Voltage divider - resistors value


Can somebody tell me how to choose correct resistor value in voltage divider circuit?


R1 = 1k Ohm, R2 = 2k Ohm OR R1 = 10k Ohm, R2 = 20k Ohm. Input voltage: 5V Output voltage: 3.33V


Assuming the output circuit takes no current, then the voltage across each resistor is proportional to its resistance:

(Vin - Vout) / R1 = (Vout - 0) / R2

The rest is simple algebra.

The voltage divider defines the ratio of the resistors (relative value). The absolute value of the resistors (1K or 10K or ...) defines how much amperes will go through it.

A 10K20K VD will draw 30K at5V => 5/30 mA = 160 uA A 1K2K VD will draw 3K at 5V ==> 1.6 mA

If the load impedance is high (say, an op-amp or Arduino input) either of those will work. You can't "power" anything with a voltage divider.

I'd generally go for about 10K total, but it's not that critical.

The load is in parallel R2 so the load impedance must be much higher than R2, or else your calculations won't be correct.

Otherwise, if the total resistance is low you'll get too much current through the voltage divider and that's a waste of power/energy and the resistors can even overheat if the power dissipation is too high.

It the total resistance is too high (maybe more than 100k) the circuit can be prone to noise pick-up.

That's it!


As a rule of thumb have ten times the current going down the two resistors as you want to take out of the lower leg.