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Rtotal = R₁ + R₂ + R₃ + ...
In a series circuit, the total resistance is simply the sum of all individual resistances. The same current flows through all resistors.
Series resistance is the total opposition to current flow when two or more resistors are connected end-to-end in a single path. In a series circuit, the same current passes through every resistor, while the voltage divides across each component proportionally to its resistance value. The total resistance is always greater than any individual resistor because each one adds its own opposition to the flow of electrons. This principle is fundamental to voltage divider circuits, current-limiting designs, and understanding how resistance accumulates in long wire runs or daisy-chained components.
Trace the circuit and list every resistor connected end-to-end through which the same current flows. Make sure no branches split off between them.
Read the resistance from the color bands, markings, or datasheet for each resistor. Convert all values to the same unit (ohms, kilohms, or megohms) before proceeding.
Apply the formula R_total = R1 + R2 + R3 + ... by simply summing every resistance value. For example, 100 ohm + 220 ohm + 470 ohm = 790 ohm.
Measure the total resistance across the entire series string with a multimeter set to resistance mode. The reading should match your calculated value within the tolerance of the individual resistors.
Series resistors are essential for limiting current to LEDs. Without the correct series resistance, an LED will draw excessive current and burn out almost immediately.
Two resistors in series create a voltage divider, which is used in sensor interfaces, biasing transistors, and creating reference voltages from a single supply.
When you need a resistance value not available as a standard component, combining series resistors lets you create precise custom values from standard parts.
The E12 series provides 12 preferred values per decade. These are the most commonly stocked resistor values available worldwide.
| E12 Value | 1x (Ω) | 10x (Ω) | 100x (Ω) | 1kx (Ω) | 10kx (Ω) |
|---|---|---|---|---|---|
| 1.0 | 1.0 | 10 | 100 | 1,000 | 10,000 |
| 1.2 | 1.2 | 12 | 120 | 1,200 | 12,000 |
| 1.5 | 1.5 | 15 | 150 | 1,500 | 15,000 |
| 1.8 | 1.8 | 18 | 180 | 1,800 | 18,000 |
| 2.2 | 2.2 | 22 | 220 | 2,200 | 22,000 |
| 2.7 | 2.7 | 27 | 270 | 2,700 | 27,000 |
| 3.3 | 3.3 | 33 | 330 | 3,300 | 33,000 |
| 3.9 | 3.9 | 39 | 390 | 3,900 | 39,000 |
| 4.7 | 4.7 | 47 | 470 | 4,700 | 47,000 |
| 5.6 | 5.6 | 56 | 560 | 5,600 | 56,000 |
| 6.8 | 6.8 | 68 | 680 | 6,800 | 68,000 |
| 8.2 | 8.2 | 82 | 820 | 8,200 | 82,000 |
No. The total resistance is the same regardless of the order in which resistors are arranged. Since current is identical through all components in series, rearranging them does not change the total opposition to current flow or the overall circuit behavior.
The supply voltage divides across each resistor proportionally to its resistance. A larger resistor drops a larger share of the total voltage. The sum of all individual voltage drops always equals the total supply voltage, following Kirchhoff's Voltage Law.
If any single resistor fails open (breaks), the entire series circuit stops conducting because there is only one path for current. This is a key disadvantage of series circuits compared to parallel circuits, where other paths remain functional.
Yes, but each resistor must be rated to handle the power it will dissipate individually. Since the current is the same through all series resistors, the power dissipated by each one is P = I squared times R. Higher-value resistors dissipate more power and may need higher wattage ratings.
Convert all resistors to the same unit before adding. For example, 1.5 kilohm = 1,500 ohm. Then add normally: 1,500 ohm + 330 ohm = 1,830 ohm (or 1.83 kilohm). Mixing units without converting is the most common source of calculation errors.