Attention must be paid to the following principles:

The series of metallized film resistors exhibit reverse oxidation reactions at dc loads, which can be more severe in hot and humid environments. When the resistor is under ac load, if the working frequency is low, it can be selected by winding resistor, metal oxide film resistor, metal film resistor, carbon film resistor, synthetic film resistor and synthetic core resistor series.

High frequency characteristics should be considered when operating at frequencies above tens of kilohertz. In general, the series of metal film resistors have three times the drift of the series of carbon film resistors. Therefore, in power reduction applications, the series of carbon film resistors appear to be less prone to drift failure than the series of metal film resistors. There is a precision alloy foil resistor that combines the advantages of both metal film and wire wound resistors. The resistance value is very stable and has good frequency characteristics. When the operating frequency is below 5 MHZ, it is basically a pure resistance, so it is often used as a standard resistance.

1. Working frequency

Frequency characteristics should be taken into account when applying resistors to ac loads. When the frequency increases, the resistance value will significantly deviate from the nominal value due to the effect of distributed capacitance, skin effect, dielectric loss resistor body and inductance effect caused by lead and other factors. The working frequency of winding resistor is generally no higher than 50kHz, while the working frequency of non-inductive winding resistor is up to 1MHz or above.

2. Resistance value stability

If the circuit function requires high stability of resistance value, such as precision attenuator, sampling voltage divider circuit, etc., attention shall be paid to the selection according to the different load conditions of the resistor. When working on dc load, the sequence of winding resistor, carbon film resistor, metal film resistor, metal oxide film resistor, synthetic film resistor and synthetic core resistor should be preferably selected.

The series of synthetic film resistors and synthetic core resistors rarely have open circuit or short circuit failure, but the resistance value is not stable enough. The use of such resistors can prevent the equipment failure caused by the sudden failure of resistors in insensitive circuits with tolerance design. Therefore, these two types of resistors are often used in circuit designs requiring high reliability.

3. Load characteristics

The resistance value shall not be greater than the power rating PR for both ac and dc loads. When the resistor is used at ambient temperature Ta greater than the allowable ambient temperature TR, the relation between the applied power P and the nominal power rating is: P=PR(ta-tr)(tmax-tr), i.e. PR shall be reduced. In the case of pulse power, the peak power of the resistor may be several hundred times the average power. The main limiting factors are:

(1) high voltage causes breakdown between layers;

(2) the pulse current is too large and exceeds the allowable current density. The order of the resistors' ability to withstand pulse load is winding resistors, carbon film resistors, metal film resistors, metal oxide film resistors, synthetic core resistors, and high pressure glass glaze resistors.

Ii. Selection criteria of resistor

According to the different materials used, resistor and potentiometer can be divided into alloy type, film type, synthetic three categories, in addition to the structure and use of other categories. The preferred type of fixed resistor is different depending on the application requirements.