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Note: The circuit diagram updates automatically as you enter values.
Note: You can add up to 10 resistors in parallel.
This analysis shows how resistor tolerances affect current distribution.
Given: Itotal=20mA, R1=470Ω, R2=220Ω
Given: Itotal=500mA, Rshunt=0.1Ω, Rsense=1kΩ
Given: Itotal=10mA, R1=10kΩ, R2=10kΩ
For two resistors in parallel, the current through R1 is
I1 = Itotal × R2 / (R1 + R2)
Note the opposite resistor on top. That is not a misprint: current divides in inverse proportion to resistance, so the smaller resistor takes the larger share. The formula is the mirror image of the voltage divider, where the resistor you are measuring across appears in the numerator.
The reason is that parallel branches share the same voltage, so by Ohm’s law each branch current is that voltage divided by its own resistance — which means lower resistance gives higher current. Two equal resistors split the current evenly; a 1 Ω and a 9 Ω branch split it 90/10 in favour of the 1 Ω.
Paralleled semiconductors. Putting two MOSFETs in parallel to share current only works if they really do share. The device with marginally lower on-resistance takes more current, heats more, and — because MOSFET RDS(on) rises with temperature — its resistance increases, pushing current back to the other. That positive on-resistance coefficient is a stabilising feedback and is why MOSFETs parallel reasonably well. Bipolar transistors and diodes behave the opposite way, so they need ballast resistors to force sharing.
Paralleled battery cells. Cells share according to internal resistance, and ageing lowers capacity while raising resistance, so mismatched cells never share equally.
Current-sense shunts. Deliberately dividing current through a known ratio lets a small sense element measure a large current, but any stray resistance in the paths — including PCB traces and solder joints — alters the ratio, which is why such layouts are drawn with great care.
I1 = Itotal × R2/(R1+R2) for two parallel resistors. The opposite resistor appears in the numerator because current divides in inverse proportion to resistance.
Because parallel branches share the same voltage, so each branch current is that voltage divided by its own resistance. Lower resistance therefore carries more current, which is the reverse of the voltage divider case.
The smaller one. A 1 Ω and a 9 Ω branch split the current 90/10 in favour of the 1 Ω resistor.
Not perfectly, but they self-correct. The device carrying more current heats up, and since on-resistance rises with temperature, its share falls again. That positive coefficient is why MOSFETs parallel reasonably well.
Because their forward voltage falls with temperature, so the hotter device takes an increasing share — a runaway rather than self-correcting behaviour. Small series resistors force sharing.
Cells share current according to internal resistance, so cells of different age or capacity never share equally. The stronger cell is worked harder and ages faster, progressively worsening the imbalance.