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Resistors in Series & Parallel
Calculate the equivalent resistance of two, three or four resistors connected in series or parallel.
Ideal-resistor calculation. Real components have tolerance, temperature coefficient and power limits. Choose resistor wattage and voltage rating appropriate to the circuit.
Formula
Series and parallel resistor rules
Series resistors
Resistance adds directly: R total = R1 + R2 + R3 + R4.
Parallel resistors
The reciprocals add: 1/R total = 1/R1 + 1/R2 + 1/R3 + 1/R4. Two equal resistors in parallel give half the resistance of one.
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