Rated power

Under the specified ambient temperature and humidity, assuming that the surrounding air is not circulating, the maximum power allowed to be dissipated on the resistor under long-term continuous load without damage or substantial change in performance. In order to ensure safe use, the rated power is generally 1-2 times higher than the power consumed in the circuit. Rated power is divided into 19 grades, commonly used are 0.05W, 0.125W, 0.25W, 0.5W, 1W, 2W, 3W, 5W, 7W, 10W.

 

The symbol for the power rating of the non-wirewound resistor in the circuit diagram is shown below:

Nominal resistance

The resistance value marked on the product is in ohms, kiloohms and megohms. The nominal resistance value shall conform to the value listed in the following table multiplied by 10N ohms, where N is an integer.

Allowable error

The maximum allowable deviation range of the actual resistance value of the resistor and potentiometer for the nominal resistance value, which indicates the accuracy of the product, and the allowable error level is shown in the following table.

A number or meaning represented by a color

Example:

1) Mark one end of the resistor with a color ring. The color scale of the resistor is arranged from left to right. The resistance in Figure 1 is 27000Ω ± 0.5%.

2) The color ring mark of precision resistor is represented by five color rings. The first to third color rings represent the effective number of resistance, the fourth color ring represents the multiplier, and the fifth color ring represents the allowable deviation. The resistance in Figure 2 is 17.5Ω ± 1%

 

 

Rules for marking units of resistors and potentiometers in circuit diagrams

The resistance value is above megohm, and the unit is marked m. For example, 1 megohm, marked 1M;2.7 megohm, marked 2.7m.

The resistance value is between 1 kilohm and 100 kilohm, and the unit is marked k. For example, 5.1 thousand euros, marked 5.1k;68 thousand euros, marked 68k.

The resistance value is between 100 kilohms and 1 megohm, which can be marked with unit k or unit m. For example, 360 thousand euros can be marked with 360k or 0.36m.

If the resistance value is below 1 kilohm, the unit Ω can be marked or not. For example, 5.1 ohms can be marked with 5.1Ω or 5.1;680 ohms can be marked with 680Ω or 680.

 

Maximum operating voltage

It refers to the voltage when the resistor works for a long time without overheating or electric breakdown damage. If the voltage exceeds the specified value, sparks are generated inside the resistor, causing noise and even damage.

 

Stability

Stability is a measure of the extent to which the resistance of a resistor changes under external conditions (temperature, humidity, voltage, time, load properties, etc.)

1) The temperature coefficient a indicates the relative change of the resistance value of the resistor when the temperature changes by 1 degree.

Namely:Where: R1 and R2 are resistance values at temperatures t1 and t2 respectively

2) The voltage coefficient av indicates the relative change of the resistor resistance value when the voltage changes by 1 volt,

Namely:Where: R1 and R2 are the resistance values when the voltages are U1 and U2 respectively

 

noise electromotive force

The noise electromotive force of the resistor can not be considered in the general circuit, but it cannot be ignored in the weak signal system.

The noise of wire-wound resistors is only due to thermal noise (caused by molecular disturbance) and is only related to the frequency band of resistance, temperature and external voltage. In addition to thermal noise, thin-film resistors also have current noise, which is approximately proportional to the applied voltage.

High-frequency characteristics: The resistor is used under high-frequency conditions, and the influence of its fixed inductance and inherent capacitance should be considered. At this time, the resistor becomes an equivalent circuit in which a direct current resistor (R0) is connected in series with the distributed inductance and then connected in parallel with the distributed capacitance. LR = 0.01-0.05 microhenries, CR = 0.1-5 picofarads, LR of the wire-wound resistor reaches tens of microhenries, CR reaches tens of picofarads, and LR still has a few tenths of a microhenry even for the wire-wound resistor without inductance winding.