound every second, and a set of measurement data will be automatically recorded. When all the timing measurements are completed, the last beep will be extended.

Maximum/minimum storage setting:

Although the maximum/minimum value setting and the upper/lower limit setting have many similarities in operation, they are different in measurement purposes. The maximum/minimum value setting is mainly the selection of time intervals. During the measurement process, each sampled measurement data will be compared in each measurement time interval, and the maximum and minimum values ​​that have appeared in this time period will eventually be retained and recorded in the memory. The extended use of this function can also detect soft fault diagnosis of instruments and equipment, explore the time period when the fault occurs, and record the worst possibility.

Upper/lower limit setting:

The upper/lower limit setting is basically the same as the maximum/minimum value setting in terms of operation, but the former does not require time interval setting. During the measurement process, when the measured value exceeds the set upper limit or falls below the lower limit, a buzzer will sound an alarm. The displayed data will not be saved.

Stopwatch timing:

The multimeter does not have a timing function. However, in actual testing, a stopwatch is often used, especially for some tests that require simultaneous measurement and timing. In addition, choosing a timer also brings many inconveniences. Considering the convenience of the user, this machine can start the stopwatch timing function regardless of the range. It adopts a secondary display method, which is displayed on the same LCD page as the measurement result, realizing complete synchronization of timing and measurement. The maximum timing is 60 minutes.

AC parameter measurement:

(Audio Millivolt Meter/Power Level Meter).

The AC measurement range of general digital multimeters is relatively narrow, because traditional rectifier circuit designs use 062 or 358 op amps, and the frequency response range is only 10Hz~400Hz, which is completely within the power frequency measurement range. When the test requires the audio range, a true RMS measurement instrument is required.

By using an operational amplifier with a bandwidth greater than 100K and a parallel voltage divider circuit, plus a capacitor compensation circuit, the AC frequency response of the whole machine reaches 20KC, which meets the needs of audio testing. At the same time, the dBm indication can be switched to indicate the power level in decibels, which provides a guarantee for the debugging of the audio power amplifier.

Automatic data retention and storage:

Data hold is a common function of general digital multimeters. Automatic hold is rare. In actual measurement, one cannot spare a hand to press the hold key to achieve the purpose of data hold. The concept of automatic hold is to set the meter to the automatic hold state in advance. During the measurement process, the A/D sampling is basically stable within the 10% reading range after the CPU determines that it is compared, and an average value within the range will be obtained. At this time, there is no need to press the button again, and the measured data is directly held on the LCD display screen, and a buzzer prompt is sounded. If there is no stability during the measurement process, the automatic hold will always look for a value within the 10% reading range for comparison.

Sine wave signal output:

Ordinary multimeters cannot output sine wave signals, and some simple signals are often needed in actual measurements. This machine is the only multimeter with variable fixed-point sine wave signal output, and the output frequency is 500Hz/1kHz/2kHz/5kHz/10kHz/22kHz; it basically covers the audio range, bringing great convenience to circuit debugging.

Setting of on-resistance value for continuity test: (ET112).

At present, there are two ways to test the continuity of multimeters. One is to use an op amp as a comparator, which has a fast response speed. Due to the discrete parameters of the op amp, the on-resistance value of each meter is different. The second is to use the A/D result for comparison. This method has accurate on-resistance value and good consistency, but the buzzer sound has a delay, and the user feels slow. In these two methods, the on-resistance value cannot be set. It is very difficult to encounter a connection network with high continuous test requirements.

The only one adopts the latest circuit design, which can accurately set the on-resistance value and test quickly. It can meet special test needs and add a colorful touch to the functional design of the new multimeter.

Error sorting function:

In the LCR measurement state, sorting the measured components is a necessary function of the LCR bridge, but the multimeter does not have this function, which brings inconvenience to the LCR batch sorting or inspection. In order to better reflect the concept of designing for users, the error sorting function is specially set. Only when entering the LCR range and setting this function, you can first measure a relatively standard measured resistance/capacitance/inductance; then select a specific value between 0.5% and 20%. During the measurement process, the instrument automatically converts it into an error range. For the continuously measured LCR, if it is within the error range, the buzzer prompts PASS. Thereby improving the speed of continuous testing. When the standard LCR cannot be provided or the error sorting is asymmetric, the upper/lower limit can be set, and the PASS prompt will be given within the set range during measurement.

High frequency inductance measurement with reduced internal resistance influence:

Using 400Hz as the driving signal for inductance measurement, and using the inductive reactance of the inductor being measured and the AC voltage divider of the standard resistor to achieve inductance measurement, it will cause serious errors in the measurement of small inductances, because when the test frequency is relatively low, its DC internal resistance is comparable to the inductive reactance, and the voltage divider taken out on the inductor being measured is the sum of the inductive reactance and the internal resistance. To reduce the influence of the inductor internal resistance, the test frequency must be increased. This machine uses 22kHz to test μH inductors, which reduces the influence of the inductor internal resistance to a very small level. For example, when measuring a 100μH inductor with an internal resistance of 1 ohm, the inductive reactance presented by the inductor being measured at 400Hz is;

ZLx=6.28x400x100μH=0.25 ohms is only 1/4 of the internal resistance. At this time, the measurement error reaches 400%.

Tested at 22kHz;

ZLx=6.28x22000x100μH=13.8 ohms, the error can be reduced to less than 10%.

To solve the problem, high-frequency drive should be used, which is closer to the frequency range of small inductance operation, and the measurement results will be more realistic.