Principle and Overhaul of DT electronic scale

DT electronic scale consists of sensor, amplification circuit, A/D transformation circuit, display circuit, chip microprocessor, memory and power source etc. Sensor transforms weight into voltage signal correspondingly, then to be amplified, thereafter enter into A/D transformation circuit, turn into digit, and at last digital weight is displayed on the indicator screen after calculated by chip microprocessor.

  Ⅰ  Operating Principle

1.       Sensor

DT electronic scale users aluminum alloy double-hole beam-style resistance strain sensor. In terms of different weight and precision, select proper sensor.

2.       Amplification Circuit

Positive and negative terminal of signal output of sensor respectively link positive and negative input terminal of the integrated operational amplifying circuit. And this magnification times is decided by R1. When no-load, voltage of output terminal (foot 6) should be lower than half power source voltage, exactly to say from 6.5V to 7.3V. If above 7.5V, A/D transformation will fail, meanwhile scale will display “F----5”. There should be more than 1.5V voltage difference between full-load voltage and no-load voltage of foot 6, and no-load voltage is higher than full-load voltage; if less than 1.5V, calibration will fail. Bigger resistance value of R1 is, bigger voltage difference is. IC2 is a filter putting out equal voltage to IC1. According to these values, we can judge whether sensor, IC1 and IC2 work normally or not.

IC3 is a sampling voltage tracking generator. For example, when R6, R5, R4 respectively are 4K, 3K, 1K, voltage on foot 6 of IC3 is 9.3V; if R6, R5, R4 are respectively equal to 2K, 1K, 1K, voltage on foot 6 of IC3 will be 11.5V.

3.       Principle of AD Transformation Circuit

 AD circuit adopts double-integral theory. At first, it integrate signal positively (notice: voltage into which weighing signal is transformed after amplified must be lower than midpoint voltage of power source in AD part; then will have a favorable integration). And then, integrate it negatively using constant voltage higher than midpoint voltage. Time clock calculates the time which negative integral needs when back to original state; this timing value is inside code namely.

On the whole, if these parameters above changed, the part of AD will not work well: Displaying “F----5” means that there is some trouble in artificial circuit. First check whether sensor broke down or not. Detect whether voltage on foot 6 is normal or not; if abnormal, sensor might break down.


     Turn on the scale first. Then it display “------”; following is from “F----2” to “F----9”.

If stop when “F----3”is shown on, this means there is some key conducted.

If stop when “F----5” is shown on, this means A cannot be transformed into D.

Usually, voltage into which weighing signal is transformed after amplified isn’t lower than midpoint voltage of power source (in AD part). Generally because sensor is not good or improperly linked, or maybe occur some problems with amplification circuit.

After self-detection, it displays “1”; one chirp of buzzer is heard simultaneously; then display “0”. If displaying “F----L”, this means base weight is too light, or very small value is transformed in AD transformation, all these may make result wrong. Common reason is zero-shifting of sensor.

And “F----H” means overload, that is to say outside upper limit of range ability. Press calibration button, it will display “C-----” normally after a while. Though “C----F” indicates zero point isn’t stable. It is necessary to reset, correct again.

Caution: before correction, pare first as usual.

When display “C----F”, there may be another problem that base weight of tray is too heavy. Sensor needs to repair.

If 5-6 minutes passed by after correction, there is nothing displayed; this represents value of AD transformation cannot get to minimum limited value (may relay when calibration time is unstable; calibration value must be gained when stable, otherwise wait until stable).

There is a minimum limited value which must be reached; then can enter into counting state, or else cannot (notice: when counting, unit weight cannot be too light, especially cannot be less than 1g, or else occurs unstable end of digit counted.




1.display F----5 not add tray

2.sensor doesnt work well

3.without 15V power source 

2.display F----H


2.sensor doesnt work well display not stable

1.sensor contacted conducting wire, and so on

2.15V power source not stable

3. units on circuit card welded virtually

4.crystal oscillator broke down

5. capacitor is leaking electricity

6.sensor went bad

7.bad reception of sensor

8.4066 broke down

4.display F----3

1.some key has been conducted or under short circuit condition

5.display lacking of segments

1.bad core of chip microprocessor

2.some bad segment of numeric code tube

3.74∠S273 went wrong

6.display lacking of figures

1.bad core of chip microprocessor

2.triode invalid accordingly

3.numeric code tube invalid

7.signal alarm alarming continually

1.bad core of chip microprocessor

2.crystal oscillator broke down

3.reset capacitor invalid

8.display 8 wholly or not display

1.chip microprocessor invalid

2.crystal oscillator broke down

9.linear weight display incorrectly

1.improper amplifying resistance

2.improper adjusting linear resistance


Copyright:ChangShu Yiou Ever-bright Apparatus Co.,ltd(Changshu Changqing Instrument and Meter Plant)