Two detection methods of common resistance and replacement of resistance

Author: Noyafa–CCTV Tester

Two common resistance detection methods and resistance replacement The resistance of objects to current is called resistance, and the components made of this resistance are called resistors, or resistances for short. The resistance is the most basic and commonly used electronic component in the instrument circuit; the two detection methods mainly used to stabilize the common resistance and the resistance of the replacement object to the current are called resistances, and the components made of this resistance are called resistances device, referred to as resistance. Resistors are the most basic and commonly used electronic components in instrumentation circuits; they are mainly used to stabilize and adjust the current and voltage in the circuit, that is, step-down, current limiting, amplitude modulation, shunt, isolation and other functions.

Graphical symbols for resistors and textual symbols for resistors see"Electrical Graphical Symbols and Text Symbols"As shown, the resistance text symbol is R, and its unit is ohm (Ω), kiloohms (kΩ), megohm (MΩ). Commonly used resistors are wire wound resistors, carbon film resistors, synthetic carbon film resistors, metal film resistors, solid carbon resistors, row resistors, etc. Resistance detection method The resistance detection can use a multimeter to measure the resistance value to determine whether it has faults such as open circuit, short circuit and resistance change.

To detect the resistance in the instrument circuit, online detection or open circuit detection can be used. ①On-line resistance detection method On-line detection is to use a multimeter to directly detect the resistance on the circuit board without soldering the resistor. This method is simple, but sometimes it will cause measurement errors due to the influence of other components in the circuit.

Therefore, during online detection, the influence of other components in the circuit on the resistance value must be considered. The operation steps are as follows: A. First, disconnect the power supply of the circuit board. Then observe the resistance to be detected to see if it is burnt, the pin is broken, the pin and the copper foil wire are open circuited or soldered. B. Then select the range of the multimeter according to the nominal resistance value of the resistance, measure the resistance value of the resistance, and remember to measure the resistance value R1.

Swap the two test leads of the multimeter, and then measure to obtain another measured resistance value R2, compare the resistance values ​​of the two measurements, and take the larger resistance value as the reference value. If the value is equal to or very close to the nominal resistance of the measured resistance value, it can be judged that the resistor is normal. If the value is larger than the nominal resistance value of the measured resistance, it can be judged that the resistance is damaged. If the value is much smaller than the nominal resistance value, it may be caused by the parallel connection of other small-value resistors in the circuit. At this time, the resistor should be welded open for detection and judgment.

②The resistance open circuit detection method The open circuit detection method can independently detect a single resistor. Compared with online detection, open circuit detection can effectively avoid the influence of other components in the circuit, thus ensuring the accuracy of measurement, but the operation is slightly more complicated. There are two methods for open circuit detection. One method is to solder one end of the resistor from the circuit board, and use a multimeter to test the resistance.

Another method is to cut the copper foil wire from one end of the resistor to the printed circuit, and then measure. Compare the measured value with the nominal resistance value to determine if the resistance is normal. After the measurement is completed, the resistor should be welded and restored immediately to avoid missing welding and new failures.

When measuring high-resistance resistors, do not touch the test leads and the two electrodes of the resistor with both hands to prevent measurement errors caused by human body resistance shunting. Replacing the resistance method The faulty resistance should be replaced after inspection. The replacement of the resistor is given priority to the same type, same material and same resistance value.

If there is no identical resistance, it can be considered as a substitute, but it should be noted that in general circuits, high-power resistors are allowed to replace low-power resistors of the same value. For occasions with temperature requirements, the working temperature of the resistor should be considered. For example, metal film resistors can be used in Long-term work below 125°C, and carbon film resistors are only suitable for working at temperatures below 70°C; ordinary resistors cannot be used instead of precision resistors. The resistance used for sampling the protection circuit should be replaced with the original value and equal power resistance. After the resistor is damaged, if there is no resistor of the same specification to be replaced, the resistors can be connected in series or in parallel for emergency.

Use the series formula of resistors R=R1+R2+...+Rn to turn several low-resistance resistors into required high-resistance resistors. The parallel formula of resistors 1/R=1/R1+1/R2+...+1/Rn can be used to convert several high-value resistors into desired low-value resistors. Whether in series or in parallel, the amount of power shared by each resistor must not exceed the rated power allowed by the resistor itself.

After the fuse resistor is damaged, if there is no fuse resistor of the same type to be replaced, other fuse resistors with the same main parameters can be used; the fuse resistor with a small resistance value can be replaced directly by fuse; it can also be used in the form of a series connection between the resistor and the fuse. Substitute.