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Recognition and Measurement of Electronic Components

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This video mainly explains the application of resistors

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Pull-down resistors and thermistor circuits

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The function of the pull-down resistor is to ground the uncertain signal through the resistor

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to set the working state of the signal

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For example, in the picture,

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after the CPU MIPI display circuit is grounded through a resistor,

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the working status of the display module can be set

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If the resistor is removed, the display module will not work,

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resulting in no display

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Pull-down Resistor Identification

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One end of the pull-down resistor is connected to the line,

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the other end is grounded,

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and the resistance value is not fixed

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In some circuits, the resistance value is around a few Kohms,

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and in some circuits, the resistance value is only a few hundred ohms or tens of ohms

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For example, resistor R1252, which is a pull-down resistor

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Thermistor

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Thermistors are mainly used to monitor the temperature of the mobile phone,

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and they are usually connected to the main power supply

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When the temperature rises,

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the resistance value of the resistor will change with the temperature

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When the main power supply detects that the signal is abnormal,

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it will cut off the power supply output, thus playing a protective role

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Thermistors are divided into positive temperature coefficient thermistors

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and negative temperature coefficient thermistors

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The positive temperature coefficient thermistor is the higher the temperature,

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the greater the resistance value

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The negative temperature coefficient thermistor is the higher the temperature,

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the smaller the resistance value

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Negative temperature coefficient thermistors are used in mobile phones

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After the thermistor is damaged,

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it will cause the high temperature alarm of the mobile phones

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Only need to replace or remove the thermistor can be solved

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Let's take the iPhone 12 model as an example to find the thermistor

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First open the component map, bitmap and schematic diagram

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Find the location of the main power supply in the component map,

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and then find the corresponding location number of the main power supply from the bitmap

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Then enter the location number of the main power supply in the schematic diagram,

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click Advanced Search, and view the module marked with NTC inside the power supply

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For example, on page 94, there is a small module marked NTC,

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which refers to the power supply temperature detection module

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The signal line from this module will be connected to the thermistor

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Find the signal connection component,

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then you will find that the resistor R4900 is a thermistor

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Different signal lines in the power supply monitor the temperature of different positions on the mainboard

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Such as the top of the mainboard, baseband power amplifier,

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CPU, battery, etc.

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For maintenance, only the resistor needs to be replaced,

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or the thermistor can be removed

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Ok, that's it for this lesson