Resistance is the most abundant component in electrical equipment, but it is not the component with the highest damage rate. The most common resistance damage is open circuit, and it is rare for the resistance to become larger or smaller.
 
Commonly there are carbon film resistors, metal film resistors, wirewound resistors and alloy resistors. How can we judge whether they are damaged in the circuit? What are the characteristics after they are damaged? This is the problem engineers encountered. Today we will study what features and identification methods of resistance damage.

First, the characteristics of wirewound resistance damage
Wirewound resistors are typically used as current limiting currents. The resistance is not large. When the cylindrical wirewound resistor is burnt out, some may be blackened or the surface may be cracked or cracked; some may have no trace.
Cement resistance is a kind of wirewound resistor that may break when burnt out, otherwise there will be no visible traces. When the fuse is burned out, some surfaces will blow up a piece of skin, and some will have no trace, but it will never burnt and black. According to the above characteristics, when checking the resistance, you can focus on it and quickly find out the damaged resistance. 
 
Second, metal film resistance and carbon film resistance damage characteristics
The metal film resistor and the carbon film resistor are the most widely used, and the damage is characterized by a high damage rate of 100 Ω or less and 100 kΩ or more, and an intermediate resistance value (for example, several hundred ohms to several tens of ohms) is rarely damaged. When the low-resistance resistor is damaged, it is often burnt and black, which is easy to find, and there is little trace when the high-resistance resistor is damaged. 
   
Third, the resistance value test
According to the characteristics listed above, we can first observe whether there is any black mark on the low-resistance resistor of the circuit board, and then use a multimeter to directly measure the resistance across the high-resistance resistor on the board. If the resistance is greater than the nominal resistance, the resistor will be damaged. If smaller, then generally ignore it.
 
Through the above points, we first judge whether the resistance is damaged from the appearance of the resistor, and check it further by the resistance test method, so as to determine whether the resistance is burnt.