Electrical – Why transformerless power supply need a capacitor to decrease the current

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Assuming we have a 12 V load which needs 1 A current, when we connect this load to 60 Ah 12 V car battery it does not need to resistor and it will ony draw 1 A.

But on the other side, assuming we have a transformerless power supply circuit connected to 220 V so the DC voltage after diode bridge will be 310 V. Why can we not connect 100 LEDs in series (each 3.1 V) directly without using a capacitor or resistor?

I know this capacitor is used as reactance (\$X_C\$) to limit the current, but why is it needed since the voltage of LEDs is equal to the output voltage of the power supply? Shouldn't the LEDs should draw the required current?
According to this equation R=(Vin-VLED)/I =(310-310)/I = zero resistor needed

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Best Answer

How confident are you that your rectified mains voltage will be exactly 310V?

How confident are you that 100 LEDs in series will have a voltage drop of exactly 310V?
Have you read the datasheet? What is the Forward Voltage tolerance specification?

What if your mains is actually 315V and/or your string of LEDs adds up to 308V?

LEDs need current limiting - they are not voltage-driven devices.

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