Calculate Voltage Divider

ALPS RK50 Potentiometer
ALPS RK50 Potentiometer

A voltage divider usually consists of two resistors at which the total voltage Uges is divided into two partial voltages. The basic form is the unloaded voltage divider, which corresponds to a series connection of two resistors.

As soon as an unloaded voltage divider is connected to a consumer, the voltage distribution in the voltage divider changes.

A loaded voltage divider consists of the series connection of the resistors R1 and R2. In addition, one of the two resistors is loaded by a consumer, in this case by the resistor RL (load resistance).
The circuit changes from a series circuit to a mixed circuit consisting of a parallel circuit (R2 || RL) and a series circuit (R1 + (R2 || RL)).

If the voltage divider is loaded with a resistor, the following changes take place in the circuit:
  1. The total resistance of the circuit becomes smaller.
  2. Because of this, the total current I increases.
  3. The partial voltage U1 at the resistor R1 increases.
  4. The partial voltage U2 at the resistor R2 becomes smaller.
How big must the resistances in the voltage divider be?
It does not matter whether the resistors are chosen to be low or high. Because on the one hand you have to be careful that the resistances are not too small. Otherwise too much current will flow and too much power will be consumed. On the other hand, the resistances must not be too high, otherwise the divided voltage is too dependent on the load resistance.

How large the resistances have to be chosen depends on the application. And there are rules of thumb for this. One of these rules of thumb says that the voltage U2 only remains more or less stable if the current through the voltage divider I2 is about 3 to 10 times greater than the current IL flowing through the load resistor. Then you can neglect the voltage drop due to the load on the voltage divider. This can only be achieved with a high-ohm load resistor.

If you use the voltage divider to control a particularly high-resistance input, and that should be the rule, then the voltage divider can consist of resistors in the kOhm or 100 kOhm range, maybe even in the MOhm range.

Calculate Stepped Attenuator

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