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The Current Divider: When the River Forks

  • 4 min
  • Beginner

In the previous article we saw how to distribute voltage by connecting resistors in series. Now we will see how to distribute the flow of electrons by connecting resistors in parallel using a current divider.

What it is and how it works

A current divider is a simple linear circuit that takes advantage of Ohm’s Law and Kirchhoff’s Law to distribute current between two components.

Physically, it is nothing more than two resistors connected in PARALLEL between the voltage () and Ground (GND).

The physical principle: the path of least resistance

Electricity is lazy. It always prefers the easy path. At a node where the current splits:

  • The branch with LESS resistance will take the GREATER share of the current.
  • The branch with MORE resistance will take the SMALLER share.

The formula

Imagine a total current () arriving at a node and splitting between two resistors ( and ). We want to know how much current flows through, for example, resistor 1 ().

The formula is:

Note carefully the numerator. To calculate the current in branch 1 (), we multiply by the opposite resistor ().

  • In the voltage divider we placed our own resistor () on top.
  • In the current divider, to calculate we place the resistor from the other branch () in the numerator.

And, logically, for the other branch:

Where this formula comes from

It is a direct application of Ohm’s Law and the fact that in parallel the voltage is the same ().

  1. Calculate the equivalent resistance ().
  2. Know that .
  3. Apply Ohm’s law to branch 1: .
  4. By substituting, we obtain the formula above.

Numerical Example

We have a total current of 10 Amperes arriving at a junction.

  • Path A: Resistance of .
  • Path B: Resistance of .

Where will the electrons go? Intuition tells us that path A is 4 times easier than path B, so it should carry much more current.

Let’s calculate :

Let’s calculate :

Result: 8 A flows through the 2 Ω path and 2 A through the 8 Ω path; in total, 10 A.

Voltage Divider and Current Divider

CharacteristicVoltage DividerCurrent Divider
ConnectionSeries (Row)Parallel (Ladder)
Constant VariableCurrent () is equalVoltage () is equal
Variable that changes is divided is divided
ProportionalityDirect ()Inverse ()
Primary UseSensors, ReferencesShunts, Mesh Analysis

Key difference from the voltage divider:

  • In the voltage divider: more resistance = More voltage (Proportional).
  • In the current divider: more resistance = Less current (Inversely Proportional).

Real-world applications

Current Measurement (the Shunt)

To measure current, a shunt resistor of very small value is often used. The instrument measures the voltage drop across its terminals and calculates the current using Ohm’s law.

  • 99.9% of the current flows through the Shunt (easy path).
  • A tiny, safe fraction flows through the delicate meter. Knowing the proportion of resistances, the device calculates the total current.

The Danger of LEDs in Parallel

A typical beginner mistake is connecting two LEDs in parallel using a single resistor to limit the total current.

Theoretically, the current would split 50/50. In practice: No two LEDs are identical.

  1. One of them will have a slightly lower forward voltage or will heat up a bit more.
  2. That LED will conduct more current because the current-voltage relationship is not linear.
  3. As it heats up, its forward voltage usually decreases, and the sharing can worsen.
  4. Avalanche effect: One LED ends up hogging all the current and burns out. The other, left alone, suddenly receives all the current and burns out afterwards.

Solution: NEVER use passive current dividers for semiconductors. Put a series resistor for each LED.

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