The conduction characteristics of the LED belong to the Zener type, that is, when the voltage applied across the device reaches a certain value, the conduction current tends to rise linearly. This Vz (avalanche voltage) of the LED is typically between 2V (red) and 3V (white).
A 12V system used in automobiles requires a series current limiting resistor. Unless it is only possible to use a single LED, generally 3 to 4 LEDs should be connected in series with a current limiting resistor.
The specific algorithm is: R = (Vop - NVz) / Iled. Vop is the working voltage, Vz is the avalanche voltage of the LED, N is the number of series, and Iled is the expected LED operating current.
Note: There is room for serialization. For example, the white LED can be calculated in series for 12V systems, but because the terminal voltage of the battery is only 12.3V - 12.6V when the generator is not turned on, it is brought into the above formula: R = (12.3V - NVz) / Iled, Iled takes 20mA, Vz takes 3V, and the calculated R is 15 ohms. However, after the engine is started, the terminal voltage of the battery will gradually rise to 13.8V-14V, and the Iled will become 120mA! The difference between the three serial connections is between 20mA and 29mA, which is acceptable.
The number of series is reduced, the electrical efficiency is reduced, and the current limiting resistor is increased.
2: Internal resistance, can't draw a picture now! The word is the equivalent impedance of an object that is connected to the circuit system. When incorporated into a circuit system, it appears to consume a current in the system or a partial pressure with the source internal resistance. When stringed into the system, it appears to generate pressure and lose system voltage.
As a measuring instrument, in the above two cases, the measurement accuracy will be affected.
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