In the maintenance practice of the 5810R desktop centrifuge manufactured by Eppendorf, Germany, two failure examples were excluded and the experience was summarized.
1, centrifuge failure example 1
1.1 Fault phenomenon When the power is turned on, the display has no display; when it is turned off, the error message appears after the display shows “OFFâ€, and the display disappears completely.
1.2 Failure Analysis First check that the fuse is normal. Power on, online measurement of the display power supply LM7805 output terminal voltage value is +2.6V, after shutdown, this voltage value becomes +5V, the voltage value becomes 0 after about 2s; turn on again, measure the voltage of the LM7805 input terminal is +6V Left and right, this voltage is output through the switching power supply circuit, which is normally +15V. After replacing the new LM7805, the fault remains, and the LM7805 itself is excluded. It is preliminarily judged that the switching power supply circuit of +15V voltage is faulty.
1.3 Troubleshooting The vulnerable components around the transformer are measured. The diode VDI is short-circuited and measured from the board. It is still short-circuited. The diode acts as a freewheeling of the main winding of the transformer in the switching power supply circuit, and replaces the diode with the diode lN4007, and the fault is eliminated.
1.4 Fault Summary The diode VDl and the resistor R2 and the capacitor C3 together form a peak absorbing circuit. During the conduction of the field effect transistor IRF830, VD1 ensures that the circuit does not affect the power supply operation: when the FET is turned off, VDl is turned on, and the reverse induced electromotive force of the main winding of the switching transformer is released, thereby effectively protecting the FET from being broken down. Therefore, when VD1 is short-circuited, the switching power supply does not work properly.
2, centrifuge failure example 2
2.1 Fault phenomenon It is often turned on without display, and occasionally it can display normal and work normally.
2.2 Fault Analysis When the power is turned on, the two 15V voltage values ​​of the main control circuit and the power supply part of the display panel are all about 9-10V. The voltage of the power supply DC voltage of the switching power supply is +309V. It is judged that the switching power supply is not fully oscillated. The DC voltage C+ (normal value is 375V) is supplied by the power factor correction circuit. The power factor correction circuit is controlled by the chip L498IAD, which is powered by the +15V of the main control board. It is presumed that there are two possibilities for the cause of the fault: (1) The DC voltage C+ is abnormal, causing the switching power supply to fail to output the +15V voltage normally; (2) The switching power supply cannot output the +15V voltage normally, resulting in the power factor circuit not working properly:
Preliminary analysis, the advantage of this switching power supply is that the input voltage value range is very wide. When the voltage value of the DC input terminal is greatly reduced, it can still work. Therefore, the possibility of (2) is large, that is, the switching power supply itself cannot work normally. The +15V voltage value cannot be output normally, and the chip L498IAD does not work, resulting in the DC voltage C+ being only 309V.
Further analysis, the centrifuge can occasionally display normally, and it can work normally after display. Disconnect the connecting board of the most displayed board and the main control circuit, and start the main control board separately, and start normally every time. The 375V of the C+ terminal and the two +15V voltages of the output are normal. At this time, the display panel is connected, and the display can be normally performed. Therefore, it is judged that the switching power supply only fails at the moment of starting the vibration, and is likely to be in an overcurrent protection state.
The switching power supply is mainly composed of a switching transformer, a field effect transistor IRF830, a current type pulse modulation integrated circuit UC2844AD, a current sampling resistor R1 (0.45 ohm), etc., wherein the 5 pin of the UC2844AD is an overcurrent protection signal input terminal, when flowing through the main coil When the current is too large, the voltage across the resistor R1 rises. When the voltage at pin 5 exceeds IV, the UC2844AD activates the overcurrent protection function and stops outputting the drive signal to the IRF830. In order to avoid the false start of the overcurrent protection caused by the instantaneous spike current surge, the peak voltage is absorbed in the circuit to absorb the capacitor C1. When the power is turned on, the peak voltage generated by the instantaneous peak current flowing through the resistor R1 is absorbed by the capacitor CI to prevent the switching power supply from starting normally. Therefore, it is finally determined that the capacitor C1 is faulty.
2.3 Troubleshooting The capacitor C1 is soldered and measured to 1 nF. It is judged that the capacitance is too small to completely absorb the spike voltage at the start-up instant, and C1 is changed to 10nF. Reboot and display normally. After repeated switching inspections, this fault is eliminated.
2.4 Summary of Faults In this fault, the main control board can be started normally after the display panel is disengaged. I have suspected that the components on the display panel are damaged or deteriorated, resulting in excessive current. I soldered the rectifier diode VD2, and connected the DC I5V power supply isolated from the mains at the two ends of the filter capacitor C2. At the same time, the centrifuge and the external DC 15V power supply were started. The centrifuge showed normal operation and other functions were started. At the same time, the current consumption of the external DC 15V power supply is only about 0.1A, indicating that the display board itself is not faulty.
Because the design value of the capacitor C1 cannot be known, the analysis leads to the following two conditions: (1) the capacitor C1 is not counted as large, and then the capacity becomes smaller, causing the fault; (2) the original design value of the capacitor CI It is 1nF. However, after several years of use of the centrifuge, the components are aged, causing the surge current to increase when the switching power supply starts to oscillate, so that C1 cannot be completely absorbed, resulting in overcurrent protection.
Fully automatic Blood Glucose meter for household export Blood Glucosemeter Rapid Test Machine ,Purpose:On each test strip there is a test area containing reaction chemicals. When blood is applied to this area, a chemical reaction takes place, then a transient electrical current is formed. The blood glucose concentration is calculated based on the electrical current detected by the meter, then the result is showed on the display. The test measures glucose from 20mg/dL(1.1mmol/L) to 600 mg/dL(33.3mmol/L). The test strip is calibrated to diaplay the equivalent of plasma glucose values to allow easy comparison of results with laboratory methods.
Blood Glucose,Blood Glucosemeter Rapid,Blood Glucose test
Suzhou Xuanweicheng Biotechnology Co., Ltd , https://www.xwc-medical.com