5V Usually this voltage should be a DC power supply. The most common way to adjust 5V DC to 3V power supply is to use LM317 integrated adjustable voltage regulator scheme, the peripheral circuit is very simple, the output voltage range is adjustable, the lowest output voltage can reach 1.25V.
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The voltage drop of the rectifier diode is about 0.7V, and the current can reach about 1V when the current is large, so the series of 2 to 3 rectifier diodes can fully meet the requirements according to the load. This method is relatively simple, do not need to change the circuit design too much, just series 2 to 3 rectifier diodes can be found according to the requirements of the load current, such as 1N4007 maximum working current of 1A, 1N5408 maximum working current of 3A. However, the output voltage stability of this method is not high, easy to be affected by the load current, and can only be applied to the load on the power supply voltage requirements are not high.