Reliability of wirewound resistors for starting

　　Using wirewound resistors as starting resistors, but the reliability is not good, this is a problem that many companies will encounter.

　　Generally, the starting resistor mainly considers not only the rated power, but more importantly, the impact resistance. However, in general, the resistors used as starting resistors, such as wirewound resistors, may have lower requirements for impact power in normal national standards than in actual applications. For example, a 6-watt wirewound resistor may normally be able to withstand an impact of 60 watts for 5 seconds (I don’t remember the exact amount), but in the actual starting circuit, the instantaneous power may be much greater than 60 watts. Therefore, in actual use, it may exceed its tolerance and the resistor may be damaged.

　　Therefore, when selecting a resistor, the first thing to consider is how much impact power the circuit may have. Of course, the general impact time is much less than 5 seconds, so a resistor that can withstand 60 watts for 5 seconds can be used for a larger impact power (but the impact time is small). There should be a certain relationship between impact time and impact power, but unfortunately, manufacturers generally do not provide this curve.

　　Due to space and cost constraints, it is generally impossible to choose a resistor with a higher power at will. At this time, you need to work hard on the selection of manufacturers and certification tests. Resistors of the same power have very different impact resistance capabilities produced by different manufacturers. Therefore, when selecting and certifying, the samples of the manufacturer must be strictly tested. You cannot simply simulate the actual circuit for testing, but must be more rigorous than the actual circuit, otherwise the reliability difference between manufacturers cannot be compared. Even if you have selected a good manufacturer, you cannot simply apply it in large quantities. Because the test standard of the resistor manufacturer is the ordinary national standard, and your application environment is beyond that application range. If there is no special agreement, the manufacturer only needs to ensure that it meets the national standard. The sample may be better this time and pass your impact test, but when purchased in large quantities, the resistor may be worse than the sample, but meet the national standard; at this time, if their resistance defect rate is high in actual application, you have no reason to return it. Therefore

　　, at this time, you must sign a special agreement with the manufacturer. It is best that the manufacturer can guarantee the safety curve of impact time and impact power that meets the requirements of your actual circuit. Of course, ordinary resistor manufacturers may not be willing to sign such an agreement with you (because they think your quantity is small or their technical level is not up to par). At this time, there is a compromise. You can limit the resistance wire material they supply in the future to the same material, and the wire diameter of the resistance wire must not be smaller than the wire diameter of the sample. As long as the above two points are guaranteed, the impact resistance is basically determined. In addition, if possible, you can provide an impact circuit and ask the manufacturer to perform an impact test according to this circuit before leaving the factory. Don't expect to do a full impact test every time you inspect incoming goods, because that would be too costly, and it is more efficient for the manufacturer to do it than for you to do it yourself.