# Application of resistance

After understanding the basic knowledge of resistance, it is to apply resistance, the application of resistance is mainly divided into voltage, current limit two.

First of all, from the partial voltage, is to use two or more resistors In series together, series circuit has I=I1=I2=I3=In, U=U1+U2U+U3+Un; Voltage divider circuit is usually used as voltage acquisition in digital circuit applications. The voltage is separated by two high-precision resistors and the voltage is not more than 5V(or 3.3V, 1.8V, etc., which is determined by the digital chip), and the voltage value is read out by ADC analog-to-digital conversion.

A temperature acquisition circuit can be formed by connecting a fixed-value high-precision resistor and a 10K negative temperature coefficient thermistor in series. (Negative temperature coefficient thermistor has the characteristic of decreasing resistance as temperature rises.) At room temperature of 25℃, the resistance of thermistor is 10K, assuming VCC=5V. Since the resistances of the fixed-value resistor and thermistor are equal, the collected AD point voltage is U=(10/10+10)*5=2.5V. When the temperature rises, the thermistor resistance value decreases. The AD point voltage can be calculated from (R5/R5+R2)*5, and then the temperature at this point can be read through the thermistor temperature-resistance curve.

In addition, in the application of DC TO DC circuit, it is also used two resistors at the output end of the voltage after the partial voltage, a voltage value after feedback TO the chip, stable output. The parameters of the two resistors can usually be calculated from the chip manual.

Can be seen from the above two circuits, the partial pressure of resistance value Settings are very big, is a 10 K or K level, mainly because of the parameters of the sampling process is voltage, not current, at the same time, set up big resistance can get very small current, basic or uA level is less than 10 ma, so can reduce the loss of resistance, use small package of resistance, reduce costs, The loss power of the resistance can be calculated by P=I^2*R.

Second, is the effect of current limiting, in fact, the resistance is used as current limiting at the same time, but also play the role of partial pressure;

For example, in this circuit, if the VCC is 24V, the fan is 12V, and the current is 0.06A, then to make the fan run normally, A voltage of 12V should be obtained at the positive pole of the fan, and the voltage allocated to the resistance is also 12V. Because the current of the series circuit is equal, the current flowing through the resistance is also 0.06A. Through R=U/I, it can be obtained that R=12/0.06= 200ω, and through P=I^2R, it can be obtained that the loss above the resistance is P=0.06*0.06*200=0.72W. Therefore, the packaging of resistance needs to choose 2512 (1W) package.

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