Using the calculator
Type the input voltage and the two resistors, top one first: `5V, 1k, 2k`. The output voltage appears with the current through the pair and the power each resistor has to handle, and Show the steps draws the divider.
The voltage needs its V so the calculator can tell it from the resistors. Resistances can be written as 1k, 4.7k, 4k7 or 2200.
How a voltage divider works
Two resistors in series share the input voltage in proportion to their sizes. The output is taken across the bottom one, R2, so Vout = Vin × R2 ÷ (R1 + R2). Make R2 bigger and more of the voltage lands across it.
With 5 V, R1 = 1 kΩ and R2 = 2 kΩ, Vout is 5 × 2 ÷ 3 = 3.33 V. That particular pair is the classic way to bring a 5 V signal down to a level a 3.3 V microcontroller pin can read.
When the load spoils it
The formula assumes nothing draws current from the output. Anything you connect sits in parallel with R2 and lowers its effective resistance, which pulls Vout down. A load ten times R2 or more barely matters. One close to R2 matters a lot. That's why a divider is fine for feeding a signal into an input but poor for powering something, which is a job for a regulator.
Smaller resistors make the divider stiffer against a load but waste more current. The calculator shows the current and power so you can judge the trade.
Questions
What's the voltage divider formula?
Vout = Vin × R2 ÷ (R1 + R2), with Vout measured across R2, the resistor nearer ground.
How do I get 3.3 V from 5 V?
R1 = 1 kΩ and R2 = 2 kΩ gives 3.33 V. Any pair in a 1 to 2 ratio works.
Can I power a circuit from a voltage divider?
Not well. The current the circuit draws changes the output voltage. Use a voltage regulator for power, and a divider for signals.
Which resistor is R1?
The top one, between the input and the output. R2 goes from the output to ground.
Sources
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