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Why do Charger Circuits use Winding Resistance?

Writer:Microhm Page View:Date:2019-04-09

There are many kinds of resistors in our life, each kind of resistor has its own characteristics and technical advantages. Common ones are  high-power resistance, chip resistance, plug-in resistance, precision resistance, brake resistance and so on. In charger products, wire wound resistor use the most. So why do chargers use wire wound resistor, what are the advantages of them?
The common wire wound resistor, made of a resistance alloy wire, is generally made of a nickel-chromium wire or a manganese copper wire. Apart from this, lots of wire wound resistors are wound on the porcelain tube by a constantan wire. The wire wound resistor is divided into fixed resistor and adjustable resistor. Our commonly used wire-wound resistance has high stability, good resistance accuracy, low resistance noise, and can withstand high temperature. It can be used normally at 170 ℃. However, the drawback of this kind of wire-wound resistance is also very obvious. It is large, low resistance, most of which are less than 100K Ω.

In addition, due to structure, the winding resistance has a large coefficient of distributed capacitance and inductance, and cannot be used in high-frequency circuits. Our common wire-wound resistance value is accurate, except for the charger output voltage reference, other chargers do not use wire-wound resistance. For example, mobile phone charger circuit is SMPS switching power supply circuit.

The winding resistance is a current limiting element. When the winding resistance is connected to the circuit, it can limit the amount of current passing through its connected branch. If the resistance value of a wire-wound resistance is close to zero ohms, the wire-wound resistance has no effect on the current, and the circuit connecting the wire-wound resistance in series is short-circuited and the current is infinite. Because of the high demand of the signal generation circuit at the high voltage end, it is impossible to adopt the wire-wound resistance, otherwise it will lead to self-excitation and affect the oscillation frequency, so that the output fluctuates greatly. Only the sampling reference circuit at the low voltage end may use the wire-wound resistance. A cell phone charger circuit is not a high-precision electrical apparatus, it does not need to calculate the capacitance and inductance of the PCB board. Moreover, the switching power supply itself fluctuates greatly, and the filtering effect of a wire-wound resistor is generally negligible.

Keywords:Winding Resi

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