How to reduce voltage using resistor:If we have to reduce voltage from a voltage we use resistor to reduce the voltage, say we have 19v on a Laptop adapter for the input voltage to laptop motherboard, the motherboard's charging IC needs some signals or voltages to enable the circuit. say the charger IC's ACDET pin needs some voltage to identify the adapter is connected. For that resistors are used to reduce the voltage and receives voltage to the ACDET pin, the ACDET needs only about 2.8v or something ( the IC designed in that way), so the input voltage needs to be reduced to get the 2.8v, to get the 2.8 voltage electronics engineers use resistors, Resistor 1 and Resistor 2 ( R1 & R2) in between the 2 resistors we get the desired voltage. lets see how this works.
The input voltage is 19v from adapter R1 is 100 Kilo Ohm resistance & R2 is 18 Kilo Ohm resistance, the formula is Input voltage x R2 divided by R1+R2 ACDET voltage (19x17)/(100+18)=2.8v we get. We get the R1 and R2 value by measuring resistor or we can use the schematic diagram to know the resistor's values.
What to do to get a desired voltage?
If we want to reduce voltage from a given voltage for example we have 9v battery we need to glow a LED, typically a LED needs 3v, so what resistor values we need to reduce 9v to 3v for that we have a formula to calculate, the R1 value can be any value, R1 resistor we may have in stock in our spare box.
So we take a resistor we have in our hand say 100k ohm resistor we have, lets keep the resistor as Resistor 1 (R1) and we have to find the Resistor 2 resistance value (R2), the formula is R2= Voltage output multiplied by R1 and divided by voltage in minus voltage out
so we have 9v and need 3v as desired voltage to lid the LED bulb, so we apply the formula above, we take 8Kilo Ohm resistor as R1 that we have in our hand in ready stock and need to know the R2 value R2= (V out)(R1)/(Vin - Vout)= (3V)(8KΩ)/(9V - 3V)= 4KΩ
in this way we get the voltage from an input voltage.