Current Divider Calculator

The current divider is the dual of the voltage divider: just as a voltage divider splits voltage across series resistors in proportion to resistance values, a current divider splits current among parallel resistors in inverse proportion to their resistance values. More specifically, the current through each parallel branch equals the total current multiplied by the ratio of total parallel conductance to that branch's conductance: I_n = I_total × G_n / G_total = I_total × (1/R_n) / (1/R_total).

For two parallel resistors, the simplified current divider formula is: I1 = I_total × R2/(R1+R2) and I2 = I_total × R1/(R1+R2). Notice that I1 depends on R2 (the other resistor), not R1 — the branch with lower resistance carries more current, but the formula uses the opposite resistor in the numerator. This is the key distinction from the voltage divider, where Vout = Vin × R2/(R1+R2) depends on R2 itself.

Current dividers appear throughout circuit design. In current mirror circuits — the backbone of analog IC design — a reference current is mirrored (copied) to multiple output branches by sizing transistors rather than resistors, but the underlying principle is current division. In instrumentation amplifiers, current flow through matched resistor networks must be precisely controlled. In power distribution, current sharing among parallel conductors or fuses depends on their relative resistance.

Practical current sharing challenges arise when parallel components have even small resistance mismatches. In parallel power MOSFETs carrying high load current, a MOSFET with slightly lower R_DS(on) carries more current, heats up more, and if the temperature coefficient is positive (R_DS(on) increases with temperature), self-stabilizes. This 'positive temperature coefficient' is why MOSFETs parallel better than bipolar transistors, which have negative temperature coefficient that causes thermal runaway in parallel configurations.

Battery cells in parallel are another current divider application. Each cell has internal resistance, and current divides inversely to internal resistance. Cells with lower internal resistance (healthier cells) contribute more current, while aged, high-internal-resistance cells contribute less. This self-balancing effect is benign up to a point, but if cell internal resistance differs greatly, the low-resistance cell may be forced to supply most of the current, accelerating its degradation.

This calculator provides branch currents for two or three parallel resistors, the common voltage across the parallel combination, and the total equivalent resistance — a complete current divider analysis toolkit.