User manual

5 V / 1,024 = 0.00488 V = 4.88 mV

If we were to raise the input voltage range to 30 V, the

resolution would deteriorate five-fold ((30 V – 5 V) / 5 V = 5).

We would »only« be working with a resolution of 0.0244 V = 24.4 mV

anymore.

Now we can determine the voltage divider for a measuring range

up to 30 V. We already know that we want to expand the input range

from 5 V to 30 V, which corresponds to factor 5. The measuring

input for voltage measurements must not be too low-Ohmic and

should be at least 100 kΩ. Modern voltage meters in contrast,

have an input resistance of 10 MΩ to put as little stress as

possible on the voltage source and to avoid falsifying the

measuring result as far as possible.

Let us assume an input resistance of approx. 100 kΩ and define

the resistor R1 at 100 kΩ. In a serial circuit, the ratio of the

voltages is identical to the ratio of the resistors to each other.

The resistor R2 only needs to be 1/5 as large as the resistance

of R1. We take 100 kΩ / 5 and will receive the value 20 kΩ for R2.

Let us now determine the current that flows in the circuit. In

a serial circuit, the current through the resistors is the same

and the voltages will be divided. The current through the

resistors is calculated as follows:

I = U / R

Total

30 V / 120 kΩ = 0.25 mA

The following voltage drop would result at R2 then:

U = R2 x I

20 kΩ x 0.25 mA = 5 V

At an applied voltage of 15 V, the value at the ADC would be:

15 V / 120 kΩ = 0.125 mA

20 kΩ x 0.125 mA = 2.5 V

Let us calculate the power loss across the resistors at maximum

input voltage now:

P = U x I

30 V x 0.25 mA = 7.5 mW

You can now calculate your own matching voltage divider and

adjust the measured value in the program by multiplying it by

your voltage divider factor. The measuring error can be de-

termined by experimenting with a precise voltage meter and is

included in calculation of the multiplication factor. Observe

the resistance values of the E-series and do not try to include

the mathematically determined values in a circuit by using

unusual E-series or even trying to switch resistors in series

or in parallel to get precisely the calculated resistance.

This calculation also does not consider the input resistance of

the ADC. In the Arduino™-UNO ATmega328, it is at approx. 100 kΩ.