Views: 0 Author: Site Editor Publish Time: 2018-12-06 Origin: Site
Voltage method:
The voltage method is a method of measuring the voltage in a circuit with a multimeter and determining the fault location according to the change of the voltage. The voltage method is based on the principle that the voltage tends to change when the circuit fails.
The principle of voltage detection method:
The voltage check method is mainly used to deal with circuit faults, especially integrated circuit faults.
The operating voltage of some test points in the measurement circuit is still not large, too small or too small, and the cause of the voltage change is discriminated. This is the cause of the failure.
When the circuit is in normal operation, the working voltage value of each part is certain (it is also possible to fluctuate within a small range). When the circuit is open circuit, short circuit, and the performance parameters of the component change, the voltage value will inevitably change accordingly. The task of the inspection method is to detect this change and analyze it.
The voltage check method mainly measures the DC voltage in the circuit, and can measure the AC voltage and the signal voltage when necessary. The voltage check method is applicable to the inspection of various active circuit faults, and is mainly applicable to checking AC circuit faults and DC circuit faults, and has good effects on other circuit faults.
When measuring the DC working voltage, use the appropriate range of the multimeter DC voltage file, the black meter pen is connected to the circuit board ground wire, and the red test pen is connected to each measurement point. The normal DC working voltage of each key test point in the whole circuit has special data. When there is no such information, it is necessary to analyze it according to the actual situation. The following various measurements are correct.
1) The DC working voltage of the whole machine is much higher (several volts) than the working time at no load. The higher the internal resistance of the power supply is, the higher the internal resistance of the power supply is. .
2) The DC voltage at the output of the rectifier circuit in the machine is the highest, and the RC filtering and decoupling circuits are reduced step by step.
3) The DC voltage at the positive terminal of the electrolytic capacitor should be higher than the DC voltage at the negative terminal.
4) When measuring the voltage across the capacitor is zero, as long as there is a DC voltage in the circuit, it means that the capacitor has been short-circuited. The DC voltage across the inductor coil should be very close to zero, otherwise it must be an open coil fault.
5) When there is DC working voltage in the circuit, there should be a voltage drop at both ends when the resistor works. Otherwise, the circuit where the resistor is located must be faulty.
2. Use of voltage method
When measuring with the voltage method, you can measure the voltage at a certain point in the circuit or the voltage between two points in the circuit.
(1) Measuring voltage at a certain point in the circuit
Measuring the voltage at a point in the circuit is actually measuring the voltage between the point and ground.
(2) Measuring the voltage between two points in the circuit
Voltage Uce; If the red test pen is connected to the upper end of R3, the black test pen is connected to the lower end of R3, and the measured voltage is the voltage UR3 across R3 (or the voltage drop on R3); if the red test pen is connected to the upper end of R2, the black test lead is grounded (ground and R2) The lower end is directly connected), the measured voltage is the voltage UR2 across R2, which is the same as the base voltage Ub of the triode; if the red test lead is connected to the positive pole of the power supply and the black test lead is grounded, the measured voltage is the power supply voltage (12V).
The following examples illustrate the use of the voltage method.
The LED VD1 is not lit. The +12V power supply is measured normally during the test. No voltage is measured at point A. Then there is no voltage after tracking to point B. When the C point is detected, there is voltage. The reason for the analysis may be R2 open circuit. The voltage at point C cannot pass through R2, or the short circuit in C2 shorts the voltage at point B to ground and the voltage at point B is 0V. When the R2 and C2 are detected by the resistance method, it is found that the C2 is short-circuited. After replacing C2, the LED emits light. At this time, the voltages at points B and A are measured.
3. Voltage method use precautions
Pay attention to the following points when using the voltage method to detect the circuit.
1 When using the voltage method, the multimeter's internal resistance will cause a shunting effect on the circuit under test, which will cause errors in the measured voltage. In order to reduce the measurement error, the multimeter with large internal resistance should be used as much as possible. The internal resistance of the MF50 multimeter is 10kΩ/V (if the gear selector switch is set to “2.5V”, the internal equivalent resistance of the multimeter is 2.5×10kΩ=25kΩ), and the internal resistance of the 500 multimeter and MF47 multimeter is 20kΩ/ V, and the internal resistance of the digital multimeter can be regarded as ∞.
2 When measuring the circuit voltage, the multimeter black meter pen is connected to the low potential, and the red meter pen is connected to the high potential.
3 When measuring, first estimate the voltage of the tested part to select the appropriate gear. The selected gear should be higher than and closest to the measured voltage. Do not use the high gear to measure the low voltage, or use the low gear. high voltage.
