Similar to Kirchhoff’s Law, the Voltage and Current Divider rules help us calculate the related voltages or currents in a circuit. There is much more to this but if you are in a rush here are the equations to use:

Important:

  • In Parallel branch
    • Voltages are the same
    • Current splits (not equally, but based on the resistance in the branch)
  • In Series (2 resistors after each other)
    • Voltage drops are different (depends on the value on the resistor)
    • Currents are equal

Voltage Divider

A Voltage Divider turns a larger voltage down to a smaller voltage.

Looking at the Voltage Divider example circuit above, imagine connecting your headphones to your phone. The circuit is your phone and where V2 (Vout) is, is the output port where the headphone jack goes.

This circuit is the basics of Voltage Dividers where we have

  • 2 resistors in series (when no load (headphone) is connected)
    • Once you add a load R1 and V1 won’t be in series anymore
  • We are taking the signal/voltage from across one of them
  • There is an EMF (E) across the resistors

To understand Voltage Dividers more we approach it form 2 angels:

Case 1: Find Vout

Let’s take the following circuit as an example.

  • 12 V battery
  • Two resistors R1 and R2 in series
  • Current is equal in both resistors
Equation: Current is the charge over time

This is the simplest case where we can just use the equation from above:

Case 2: You have a 12V battery. Design a voltage divider circuit with an output of 5V and 50mA

Equation: Current is the charge over time
  • We know the total voltage: 12V
  • We know the current in the circuit: 50mA

    • Since there is nothing connected at the output at this time, R1 and R2 are in series, thus, the current flowing across them is the same.
  • Using Ohm’s Law : V=IR we can calculate the total resistance (R1+R2).
  • The voltage drop across R1 is obvious: 12V – 5V = 7V
  • Now simply use the voltage divider rule to find R1 and R2:

If R1 = R2 in a voltage divider, the voltage output Vo is the half of the EMF. With a 12V battery and R1=R2=100Ohm, V1=Vo=6V.

Above we calculated the output voltage when no load was connected. When a load is connected, simply calculate the equivalent resistance (R2 in parallel with Rload) and use Requivalent in the equation.

If you want to get a voltage drop across the load as close as possible to the voltage drop across R2 when no load is connected, chose a load with large resistance. Rule of thumb says: 100x.

So if R2 is 100Ohm Rload = 100,000 Ohm.

optional reading: Success in Electronics book by Tom Duncan
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